I have been working on a better mousetrap for use on our smaller engines.
We all know they came with 1 BBL carbs from the factory. A common thing to do is drop a 4 BBL or a 2 BBL on and get it going and have fun. The performance is increased but most of the time fuel economy goes down.
I have ran 2 BBL's, 4 BBL's and even progressive 2 BBL's. The one that gave the best mileage was the progressive 2 BBL.

One thing the progressive 2 BBl lacked was full CFM at full throttle. The one I used was a Holley 350 CFM and later a Holley 500 CFM. The 2 BBl's are rated at a higher vacuum of 3 inch as compared to the 4 BBL's rated at 1.5 ". This would make my 500 be similar to a 375 CFM 4 BBL. Not enough for a healthy 250 let alone a good running 292.

To run a 450 CFM 4 BBL would be good. There are a few problems. They are expensive. And they provide too much air flow from a little foot feed needed to keep a 250 engine running at highway speed. For instance: to maintain 55 MPH with the old 1 bbl required the throttle to be open about 1/3 open. This put the carb in a state that allowed proper mixing of the air and fuel. A 4 BBl would only be operating on the primary's for 55 MPH and would only need to be opened about 1/4 or less to maintain speed. This does not allow the carb to mix the fuel/ air as well. Less efficient.

Here is the idea I came up with and am testing. Let's take a common 600 Holley vacuum secondary carb. Block off the one of the primary throttle blades and the associated air/ fuel circuits. Also the corresponding accelerator pump nozzle. Essentially make it into a 3 BBL. It would then be a 450 CFM with a small single primary for cruising and have a large secondary to kick it in the *ss.
A 750 would convert to a 562CFM , good for a healthy 292 or strong 250.

I have converted a 600 and tested it. So far it is working better than I had imagined. Still have some tuning to do with the secondary opening rate. On my wagon it pulls harder and is able to hit RPM's quicker than with the 500 2 bbl Progressive.

What are your thoughts?

Sounds good Tom I will let you know how my 292 goes when it is together with your rods and pistons. With the MSD Atomic 4 barrel efi

That's cool, Tom!

Pics? I'd like to see how you're blocking off one of the bores. Also are you killing one side of the squirter on that bore? That would be simple enough to do.

...and now I'm going to need to hit up a couple of swap meets it seems... I'm thinking 650's are a dime a dozen.. That'd put me around 487 CFM. Only 37 more than your 600 but a whole lot easier to find! If you plug in a carb calculator a 250 with a VE of 1 needs 488 CFM. Sounds to me like a good cam, oversized valves, lump ports, headers, and a 2.5" exhaust with this carb would be about perfect.

What size squirters are you using?

I guess if you gutted that barrell, got a big washer, some RTV, a small bolt and nut you could make a plug that way. RTV it so you don't have vacuum leaks.

...very interesting...

I'm interested in seeing a video made! Would like to see the wagon in action again.

Hi tlowe . . . instead of blocking an entire throttle bore (and re-jiggering fuel flows) why not just fit a tapered restrictor-plate:
Building Speed to dial the throttle area down to the desired CFM? Both the primary and secondary bore diameters could be tuned.

regards,
stock49

Good point. How is that going to help with having a better mixer when the dual throttle blades are barely open at low highway speeds?
I am trying to get the carb to be more effective at mixing the air/ fuel for better combustion.

Am I going about it all wrong?

Tonight, we went out with the wagon again. Yesterday, it had a oak desk, chair and a oak file cabinet for my daughter in the back. It really weighed down the back. The car felt faster than normal with all that. Took the desk stuff out and drove it again on my usual roads. It is definitely faster. It hits 5 K in no time. It also pulls off the line smoother, at part throttle. Almost like it has more torq at part throttle.
I have a black spring in the secondaries and you can feel them kick in a little late. Need to back off to a lighter spring.

Now to find out if the mileage is increased.

Interesting, would be cool to see some track times comparing this idea vs. other carbs.Wonder if there would be quicker 60' and 330' times??

Tom

I wonder how that would stack up against a 2 hole TBI - like we did in texas? With the auto tune now - starting with the base tune I got in Texas - its not so hard to do a TBI.

Hardest part is putting in the fuel system.

If you want a carb that mixes the fuel and air best look into a CandS specialties Aerosol 2 Barrel.

I would love to see someone try the manifold I found on the internet that has the center port separated, with an Aerosol mounted side ways rather than across like a V8.

I think this simple setup would work great.

I make almost 1000 HP with 2 of the Aerosol 2 barrel carbs.

They sell a ton of these to roundy round cars that have to run a single 2 barrel carb.

Also Roger the owner is super helpful to give you the correct information on what will work best.

Best money you can spend.

It's slightly different angle on the same idea. The problem with the 600 CFM (as you have observed) is that the two primaries 'in concert' offer too much throttle area for the CFM demands of the engine. Hence the partially open throttle plates when cruising. By blocking off one of the throttle bores you have effectively halved the throttle area:
..bore......radius........r-squared............area..............2X
1.5625....0.78125.....0.610351563...1.917475985.....3.83495197

The tapered spacer can achieve the same effect but using both bores:
..bore......radius........r-squared............area..............2X
1.109375..0.5546875...0.307678223...0.966599644.....1.933199288

The stock throttle bore for a 600 CFM is 1 9/16" - the example above tapers/restricts the bores down to 1 7/64.

Do you mean a spacer like this?

http://blp.com/cart/index.php?main_page=product_info&cPath=72_126_304&products_id=1378

Combined with this?

http://www.ebay.com/itm/Holley-4777-650-...y-/261997344277

$260 would be pretty cheap if it works well. If someone tries it and finds out I might be putting a nice 390 up on Flea-Bay at the $260 mark and see what happens...

The secondaries on the spacer would need to be opened up to allow full bore.

...or I can make one with my buddy around the corner...

Now I wonder where I can find a 650 I can experiment with... Hershey's coming up! I'll dig around there.


edit: Wouldn't that kind of tapered spacer just increase the velocity without reducing the flow very much? I don't think it's as simple as tossing in a spacer like this. The bottom holes would have to be much smaller I'd think.

Gbauer,
A 650 Holley is a double pumper with mechanical secondaries. Not a very common carb. The 600 is a vacuum secondary and is very common. Probably the most common Holley carb on earth. Available at swap meets in worn condition starting at 20.00. Good ones can be had for 75 on up.

Stock49,
I do see how a restrictor would help lower the CFM available, but I don't see how it would increase velocity past the venturies to promote air / fuel mixing.

Turbo6,
How will the Aerosol type carb work for normal street use? Do they have a choke?

I am trying to develop this 3 barrel carb setup to be a great alternative to the expensive 390 but yet help drivability and fuel mileage. And have the ability to feed real horsepower for the street.

Last night, I removed my intake/ exh to replace the leaky gasket. I hate header leaks, they sound like crap. Trying a Remflex gasket.

Going to change the secondary spring to allow a quicker pull in rate.
After this carb is finished testing, a modified 750 is ready to go on next.

Can you use an app on your phone and do a 0-60 comparison or something? I've found my butt-sensor is constantly out of calibration. Tends to read high right after a change and low once it's settled in for a while.

Tom,

Just curious how about a holley 4165 ? Its a Qjet replacement so the primaries are small.

Make a slight mod to your intake pattern to allow use of a spread bore carb... make the carb flange bolt on and then machine the plate to suit 2, 4sq or spreadbore carb's

Or a QJet ? GM put them on some V6 engines.

Aerosols work great on the street.

What's a choke?

Turbo6,
Choke? Funny! My current 2 BBl does not have one either. Cold mornings can be rough.

EFI-DIY,
You are correct about the Quadrajets, but they are now rare to find. I am just trying to make a very common, easy to find, cheap carb be better suited for our small engines.

Messed with it more tonight. Changed the secondary spring to a lighter one. Took it out. The secondary's are opening at about the right time while accelerating. No bog, just acceleration. I like it. Nice setup.

Back at the shop, changed over to the 750 based carb. Had trouble with the dual feed fuel line hitting on the head. I am mounting the carb oriented as normal. Primaries forward and secondary's back. Had to bend the fuel line up and out at a 45 to clear the head. Got it all done and fired her up. idled the same, but has fuel leaking into the secondary's. I should have known better. Did not take the back side apart and replace gaskets. Took it for a drive. It seemed the same as the 600 but maybe not as responsive at low RPM. The secondary's came in too late. Need to change the spring too.

With these modified carbs, it is definitely a different mannered engine. The idle was rougher with the 2 BBl progressive and taking off with 2.56 gears was a test of skill on the throttle and clutch. Rpm's quickly went to 4K and would still climb but slow.
The modified carbs idle better and hit 5 K in no time, infact faster by far than before. Coming off the line, no more test of skill, it cleanly pulls the gearing. Jake pointed out to me how he thought it pulled harder. It is because the engine has 1K more in the RPM range for shifts.
I do have a G-Tech Pro RR ordered. Should be a good tool to help verify things.

Will get the 750 3BBl rebuilt and retest with no fuel leaks.

If you have an android there's a couple free apps that'll do 0-60 times.

...or a stopwatch.

I'd still love to see pics of what you're doing. How are you blocking off a bore? How are you plugging one side of a squirter?

My 65 Nova has a early 283 Military intake with a WCFB carb pattern, the adapter that is on there has tapered hole, something like this , but to more of an extreme on the primary side , but leaving the secondary side open, or non tapered???
The adapter I have is like this.
http://www.55chevys.com/chevytalk/carbadaptor1.jpg
I am changing my intake out soon, so if you would like my adapter to play with let me know

Really hoping to see some driving videos again! lol!

I feel like I'm witnessing the gradual build up to a summer block buster movie that I've been anticipating.

Just a little bit every so often to feed the curiosity but nothing really showing what's going on!

Pics?!? Please?!!? Pretty please!

Worked on a few carbs this weekend. Have 4 done. 2 600 and 2 750. Tested the modified 750, it performs nice. Going on a long trip soon and will have fuel mileage reports. Have one of the Modified 600 going to a fellow inliner for him to test.

Hope to get some video this evening. The wagon definately responds well to more carb.

Pics of a modded 600.


This is the stuff that keeps me busy.

Ahh... Now I see how you're doing it! Or at least 90% of it.

What attaches the plug to the carb to hold it in place? Also it looks like you drilled a small hole in the plug to allow a bit of air through. Is that the help idle air mix? I wouldn't think you'd need that.

How'd you plug the squirter?

Slick move on removing the butterfly the way you did.

I bet you're getting more torque and still have nice top end like this.

There is much more done than plugging the 1 bore. Plugging the squirter, air and fuel ports.

The small hole is simply a drain if fuel were to ever be present.

Now that I have slept on this a couple of nights I think you are right. While the restrictor does decrease the throttle area and thereby the velocity through the throttle bore - it also increases the ratio of venturi-area/throttle-area effectively slowing the velocity through the venturi . . . which is exactly opposite of what you are accomplishing by diverting all of the primary CFM demands through one bore.

This is one of the most interesting threads I have read in some time. If when going down the road at 55 or so with the throttle opened just a small amount the carb will be using a lot of idle/off idle fuel circuits. This is one of the reasons fuel economy would be less than ideal. From my very limited understanding of Holley carbs, they are very poor at operating in this area but very good when "loaded" like at 1/3 to1/2 throttle for example. Having a 1 bbl on takeoff I would think would be a lot crisper than both primary barrels of a 600 Holley. IIRC the engine in the wagon has a 270 duration cam? If so it could use a lot more air than a stock crammed engine. This is why I am trying out a duel jet. A little more air than a small base 2gc but the triple venturi set up will keep volicity high for good response and gas mileage. The downside is I won't quite have enough air even for a stock cam. Please keep us informed and thanks for comming up with a new idea. I remember a test by Deuce. Coupe on a stock 292. I believe he posted that the engine only "wanted" about 300 cfm based on a paper clip test. This would be about 423 cfm in two barrel terms. Jay6155

Just got back from about a 1000 mile round trip. The wagon performed flawlessly with a full load. We went to the HAMB Drag Nationals in Joplin Mo. Saw a few inliners and talked with plenty of inline guy's.

We ran the modded 750 on the trip down. It took 2 tanks to get there. It averaged 15 MPG and the tailpipe is black. I did not bring any jets to lean it down. Drats! But it ran great. I would expect it to be able to duplicate what the progressive Holley 500 did for mileage (19-22MPG). So in the parking lot, the boy's and I changed to the modded 600. The next morning we took off for the drags. Going thru the traffic in Joplin was good and it started raining. About 4 miles of stop and go and it started idling higher. We pulled off and I idled it down. Took off and it fell on it's face and died. Popped the hood and looked down the carb, my plug had worked out, exposing the hole bore with no fuel. Luckily, the plug was stopped by the throttle shaft. We waited for the rain to cease and swapped back to the 750 and continued the rest of our trip.
Got home this afternoon and replaced the plug and swaged it in much harder. It is now installed and is working fine. Will test the Modded 600 for fuel mileage also.

If you can,
I would try & do this mod to a spread bore carb, really small primary, better mileage, large secondary to possibly give you enough CFM to feed your engine.

Just a thought.

MBHD

A spreadbore that isn't modded may provide near the same or better as the modded non-spreadbore since the premise of the spreadbore is smaller venturi for better mileage and better driveability in cruise RPM's, then the big kick in the a$$ when you open up the back barrels. You could also make the primaries progressive on the 4 bbl your messing with, you may be giving up performance and other benefits without realizing it by blocking off the one barrel completely. Then you can delay how progressive it opens to tailor the engines needs even better. I think that would give you the best of both worlds.

IIRC the front 2 barrels of a Q-jet with the small Venturi is only something like 229 cfm at 3.0 vs 210 cfm at 3.0 for a mono jet used on stock 250s. Not much different. By limiting the rear air valve opening the total carb cfm can be not quite 500 cfm at 1.5 hg. However a lot of people would rather have a Holley or a carterbrock because they are easer to tune. Nothing wrong with any choice, most people like me stick with what they know. How much cfm does the Primary's of the Holley Q jet replacement carb flow? I think it is a 4165? I am talking about the Holley that was bought as a bolt on Q jet replacement. Is it a double pumper? Clearly I don't know much about Holleys but am eager to learn. I didn't even know that the 390 was expensive until this thread. Thanks everyone for helping me learn about Holleys. Jay6155

I can't really tell from the pictures, are all four Venturi sizes the same on both the 600 and 750? How big are they? Thanks in advance jay

All four are the same size. Not sure how big they are.

Thanks. Just guessing here but the only difference between 600 and 750 is Venturi size, the throttle bores are most likely the same on both carbs. Someone, perhaps you Tom said that the 500 cfm Holley 2barrel is the same as the front half of a 750. A 1 barrel front half of a 600 would be 211cfm in 2 barrel terms. The same as the mono jet. This could easily be the best of both worlds, a small 1barrel for gas mileage and crisp response in normal driving, with a little more cfm than the 390 when you want power. Jay

The throttle bores are bigger on the 750, and they are the same size as a 500 2 bbl.

Video of the modded 600 in operation. Sorry about the poor audio during the car ride. The camera case muffled the sound and then it also vibrated.
https://youtu.be/iXdFLCIz-BA

I did not mention this in the last post. The modded 600 gave almost 21 MPG, practically the same as the progressive 2bbl. The main difference is the ability to provide more power too. It also has more low end tq.

New video of it running, https://youtu.be/iXdFLCIz-BA
Old video of it running, https://youtu.be/3NU7n3kOWe8?t=9m20s

Thanks for the videos Tom. That explains it a whole lot better!

Sent the modded 600 out to be tried by another inliner (independant testing).

Rejetted the modded 750 last night and put it back on. Went from the stock 72 down to a 66 jet. Will recheck the mileage and performance on a road trip soon.

Can you mod a spread bore Holley?

MBHD

Bet it could be modded. But they are somewhat hard to find. Also, I think the transition from single primary to dual secondary would be hard to tune to work right.
The standard Holley 600 and 750 are the cheapest, easiest to find and parts are plentiful.

21 mpg out of a 600 Holley is hard to argue with! Even with the one barrel mod.

MBHD: I thought about this before but never actually tried it.
I too thought a spreadbore Holley would be good with it's small primary and big vacuum secondary.
Of course as Tom points out the 600's (and various unidentified 450-ish sized carbs) go cheap at flea markets.
But then I found a cheap 390 and forgot all about it. : )

The spreadbore idea would work fine, just think outside the box about how to make this concept on a smaller scale and still fit the standard Holley bolt pattern. I have it figured out, just seeing if others have their thinking caps on.

Curious, will your idea be cheaper or work as well as the modified Holley's?

Jake and I took the wagon out tonight to a HAMB party about 30 miles away. He again remarked on how the car pulls harder than it ever has. So far the testing has turned out to be positive. I don't consider it a experiment anymore. This is a real, reliable alternative for a carb to give great fuel mileage, easy to get and keeps the initial cost down. Of course you will need to be able to perform a few mods to make it work.

Doubt it would be cheaper, but it will be less unorthodox than your approach. Unless you are replacing the baseplates on these carbs your modifying, which usually have the throttle shafts worn out, its really going to be a hard sell to convince people to buy a modified worn out carb. Its not always about how cheap you can do something, most people don't mind spending more for better quality.

So far the carb's I have been buying are nice ones. They usually bring 100 or less for them.

So what is your idea? Will it be easy to tune and acquire?

Well, I can't take full credit for this idea completely, it and similar concepts have been done for 20-30 years already in different motorsports of racing. If you have ever put a baseplate from an 850 onto the main body of a 750 Holley, then you have already inadvertently performed a similar modification. This was a popular mod 25 years ago or longer that improved performance. In a similar way, in racing classes where you are restricted to a baseplate of a certain butterfly size, usually a 750, then it was common to bolt an 850 main body onto the 750 baseplate to gain an extra edge, and it also was worth some HP to do this. We had a local circle track that required this for a class at one time that we did engines for, and I modded a bunch of these. In the same way, you can create a spreadbore type carb with the standard Holley bolt pattern baseplate by changing the primaries to ones that are smaller just like the ones in a 390 or spreadbore carb. There are several ways you can do this, it's not hard to figure out. But you basically end up doing the same thing you are now, reducing the effective size of the primaries, except now you would be doing it in a way Holley already has done it. You won't have to plug passages here and there and block off one hole barrel completely. And you still have the benefit of a larger secondary with the butterfly size of a 600 like you have now. And it will be as easy to tune as any other Holley, just simple jetting and squirter selection, idle mixtures, etc....

I run a homemade 3 x 2 setup on my Logghe chassied Nostalgia racer that uses three 350 2V main bodies taper-bored out to accept 500 2V base plates. Good throttle response and on a U-flow head they work OK, but my X-flow head needs (3) 540s. Similar principal as what can be done on your 4V.