I have been trying to make a 395 cfm Holley 4 barell run smoothly on my 302 GMC engine. It hesitates at low rpm and during takeoff. I have checked for vacuum leaks and changed the accelerator pump nozzels and cams but the hesitation is still there. The car really goes well once it gets past the hesitation problem, but it is very difficult to drive.

Does anyone I know if there is a low cfm Rochester or Carter 4 barrel that I could use as a direct replacement?

Check the power valve/s. Too weak and it'll flood slightly. Too strong and it leans out.

Is the intake heated?

IMHO there are several options. One would be a TLowe "3 barrel". Basically it's just a bigger Holley carb than you have with one of the primary barrels plugged. Another option, would be a Rochester Q Jet. By limiting the amount of secondary air valve opening the CFM could be adjusted. This was done from the factory on smaller engines like the 305 Chevy. By putting a screw in the air valve the CFM could be adjusted from 750 to well below 500. There is also a Carter AFB. There is a 500 CFM model and a 400 CFM model. The 400 might be hard to find. In the marine world, the 625 model was put on a 262 cubic inch Chevy v6. I don't know much about 302 GMC engines and their air flow needs, just throwing some ideas out there to think over. How about an Autolite 4100 for a 289 ford? I think that they are some where in the 480 CFM range. I hope this sparks some debate and exchange of ideas. Jay

A properly set up and adjusted Q-jet would probably be the most drivable!!

I agree about the Q Jet. A small 4bbl that I forgot about is a Rochester 4G series. This was the GM produced 4 bbl from the early 50s until about 1966. The model used on the power pack 283 should be in the ball park for size. I have only owned one of these carbs in my lifetime. It ran ok but I had someone with 35 years carb experience help me set it up. Very sensitive to float adjustment and drop. I was also told by people who were around this carb when it was new that it was considered "thirsty" on gas. A 302 GMC with a Q Jet or 4G would be pretty unique. In my mind a 302 GMC is pretty unique by ITSSELF in a good way. Jay

The smallest 4GC carbs are still about 485 CFM. The early WCFB Carter 4 bbls. are around 375 CFM.

Are WCFBs hard to find? I wish I knew more about them. From what i have read they were used on 283 Chevys also. IMHO the combo of a 302 and a carb in use from that period would be pretty cool. If anyone here ever ran one I would like to hear how it turned out. From reading at the GM heritage center the primary Venturi had a 1" diameter. Certinally in theory at least small enough for a good airflow siginal at low engine speeds. Airflow signal is one of the reasons that TLowe invented the "3 barrel", trying to move air fast enough to make the carb work more efficiently. If I had a stack of 4 bbl carbs I would gladly loan them to the OP to see what worked best. One question, how much carb does a 302 GMC need compared to the same size SBC or SBF? Jay

No, the WCFB's are not rare by any means. Also, many of the Pontiac OHC 250 engines had Quadrajets on them and were very driveable and had good street manners. The idea of improved air signal is one reason why that design of a spreadbore carb works good because of its greatly reduced primary bore size. Given that, a Holley 650 spreadbore carb would likely be a better street performer on many of these 6 cylinder engines including the 250's and 292's than a Holley 600 squarebore carb. Even though overall it flows a higher total CFM, simply because it utilizes that design of smaller primary vs. secondary throttle bores to be more responsive, efficient and driveable in slow street driving RPM's. One thing that is often overlooked today and in times past with hotrodders is using the right part for the job, many do not. Also, Holley invented the 3 bbl back in the early 1970's, not Tom.

Holley may have invented a 3 bbl before me, but theirs was for a whole different purpose. It was a big performance carb for big engines.
My 3 bbl is trying to provide economy and performance for small engines like ours.

One more small 4bbl option is the old Autolite 4100 4bbl. The smaller 1.08 venturi version has a little better manners, but the bigger 1.12 venturi works good too, it is my go-to carb on my 292. And since they are OEM Ford carbs you can generally re-sell them later, especially the 1.08s, the Mustang guys like em.

That said, I wouldn't give up on that 390 Holley yet!
* What list number is it? A 4150 (2 metering blocks) or 4160 (secondary has no block, just a plate). What number is the metering block (just to see if things match up).
* For now, just TRY locking the secondary SHUT, that rules out the possibility it is opening early & causing trouble.
* What jets are in the primary, what number power valve, and how big are the PVCR holes (Power Valve Channel Restriction) the fuel feed holes when you take the power valve out?
* Clean the carb and metering block in brake fluid or diesel, then blend over to gasoline, then over to laquer thinner, then over to acetone, blow dry at each step, kinda like prepping a Super Model's hair, which is how Holley's like to be treated, they are spoiled cuz they're fast and so fussy.
* Check the body-to-baseplate gasket carefully, is it newish & do all the holes line up - the 390 gasket is different so that can cause trouble.
* Make sure the float levels are right up there at the sight plug hole, you've probably done that already.
Well that's a start anyway.

Deuce, there are plenty of good options out there for these engines. The Autolite is often overlooked, but ideal for inlines because most of the engines they were designed for were about the same size as most inline 6's. Also, the WCFB's were actually intended to be used on the Clifford's and Offy inline intakes as well as the Flathead Ford versions of the Offy, that's what the bolt pattern is without the bolt-on adapter plates. I guess they knew a thing or two about carb sizing back then before we got all smart and started thinking "bigger is better".

Wow! Do you mean that if a WCFB were used on an Offy or Clifford on a 250 Chevy the adapter plate that everyone talkes about that goes with the manifold would not be needed? What other carb would fit without the plate? 4gc Rochester? From my limited experience with the 4100, it's very simple and easy to tune. I ran a 2100 on my 79 250, the 4100 is almost two of them stuck together. I looked on E bay for WCFB carbs. They aren't all that bad price wise unless the word Corvette is involved. I agree that bigger is not always better. That 1.08 autolite 4100 sounds like something I would try if I had the 302 in question. One thing that IMHO needs added to the discussion is what are the airflow requirements of the 302. Does it have head work or a hot cam? Is the head design good or bad? Jay

Many online carb CFM calculators are out there, but a 302 such as this street driven one shows a 399 CFM at most with a redline of 5000 RPM. Of course that is way too much RPM for these engines on the street, so a 390 carb should be more than enough for it.

Jay, most of the Ford V8 engines that used the Autolite's mentioned above were anywhere from 260-302 cubic inches, so ideally the same as the inline 6 Chevy 250 and 292's, and of course most of the Jimmy's from 270 to 302.

That is kinda what I figured. Any small 4 bbl that would work good on a 283 Chevy or a 289-302 ford would be good for a 302 GMC. I know that in the past I have been guilty of putting too big a carburator on whatever I was running at the time, with all the bad manners that go with that kind of desion. I went to a pretty big swap meet today, unfortunately no WCFB carbs for sale. Not trying to hijack the thread, but how about a 2bbl instead? If a good balance could be had between size and low speed air volicity that might work also. Just a thought. Jay

Another thing we are guilty of many times is "tunnel vision". Which quite honestly is failing to see what other engine groups are doing to combat this issue. The Slant 6 guys battle and overcome this problem daily with there engines that range from 170, 198 and 225 cubic inches. By all accounts, even a 390 carb on most anything but the most radical firebreathing 225+ cubic inch Slant is too large for these engines. They most often use a 350 CFM 2 BBL. carb to handle their street driving and racing chores with no problems.

Its also a compound problem as well. The solution starts with the intake, and if the plenum is too large, this begins the spiraling downward of slow airspeed for the fuel charge. At this point many times, even if the right carb is chosen, the wrong intake often is overlooked as the true culprit in this situation. And then the carb gets the bad rap for being the problem. I guess the main thing is the lack of knowledge to make the right chooses for parts selection and understanding how they must be compatible and work together.

I try to read the slant 6 forum when I can. Running intergrated heads, I at least don't have a manifold problem/ choice! The only problem left is trying to adapt a Holley 1100 CFM dominator carb to a 1 bbl hole! Seriously though, are Holley 2bbl carbs rated at 3.0 HG pressure drop like Rochester carbs? If that is the case then the 350 Holley would be a close match for the smallest 2GC which is rated at 352 CFM @3.0 HG. That would be about 248 CFM in 4bbl terms, while the 500 Holley would be About 354 CFM. IMHO the 500 would be a poor choice because if you suddenly floor it at low speeds it would almost be like a 390 Holley as a double pumper! Jay

All the Holleys from the Old Days say pre-1980 or so are rated at 3.0" Hg for 1bbl or 2bbl, and 1.5" Hg for 3bbl or 4bbl.

Some 4bbl carbs were rated at 2.0" Hg even back then and some are even rated at 1.0" Hg today so it helps to be sure.

So yes, the smallest Holley 2bbl is about the same as the Primary of a Holley 390cfm 4bbl.

And, the big Holley 500cfm 2bbl is exactly the same as the Primary of a Holley 780cfm 4bbl.

I will try to throw on some bigger carbs during the next carb shootout. Problem is, at my level about 175hp currently, I can run eg a 600-625cfm Carterbrock, but the floppenvalve basically just doesn't open, so its running on the primary plus secondary transition circuits, about 300cfm.

When I run a 450 Holley, the secondary opens maybe half way, so again its at 300-plus cfm.

On a 600 Holley my bet is the secondary wouldn't open at all so in effect I'd have a 1-1/4" primary 2bbl.

And the 780 would be a good test as it would be the same as running a "500cfm" 2bbl, only heavier. It would be a good test of launch and low-end though.

DeuceCoupe, I remember the carb shootout pretty well. I believe that you were running a near stock 250. IIRC the 390 Holley was able to be stomped to the floor off the line, the AFB could not. How big is the primary barrel and Venturi on a Holley 780? Are the primary and secondary sides of the 780 the same size? One thing that I have read about is a new to me size of the small base Rochester 2gc. The one that I am fimiliar with has a 1 7/16 bore with a 1 3/32 Venturi that flows 278 CFM @3.0. The one I just read about has the Venturi size increased to 7/32 and was rated at 311 CFM @3.0. Has anyone on this board actually seen one? Jay

Thanks for all the inputs. I will look for a replacement for the Holley but I would like to get it to run decent as well.

I'll try to reply some of your comments and questions below in order of your replies.

1. Blackwater. Power valve seems to work well. The car pulls very well from mid and high rpm. But it almost shuts off transitioning from idle to around 1200 rpm.

2. tlowe. Intake is not heated but its runs in mild Calif. climate.

3. integrated j 78. Good options. Are adapters available to mate these carbs to the Holley bolt pattern on the manifold?

Radar, I believe the autolite 4100 is the same bolt pattern as the Holley. There are adapters for the WCFB, 4GC, and the Carter AFB. If you want to try a 2bbl there are adapters for that also. Using an adapter to run a 2bbl on a 4bbl manifold is ok. My boat came from the factory that way. Running a 4bbl carb on a 2bbl manifold is not so good. Same thing with running a smaller pattern carb on a Holley pattern manifold not a problem. Do you have extra hood clearence? Jay

"\]More replies from Radar (Frank Hainey).

4. Blackwater and 5. Blackwater - Sounds like the Q Jet is a good option for me.

7. 7integrated j 78 - The 283 SB Chevy would take more CFM than the 302. It has better volumetric efficiency with 1 intake and exhaust port per cyl and the top rpm is at least 500 rpm higher. 400 CFM is about right for the GMC.

10. Duce Coupe - I have is a factory rebuilt Holley 8007 for six cylinder applications. It is has smaller bores and venturi than the 4610 600 CFM carb with vacuum secondary. I wired the secondary barrels shut but it still dies between idle and about 1200 rpm.

More replies from Radar (Frank Hainey)

12. integrated j 78 - The WCFB and Autolite 4100 look good too. The air flow requirements for a mild GMC 302 are about 400 CFM. It's like an overgrown 235 or 261 Chevy, with a 4 in bore and stroke. The head design is poor with 3 intake ports and 4 exhaust ports. It's ported with larger valves. The cam is a mild Isky 3/4 cam that idles very well. It pulls my 3400 lb 40 Chevy coupe with a T5 and 4:11 rear nicely. Currently run 3 small progressive Webers with about 600 cfm total, but 395 CFM Holley seem to pull better once it gets through low rpm flat spot.

Radar,
I have your 40 Chev in the Gonkulator database as running triple Pinto 2bbl's (the Holley/Weber 5200's). Looks like you swapped intakes? Is the rest of the engine the same ie 228-228 duration cam, headers, etc? With the ported head I had it Gonkulating to about 250hp, in which case it would be happy with 450-500-550cfm on top. A 390cfm should be ok, but what I'm thinking is are the idle transition circuits too small and/or not getting a good signal due to that big cam?

Simple fixes:
1. Turn the idle mix screws OUT as much as you can, not til it slows back down, but get your best idle, then go out another 1/2 turn or so. Maybe more, its easy to try.
2. Make sure the float level is high, gas should dribble out of the sight plug when you take it out.
3. Jet it richer, from the Holley book a #51 comes stock - between our CaliCrap Gas, the cam, and the headers, I'd run at least a #53, maybe #54 main jet.
4. Also make sure you have plenty of initial timing, say 12 degrees, and a fairly quick curve, and that your vacuum advance works. If its not getting enough advance it will complain off the line for sure.

Beyond that the fixes are more complex but I would try those first. Then again its good to have 2 good running carbs anyway so looking for another one wouldn't hurt.

Now and then I sell 4100s but the only one I have ready right now is a 289 HiPo Fairlane carb and its $$$$. I like to get em running first, usually on my 292 six, then make a video of them running & driving. Not a guarantee but a start. But right now I have 3-1bbls on the car so I cant get any carbs ready for a while.

The 4100s aren't too hard to find, if you score one at a swap meet that hasn't been left out in the rain, they usually clean up pretty good. They are so simple you can literally run them with the lid off to check the float levels. And like I said they are pretty re-sellable cuz most of the pre-1967 Fords used em.

Another good pick for that hot 302 would be a 4bbl from a 1970s Ford truck, these were a 550cfm 4bbl, still just about right for your rig. They are good runners, made for a mild 390cid so plenty of idle circuit, and again they are around at swap meets.

Thanks Radar, for helping me learn about the 302. I wish that I had more Holley experience so I could help. DueceCoupe is on the right track IMHO. At some point between idle and man metering you are either running out of fuel or some passage is plugged with dirt ect. On my boat which has a Holley 2bbl it started surging and bucking at about 1300 rpm. Idle and anything else was fine. Took it apart and found a small piece of dirt in one of the passages in the metering block. Now it runs ok. One question, if you open the throttle very slowly in first gear from idle does it still have a flat spot? Jay

The slant six guys have had good experience with the 390 CFM Holley.
http://slantsix.org/forum/viewtopic.php?t=18931

I upgraded from a 9625 AFB to a Quadrajet on my Barracuda.
write up here: Slant Six Quadrajet

Just to mess things up what are your thoughts on something like the small fitech unit. Seems like it might be a good match and not have issues with being over carbed.

the 1985 4.3l v6 were qjet equipped. Thats what I am running now, and love it so much , I hunt them down off fleabay and hoard them....lol

Thanks for all the replies suggesting low cfm 4 barrel carbs instead of my Holley 390 cfm which boggs on takeoff.

I took the time to read "Holley Carburetors" by Dave Emanuel from S-A Design and learned the Power Valve (PV) plays a big role in the transition from idle to cruising speed. The PV adds fuel when the vacuum drops below a set level to enrich the mixture and keep up smooth acceleration from idle to cruising speed.

PVs come with one vacuum setting, but are available from 3.5 to 10.5 in Hg. settings. The trick is to find the right PV for your engine. Valve spring constants determine the PV settings.

My Holley carb came with a 6.5 in. Hg. PV. I change to a 7.5 PV- less bogging resulted. Went to an 8.5 PV and bogging was almost completely gone. I just ordered a 9.5 in. Hg PV and think that will fix the problem. Holley PVs are about $9 plus shipping. Hope this helps others with Holley bogging problems.

Or for a low buck efi - megasquirt II (MS II) with a TBI off of a 4.3 V6 - dirt cheap. The whole deal can be done for ~$500. Full fuel and later add ignition control - use the existing dist. with a pertronix conversion to trigger the MS II.

$9.00 to fix the Holley or $500.00 to put EFI on instead? Hmmmmm.......

If you include the cost of the Holley carb plus the 4 barrel manifold plus my power valve purchases, the EFI cost is probably comparable.

I'm just saying that you're already this far into the Holley setup. Fixing what you have is likely a MUCH better plan, especially since you're already down to fine tuning!!!

You likely don't have the squirter adjusted properly. The bolt on the black plastic arm has to be adjusted so it's touching when at idle.

Try there before spending $500+ on efi.

Thanks for the input. I tried many adjustments to the accelerator pump circuit changing nozzle sizes,cams and arm adjustments but it still bogged. Changing to higher vacuum level power valves made the bog problem get better.

It seems the accelerator pump gets you going over a short rpm range when the accelerator is pressed. The power valve picks up from there and covers a wide rpm range. If vacuum drops more gas is added to keep the rpm up without bogging.

I do not intended change to EFI, especially now that my Holley 390 is working better. I was just thinking I might switch to another carb if the Holley did not work out.

Don't give up yet. I had to really play around with mine to get it right.

Try playing a bit more with the accelerator pump.

If I recall on my 250 my jets are a 49, squirter is a 25, and I have a orange accelerator cam. Yours will likely be different but it should get you in the ballpark.

The pump arm should have no gap at all when at idle and should have some tension in fact. That's what caused my bog. Well that and a vacuum leak.

I have not given up - like the mid range performance improvements even though the stumble on takeoff is driving me crazy.

Changed to Accel. Pump cam from white to orange. The orange cam has the 2nd longest shooter time of the 8 available and the 3rd largest cam rise. Also adjusted the throttle lever to have zero clearance with the pump arm. Adjusted the idle mixture screws to give max. manifold vacuum reading at idle (18 in. Hg.).

Less stumble noticeable but still there. I now plan to increase the shooter from 25 to 28 or so to see if that helps.

The power valve was changed from 8.5 to 9.5 in Hg. opening but that did not seem to do much.

Thanks again for all the inputs. I'm learning something useful.

You said there is no heat applied to intake.

Can you apply heat to it?

Heat would help for sure. I would think with a 9.5 power valve gas mileage would suffer because of how easy you get into power enrichment. Would one of those carb spacer plates that hook up to heater hoses work in your application? Just a thought. Jay

I had a stumble with my Holley 390 as well. After going through the carb and getting it fully rebuilt professionally by a racing specialist of some note, I moved on to related systems that can effect drivability. I discovered that my ignition timing was good and under normal operation, it functioned properly. It wasn't until I had a chance to see the vacuum advance in action that I discovered that on moderate to hard acceleration the timing was dropping due to a loss of vacuum. It was only for an instant, but it caused the engine to bog badly for that instant.

I added a small reservoir to the advance system and the problem went away. Don't rule out a timing issue!!

Our 240 Ford with a Holley works best (no stumble) with #37 shooters.

Ported signal vacuum works better as far as eliminating stumble than straight manifold vacuum.

If your carb does not have a ported connection you can "band aid" the problem by installing a Ford Spark Delay Valve (SDV)in the vacuum line. These come in various restrictor sizes and are color coded by bleed-off time.

Is that maybe the purpose that this valve in the vacuum line to my distributor advance serves?



It's hooked to a thermal sensor valve that is separate from the one that controls the vacuum for the EGR and EVAP, which is only opened when warmed up. So the one on top for the timing advance may be only allowing vacuum when engine is cooler? But the round plastic piece, attached to the water neck valve, may be a check valve as well to prevent sudden vacuum lose and bog?

YEAH!! My 390 ha no provision for ported vacuum. I plan to put a check valve in line this spring to help maintain full vacuum to the advance. My advance is set up to add timing as soon as the engine starts and idles. I can kill vacuum and raise the timing and three's no bog, but it makes it hard to start when the engine temp is on the hot side.

The engine in question is an old 153 cid. 4 cylinder out of a '67 Nova.

This is what I worked out for switching my Monojet on my Chevy 250cid with the Holley 390cfm:

My 390 is one of the NASCAR racing carbs. It doesn't have ANY vacuum ports. It is a double pumper with jet plates and no power valves. The PCV is tapped into the intake manifold plenum. I have a tee in line to supply vacuum to the advance servo. I fabricated a reservoir for the vac. advance and I'm adding the check valve as I said.

I am not really all that displeased with how it is working now, BUT!! if I am not more satisfied after adding the check valve, I will drill a vacuum circuit for ported vacuum later this summer.

The valve in the picture is the "cold trapped spark advance". What the system does is "trap" the vacuum to the vacuum advance unit when the engine is cold so that the timing doesn't retard when the engine is cold and you step on the gas pedal.its supposed to help with a cold engine stumble problem. Once a certin coolant temp is reached the switch opens, causing the vacuum advance unit to work normally. Jay

You might consider putting on a 50cc pump from a larger Holley. That would feed more fuel than the 30cc stock size. I've done it before when I had bogging problems. It wasn't a magic bullet but it did help. I think it would you to go back on the shooter size and get more duration out of the incoming shot. I didn't do that, I drilled my shooters out, which is not recomended. But between that and the power valve being swapped later the bog problem went away. My problem was with a cold engine. Once warm I didn't have the issue, but NH is coler than CA.
Good Luck, you are getting there.

Blackwater,
Do you know what jets you're running in that 390 Holley since it has no power valves for extra fuel? Is it rich enough to give full power and still lean enough to cruise?

My buddy, Jimmy Bridges from down in Nashville built it and I have the jet sizes written down, but I don't recall them. He sent me four more sets in two step increments up, but I haven't gotten back to the project yet this spring. I do know that it is set up square, that is the same jets on all four corners.

It is probably a little lean on top end, because if I run it over 80 mph it starts to run a little hot in the summer heat, but if I back down to 70 it runs fine.

If you eliminate the power valves, you should step the jets up at least four, and as many as eight steps to start with. I don't like running power valves because they tend to rupture if the engine backfires and can flood and wash down an engine.

Mine is set up on a sheet metal and tubing tunnel ram intake and is mounted sideways with the bowls to the left and right! I'm in the process of collecting up the parts to bore and stroke my little engine out to 175 cid. Bridges has already built a big port head with major porting, larger valves and a hump kit, as well as new valve springs and keepers and retainers and 7/16" rocker studs.

I'm a long way from finished with the tuning and it still has a stumble if you slam the pedal to the floor from anything less than midrange RPM. I am going to put the big accelerator pumps back on.

Power valves no longer blow out on modern carbs. I've backfired more than a few messing with things and haven't blown one yet.

I blew one on this carb!! It is why I had someone else rebuild it! Of course, the NASCAR 390 is a lot different from most of the other 390 Holleys.

It looks more like the old 850 double pumper. It is all mechanical. No vacuum secondary. no choke. Not even a choke horn. No vacuum ports of any kind.

If you're dealing with one of those with the vacuum secondary, you can change the spring in the secondary servo/dashpot to open faster or slower to tune the stumble or bog, or to bring the secondary in faster. I've also seen the vacuum passage drilled or bushed to increase or decrease the vacuum signal.

More RADAR comments to your recent replies.

Tlow #1716: I may add heat. My unheated triple 2 barrel Webber setup does not stumble, but the Holley does hot or cold. The 3 Webbers work fine but the Holley seems to pull much better once the stumble passes.

Blackwater: I'll try adding a small vacuum can in the secondary vac. line. It could be the Holley 8007 has a spot where ported vac. drops off and messes up the spark advance.

Also changing from a 0.025 to a 0.028 squirter and going back to an 8.5 Power Valve.

Thanks again for all the comments.

I'm running a 47 squirter...

Inches is inches, be they inline or side by side.
for a 250 (close to a 265 V-8) or a 302 (close to a 283,289,292,307,312, etc.) All are good starting points to adapt to our sixes. Use the latest (Up to and including 1967) for best power valving and smoothest operation....1968 and later begin have many concessions to smog control...making adaptation to the sixes more difficult in the ways of drivability especially for the automatic cars where you don't have the ability to "feather' the clutch to prevent a stall or a stumble. With Doug Roe's Rochester book in hand the Q-jet is everything you could need in the way of a single carb. That book will tell you how to modify every circuit in it, both smog and presmog models. 750 cfm should be plenty for any six. This carb was used on 500 inch Caddys...down to the 230 inch Pontiac OHC--Doug Roe ran one on the four cylinder Baja mini Boots. We have a member in the North Texas Chapter running a Q-jet on a pinto four cylinder 1600 (100cubes)maybe I'm lying-it may be a 2.0 liter. The secret is the tiny primary barrels--the 2.25 back barrels only come in as needed by the adjustable air valve--every thing adjustable on the Holleys is also adjustable on the Q-jet with Roe's book. I've seen Q-jets at the swap meets for as little as five bucks--you can't beat that, the same primary size of a 390 Holley with a bonus of 360 more air flow controlled cfm. This would be my choice for size, flexibility, and price.....just sayin'

yep, theirs was 950 CFM, with the back "barrel" being a 1-3/4" wide "slot" with the same old front barrels of the 850. Yours is a clever simple idea. And if done on a vacuum secondary carb, is a real good idea...lots of easy to get Holley tuning parts available...but I still like Q-jets

Preacher, what has happened for so long is that too many inline tinkerer's have simply chosen the wrong carbs for the task. Over time, this has skewed manufacturers perception as to what carbs are truly in demand. This unfortunately has driven down the costs of the "wrong" carbs, while the "correct" ones are still pricey, but still the better choice. Holley has invested hundreds of thousands of dollars in Research and Development to provide carbs to fit and suit every need including performance, economy and both combinations together, but the "bigger is better" mentality is ingrained so deeply it still makes many choose the "wrong" carb even though voices of experience tell them otherwise. Over the last 40 years, if all the inline guys had purchased the "correct" carb to start with, this supply and demand for manufacturing would have brought the price of them down to where the price of the "wrong" carbs are priced today. Most of the original aftermarket 4 BBL inline engine intakes were meant to use the WCFB carbs because they were closely sized to the needs of these engines, then and now, that's why they are drilled for that bolt pattern. So trying to re-invent a solution that Holley and other manufacturers have already done is really futile.

I'm poor so I only have half of a Q-jet. Jay

The Rochester 2GC are an excellent choice and better suited cfm-wise than most other choices.

I totally agree about the 2gc. With a little machine work either, but especially the small base could be made to fit a stock 1bbl manifold without an adapter. Or if that nice 2bbl intake that you drew up is ready. Is it in production? Either of the two small base sizes (one of which I have never seen) or the 352 or 381 cfm large base would work well with a stock or near stock engine. There is even a large base 2gc that has 1 7/16 throttle bores and 1 3/32 Venturi like the small base. I didn't know that such a carb existed til I found out I had one in my garage! I went with a Dueljet instead. Took a special adapter but I am very happy with the result on a stock engine except no cat converter and a glass pack. Very good throttle response. Jay

Jay, I just got my third intake manifold design cast from 2016, so that is going to be the next one. Should have done it first! LOL

Ok I'll bite. What intakes for what engines are ready? Slant 6? I just wish I knew more about Holley carbs to help out the OP. It seems like the 302 doesn't breathe very well when compared to a simular size SBC even when the SBC has low performance heads. If I had his engine I'd go with a Q Jet or a 400 CFM carter, mostly because I have more experience with these two carbs. Jay

Jay and I have been discussing the Rochester 2GC options using an adapter on the Monojet intakes. What we've found to be available:


Base (bores) - Venturi - CFM 2bbl (4bbl equivalent)
1.25" (1-7/16") - 1-3/32" - 278cfm (197cfm)
1.25" (1-7/16") - 1-7/32" - 311cfm (220cfm)
1.5" (1-11/16") - 1-3/32" - 278cfm (197cfm)
1.5" (1-11/16") - 1-3/16" - 352cfm (249cfm)
1.5" (1-11/16") - 1-1/4" - 381cfm (269cfm)
1.5" (1-11/16") - 1-5/16" - 423cfm (299cfm)
1.5" (1-11/16") - 1-3/8" 435cfm (308cfm)

The largest one, used on big blocks, seems to be available for around $94 as new part quite readily on eBay. But the 381cfm one, used on the L65 Chevy 350, is much harder to find as a new carb.

The 278cfm seemed to be available with either a top that fit the 2-5/8" air cleaner base, or the 3" air cleaner base of the other large base 2GCs.

Jay, two of them are Weber sidedraft intakes for Chevy inlines, one for the 153 Chevy II 4 cylinder engines and one for the 194-292 engines. The third is a true EFI intake with a front mount TB, and designed with forced induction in mind.

What is next on your hit list for casting? Intake manifolds, valve covers, side plates, thermostats housings?

Many more of all of the above! I designed, made the patterns and had cast all 3 of these intakes in just 6 short months in 2016 beginning only in March. Plus having so many other distractions in the way of completing several other designs. 2017 should be very bountiful because I have cleared my plate of these other distractions now.

Here is the new milling machine I just bought. Its a beast at over 10,000 lbs. It will take care of all the heavy duty milling operations I have in store for these parts on the drawing board. I also have gotten into a lot of heavy fabrication chores for the LS racing crowd along with some intense block modifications these guys are doing, so i'm no longer challenged with having the inability to do certain tasks now.

Nice looking parts. The Mill looks nice. Seen one of those in person. Massive. You might need 2 more arms to run it.

Thanks, I couldn't pass up the deal, it has 60" of table travel in the X axis and power feed in all 4 positions as well as rapid traverse. Working on getting it moved right now. It is Massive that's for sure.

Nice looking milling machine.
Does that run on 3 phase 220?

pete

Jay,
You know there's a lot of empty space in the intake that is the exhaust passageway. There's more to it than just drilling 2 new holes. (Did it for a Holley/Weber 2 bbl on my 194)
For small carbs I'd suggest using an Offy intake with a custom drilled carb plate to replace the stock 4 barrel plate.

Pete

Thanks! Yes, its 3 phase 220V, so i'll need to get another phase converter to run it.

It's Radar again playing with the 390 cfm Holley . I tried a 0.028 accel. pump nozzle and added a small vacuum chamber in spark advance line. The vacuum chamber made the engine diesel when turned off. The stumble is still present. Taking off the Vacuum Chamber now.

The earlier changes made to the Power Valve and the larger nozzles has made he stumble less. The stumble occurs just after the car is rolling and the accelerator is pressed down quickly to gain speed. Taking off in traffic is a problem. If I really rev up the engine before the clutch is engaged it takes off fine.

Just ordered 0.032 and 0.035 nozzles. Hope they work- I'll let you know.

Radar, it seems like going richer with power valve (sooner as vacuum drops off) and bigger squriters for the pump is making it better. Chevelle 292 Wagon, isn't there some kind of metal sleeve inside the stock intake that directs fuel flow? IIRC someone here opened up the stock intake to the Holley/autolite 2100 bolt pattern. He had to epoxy on the inside of the intake I believe. On my 1bbl intergrated head the bottom of the intake IS the exhaust manifold! On a 2bbl intergrated head, there is a metal exhaust gasket that forms the bottom of the intake. Jay

This is a delayed follow up to my earlier posts above.

Here is what I have learned about tuning the stumble out of a Holley 4 barrel. The fix depends on when the stumble takes place relative to initial takeoff.
Immediate Stumbles are fixed by increasing the squirter size with placing cam screw in hole 1.
Delayed Stumbles ( 1 or so seconds after takeoff) are fixed by decreasing the squirter size and moving the cam screw to position 2.

I had a delayed stumble in my Holley 8007, 390 cfm carb. on a mild GMC 302. It was caused by emptying the accelerator pump chamber too fast. Going to a smaller squirter extends the stream of raw gas longer. Switching to Cam screw hole 2 delays the cam action slightly, which also extends the stream. The extended stream eliminated the delayed stumble problem.

.037" - .040" squirters cured my 390 woes. It may be killing ants with a sledge hammer but it worked. Shame on Holley for letting this problem continue on the 390 for years.

what a great thread, excellent read. radar, how is your carb tuning going?

Side question, what are your thoughts on running a Weber 2 barrel carb like the 38/38DGES Clifford sells?

AFAIK that's synchronous (both barrels open together), either 360 or 390 CFM @ 3.0 Hg" (if rated as a 2-bbl.) depending on the venturi size (27mm or 30mm).

I'll have to measure my venturi's one day to get the exact size but I did read the carb is synchronous. Seems to be mixed feelings about running/fine tuning the Weber carbs.

Come to think of it, my internet research shows that's how people tend to feel about all carbs while everyone's favorites are different.

Many years ago I had a 327sbc bored 20 over w/a Chet Herbert cam, camel back heads, headers bla bla bla....ran incredible on a rebuilt Quadrajet, only ok when I swapped it for a Carter. Although I had a Carter tuning kit and was able to swap the needles in the parking lot at will and that was pretty cool, never did run as strong as the Quadrajet.

The 38/38's 27mm venturis are the same size as the 1-1/16" primary venturis on the known-to-work list #8007 Holley 390 CFM 4-bbl, which means the off idle and response should be good.

In the hands of the right tuner, the pre-electronic QuadraJet carburetor is one of the best street carburetors extant!! It isn't simple, but it is so tunable that GM made it work for engines from 230 cubic inches to 455 cubic inches with the same basic configuration. This isn't including the larger unit made for the 500 cubic inch Cadillac, (which,by the way, can be made to work on even larger displacement engines)!!

Tuned properly, it works better, (all around) than any other non-electronic carburetor ever made for the street. Once you get it sorted out, you close the hood and forget about it!!

Granted! Tuning isn't "so simple a caveman can do it", but it ain't "rocket surgery" either!! John Lingenfelter wrote SVERAL articles on it and explained it step by step in vivid detail!!

For smooth, everyday use and performance, I STILL like a Qjet for non fuel injected projects.

I'm a big Quadra-Jet fan too. I never ran one on an inline. The key to tuning them is the "step by step" part. Good MPG if you keep your foot out of it and hang on when the secondaries open.

I run a quick fuel 450 on my 261, good mileage and adjustable fuel and air jets.

I thought this thread ended in mid 2017 but it's back - great.

Moregrip - I reverted back to the three Weber 32/36's. They seem to work better on my car. Took car to drags in 2017 but car ran slower on the Holley. Don't know about the Weber 38's.

I have an old pre-electronic Q-Jet and intend to adapt it to my GMC. It sure sounds good.

The threads never end . Sometimes they rest for a while. That's the point of having a search feature.

Another vote for the QJ, tiny primaries for good tip-in and low speed, big secondaries for max air delivery.
Difference of opinion as to which way to face it on a 1 X 4 manifold: the factory Sprint faced the primaries away from the engine for best mixture distribution.

radar, thanks for the update. I bought the Clifford intake/Weber 38 combo so I'll run that as she sits. If I need more power I just might have to rebuild this 4 barrel Quadrajet I have. Come to think of it, I have a 2 barrel Varijet as well.

that's a nice piece!

IIRC it can be adapted to the Chevy 250 etc. (not a direct swap since the Pontiac-only ports are different, although I think the port positions match).
I did some searching, and no explanation as to why the huge volume difference between front and rear cylinder group plenums. Not visible but possible: the front leg (shown right, above) may be deeper, the rear leg is shallower for some reason and makes up the same X-section with width? The firing order (on this picture) is #1 (right), #5 (left), #3 (center), #6 (left), #2 (right), #4 (center), is this significant?
Beware: QJ to Holley base adapters notoriously have small gasket surfaces mating to your manifold, and are leak-prone.

I am running the front half of a Q-jet on my 1bbl intergrated head. I have the carb turned the same way that it would sit on that Pontiac manifold. Although it’s a little short on CFM the drivability is very good! I would like to try a WCFB on a little more aggressive engine. Apparently they were made in different throttle blade and Venturi sizes. Looking at the GM heritage center, some are shown with SMALLER secondary Venturis than the primary side. Jay

any detailed pics of this setup?

I can take pictures but can’t post them. Pretty sure I could e mail them. Not too great with tech stuff! Jay

cool, my username @ gmail

I wonder on the Pontiac if they put the primaries away to give those runners fed a little more length for better lower end performance.On the Big Ford's they recommend primaries close to the engines. Also I think the Aussies have carb orientation different aswell.

The explanation I read (Hot Rod?) 50 years ago: the primaries would be so close to the #3 & 4 that the vacuum difference with those ports only an inch or two from the manifold runners couldn't be fixed any other (cheaper) way.

Tom Lowe - How do I get written instructions on modifying a Q-jet to a 3 barrel? Plan to try one on my GMC 302.

I'm working on the intake manifold head brackets and tube runners now. Any suggestions cutting 3 vs. 4 holes in the carb plate and on hole orientation?

Thanks, Frank Hainey

Here are some pics of Jay's setup!

Thanks for posting the pictures. Sorry about the dirty engine compartment, a lot of it was sprayed with black rustproof when the car was new. Jay

modifying a Q-jet to a 3 barrel
Why?

To give it better street manners, off-idle throttle response, gas mileage etc.

Details here: Inliners Post circa '15

The Pontiac (Chevy) 230" used a QJ, why does it need to be smaller for a 24% smaller engine?
w/r/t cruising CFM: assuming the mixture matches, the CFM used will be the same regardless of how many throats. Depending on the venturi type the manifold vacuum could go either way.
Disabling 1 primary barrel will also have another effect: the operative throttle blade angle will now be higher (more rotation) in the throat, and the idle/transition slot relationship will change quite a bit. If the blade is above the transition slot the tip-in response relies on pump shot. The usual cure is a small hole in the disc right next to the slots to allow air to enter with a smaller angle.

I’m pretty sure that Tom made a 3 barrel out of a Holley 600 CFM carb not a Q-Jet. Please correct me if I’m wrong. According to Tom, Holley carbs perform better when
“Loaded” more with greater throttle blade opening. That and the high cost of the 390 cfm model is why he said he did it. Tom says it works well and I believe him. Jay

The hot idle position of the edge of the throttle disc (approximately mid-way between the "idle" and "transition" holes in the barrel) is universal among all carburetor brands, models and sizes. It's one of those factors that makes switching among non-OEM carbs a crap-shoot, and Holley has and will disagree that "loading" it more is an improvement (read their literature). If that were true, it would have occurred in the entire line 60 years ago.
My guess: it corrected an existing (bad) condition, such as the blades were too far closed (in or below the "idle" hole) with that small engine to compensate for lower idle CFM demand.

I think I got confused in reading the positive comments about the Q-Jet along with those for the 3 barrel carb. I'm in the process of making Q-Jet manifold for my GMC. Won't worry about the 3 barrel mod now. Sorry for the confusion.

looking forward to seeing that!

I am glad you are interested in my project. I hope you will participate in the design process with me me.

I have built GMC intake manifolds in the past. My best effort is my triple Weber 32/36 manifold for my GMC. This manifold provides a very smooth intake system. Please see the Harpor SP? intake manifold subject below.

I also built a manifold adapting a Holley 395 cfm 4 barrel to a GMC. It never really worked well. The biggest mistake I made was to use a smooth transition between the plenum and the runners.In the process I ignored the need to keep air/fuel mixture moving rapidly toward each intake port. The result was low speed bogging because the manifold runners were too big, especially at the point where plenum and long runners meet. I also made a very large plenum which did not help.

This time around, I plan to design a Q-Jet to GMC manifold where the the cross sectional area of the runners is constant and equal to the area of the head intake port. This is = 0.785 x(1.75)^2 = 2.404 sq. in. The runner will be made from 1 7/8 OD tubing which has an I.D. of 1 3/4 inches. The critical point of this design is the transition from the the long round 1.75 I.D. runner to the rectangular plenum.

The plenum volume will be made as small as possible, like the production Q-Jet manifolds shown above. The outlet opening on each side of the plenum will be wide enough the allow the smooth flow of the air/gas mixture from the Q-Jet's small and large bores simultaneously. This outlet opening bottom will match the bottom of the flat plenum.

I am thinking about a a rectangular opening into the plenum 1/2 inch high by 4.8 inches wide. This opening maintains the 2.4 sq. in. area like the long round intake runners and hopefully keeps the air/fuel flow rate up for good performance.

I have purchased the steel tubing and steel plate to mount the carb to the manifold and to bolt the manifold to the head. I have also roughly formed a rough a sheet metal transition piece for use between the 1 7/8 O.D. runners and the 1/2 x 4.8 in. openings on each side of the plenum. I have not done anything about the plenum wall material but think this will be a strip of 1/16 steel x 1 inch high and bent around the perimeter of the Q-jet carb bores.

Any comments or suggestions on my theory about keeping the air/fuel mixture flow rate up or on this design would be appreciated. I'm in the preliminary design stage so changes can still be made.

I know you might think I have too much time on my hands but I really enjoy building his kind of stuff. Thanks. Frank Hainey, (Radar).

Interested to see how the q-jet project turned out

Thanks for the interest in the Q-Jet manifold. I have had some setbacks in building this thing.
Two different attempts at welding the plenum to the carb mounting plate warped both plates. My plan is to increase the plate thickness from 5/16 to 3/8 and to bore the large holes after welding.
Secondly, the rounded plenum corners makes it very too difficult to build the tapered transition cones connecting the plenum to the intake runners. So now plan to make the plenum rectangular with straight sides. This should simplify the building the transition cones.
I hope to have something built by the end of the year.

Frank Hainey - Radar

Post up some photos once it's done, it will be cool to see.

Still following along!

I been running Toms 3 barrel for over 3 years now with no problems. That's on a 261 ci with a 700r4.