Inliners International Forums General Discussion Engines Idle Speed - Dual Rochester DC Carbs

Help! I'm going crazy! My carbs run great EXCEPT the engine dies at idle once the engine warms up. Please consider the following and share any thoughts you may have, THANKS!

The engine is a rebuilt 1954 235 with a 848 head; less than 8,000 miles on the engine since built. I installed a Bulldog cam 2 years ago, use a 12 volt system with stock coil, rebuilt distributor, new vacuum advance, new points, condenser, rotor, plug wires, hot plugs at .045 gap. Timing at 8-10 degrees advanced.

I have a pair of Rochester BC carbs installed on a Fenton intake and headers.

The system includes heat to the base of the intake from the headers, insulating spacers, stainless steel tubing from the side of the thermostatic coil housing to a small hole drilled into the header to create a well to draw heated air to the choke housing. The floats are set to factory specifications. The jets are # 51s.

Choke/choke cover is rotated to move the plate until the plate in the throat of the carb just touches the wall of the throat (closes off the air passage). The choke rod to the idle cam is bent so the screw is sitting on the 3rd high step.

Both carbs are identical; every part has been matched to ensure balance and identical functioning. Both were rebuilt at the same time. The carbs were installed onto the intake at the bench and once installed on the 235 engine were balanced with a nanometer.

Using regular gas. Fuel pump is rebuilt pump from catalog with glass bulb filter, have an additional glass bulb inline fuel filter. All gas lines from pump to carbs are at least 5/16 diameter stainless steel, all 90 degree turns are smooth and maintain line internal clearance.

The linkage is custom; from block to carbs linkage is free moving without touching any part of the engine/exhaust; linkage is spring loaded and under tension when in use. Connection to each throttle lever is identical and fixed.

Have a great day,

Ted

What idle speed are you trying for? Is it stick or automatic? Have you checked for vacuum leaks? Did you do initial timing with the vacuum advance disconnected and the line plugged? Just because things are new doesn't mean they work. I bought a new Mallory coil once that didn't work at all. It sounds like you have been pretty thorough. These are just some thoughts.

As a first thing, I would probably tell you to put hot water to the bottom of the manifold, the exhaust heat is not all that efficient. Second, I would recommend that you purchase a Unisyn, I think edelebrock still sells them. It is a tool that uses vacuum to help synchronize the carbs at idle, same air coming in at idle. You warm up the motor, disconnect the linkage, use the tool to synchronize the carb air at idle, reset the idle from that point, readjust and reconnect the linkage. Finally, make sure the gas pedal is free and the linkage is not at tension when the carbs are at idle. Otherwise, it could be that you are setting the idle, but when you actually get off the gas, the linkage closes the throttle further then you want, shutting the motor off. That's why you sync the carbs in with all the linkage disconnected.

What they said, plus: does it die from lack of fuel or lack of spark? Isolate the problem to one or the other. I once had a "mystery stall" engine I was working on where the owner had replaced the points himself, "brand new so it can't be the points". They were even gapped properly. But the engine consistently stalled once it settled to idle speed. Turns out the points were wrong type, the wire connector was able to touch the distributor ever so slightly as the bottom (advance) plate turned. Ran fine at higher rpms.

Secondly: If you have manual choke, you can try to recreate the problem with the choke partially on. If the problem goes away, then it indicates the cause was fuel starvation and driving with the choke on provided the "missing" fuel. This might help narrow it down.

And as already mentioned, what idle speed are you targeting?
I had a customer come in with a recently bought, souped up 70's Chevy with a small block in it. Had tons of power but kept stalling at idle. I told him to start the engine while he was still on the parking lot, and I soon learned his idle was like 1300rpm. He had kept turning the idle speed up to fix his stalling problem. I adjusted his idle speed way DOWN and he thought I had lost my mind... until he saw the stalling went away The automatic transmission was starting to engage /would not disengage with such high idle and stalled the engine....

Thanks guys, I appreciate the responses and the information.

Beater, the transmission is the stock 4-speed but I have not checked for vacuum leaks, yet. The timing was done, using an adjustable light, by someone else while I was at work so I'm not sure but will find out. The engine runs strong, without missing, all the way through from initial accelleration to as many r's as I dare with plenty of throttle left to go.

M, I used a nanometer (seems to be what you're referring to but with a different name?) to balance the carbs; the linkage was disconnected, idle set to provide matching vacuum/air draw into the carbs and linkage re-connected. The gas pedal is free BUT THE LINKAGE IS AT TENSION WHEN THE CARBS ARE AT IDLE. THEY WILL NOT BE AT TENSION WHEN I GET HOME!

70, feeling good about the points at this time but will keep your comments in mind. Carbs are automatic choked. The desired idle, at operating temperature, is 800 RPM but I'm running 1000 - 1200 RPM

One of two other issues I'm exploring is finding out how much fuel pressure I have to the carbs. I've got two filters in the fuel line. I'll install a gauge port this week, will let you guys know the results. The other issue is switching from regular to high-test gas.

The real issue for me is trying out each of the changes one at a time so I can evaluate the results.

Thanks a lot!

Ted

I think the plug gap is a bit wide for a stock distributor-I believe it should be more like .035". The .045" gap is more for HEI fired stuff. However I dont think that would cause an idling problem.

If each model B carb ran fine by itself on a car then I would think that you should try to change out the power piston springs like they say to do in the tech articles on this site--this will make for a crisper throttle and keep it off the power stage (rich & bad milage) because its not uncommon to have less manifold vacuum with multiple carbs (more places to leak vacuum and more potential worn throttle shaft leaks) and with less vacuum the power stage could be coming in too soon and even possible to be there at idle. I've had that happen to me on v8s using two "perfectly" tuned stock 4 bbls--at idle these "perfect" carbs would run so rich your eyes would tearup from the black exhaust. It could be the motor dying maybe from an overich mixture when warmed up. When cold the extra richness wont affect it too badly, but as it warms it dont need the extra richness. good luck! I wouldn't worry 'bout the grade of gasoline, nor the fuel pressure from a stock pump.

The manometer is a step up from a unisyn-so no problem.

See if its a mechanical linkage issue, or a mixture issue or maybe an ignition problem. You could check for plug spark by looking at a timing light (no need to be a machoman an' grabbin' a plugwire-besides it would play hell with your pacemaker and make your socks yellow!!)

Made great progress this weekend but am still dealing with hesitation as I accelerate (go from idle to off idle) and an idle speed of 1100-1200 rpm.

Purchased two manual choke adapters and installed them.

Still starts easily but this time I can move the butterflys all the way open - the rich fuel mixture went away. I haven't given up on the thermostatic choke system, just inserted some improved control so I can drive it. The other change was to install E-3 plugs. (A new design where the spark travels to a "C" shaped ring standing around the outer edgge rather than a single bar standing above the porcelain) Pulled a plug after driving 3-5 miles and they look great. Am going to an out of town car show over the holiday and will know better when I get back home on Saturday (120 -150 miles round trip)

When I manually move the choke assembly's butterfly to wide open the internal lever controlling air movement across the coiled spring and into the intake manifold closes so the vacuum loss during cold start is closed off when the engine is hot. It seems to me there is some small amount of vacuum loss still occuring at this point. Would it be worthwhile to plug the copper piping going from the base of the carb to the thermatic choke assembly?

I need help to understand the components of the stat cover and spring assembly. Are there other parts in addition to the cover, cover gasket, coiled spring and thin wire wrapped around the cover post and over the edge of the spring? When the spring is under tension to adjust the butterfly position it relaxes due to heat but it does not drag the piston / butterfly assembly back to the location where the butterfly is wide open. Is it only vacuum that finishes moving the butterfly to the verticle position? If not vacuum, how is the arm of the piston connected to the spring?

Last question, how many coils should I remove from 1- the power piston or 2- (from above or below the pump plunger)? Transition from off idle still has not improved.

Thanks, Ted

Sounds like you are ready for the next step. Find the article written by pat smith on the tech side of the incliners site. Pat gives you all the detail you will need to dial those carbs in.

Thanks M,

I just went over Pat's article and from what I can see I've all ready been there.

The spacers between the manifold and the carbs have been installed and are about 3/4" thick , hard black shiny plastic.

The gradual increase in the drill size was done with the entire restriction being completely removed from both carbs. The drill bit was drilling the entire shaft when I stopped enlarging the hole. (No change in hesitation off idle)

Every issue identified in the article has been done, both carb's components match identically.

I keep coming back to low vacuum but I don't understand how. I do not have vacuum leaks (used carb cleaner at every connection) {I did not disconnect the vacuum line to the wipers but I will tonight when I drive the truck) and the compression on all six cylinders averages 115 pounds with no more than 10 pounds variance between any two cylinders.

I've got plenty of power piston springs, the next step for me is to cut 3 coils off to start and then one coil at a time until something changes when moving from off idle.

I appreciate all the help guys, any and all further comments / suggestions will be greatly appreciated.

Does anyone out there have the ability to contact Pat Smith, perhaps talking with him about my setup might help. If Pat could call 801-657-8442 I would happily call him right back.

Thanks again and have a great 4th of July Holiday

Ted

Ted,
Sounds like you have the "S=Small" size BC carbs (they come in XS, S, M, L, and XL sizes!)
Have you tried richer jets, say #54 instead of #51?

I usually run the M=Medium or XL=XtraLarge jugs, but once ran the XS=XtraSmall carb and it liked a much richer mix than the factory setting.

Only other thing Ive had trouble with is, the seal from block to intake (tho I have the newer six which may be trickier). Sometimes they seal good cold but when they get hot, a leak develops and the idle goes bad.

Or, try a couple different carbs, those B/BC/BV carbs are still pretty cheap at the swap meets....

There are 4 different throttle disc diameters, in 2 bolt patterns: 1-1/2", 1-9/16", 1-11/16" and 1-3/4".
In addition, several were made with multiple venturi sizes:
1-1/2" with 1-1/16", 1-7/32" venturis
1-9/16" with 1-3/16", 1-9/32", 1-11/32"
1-11/16" only with 1-15/32"
1-3/4" with 1-9/16", 1-5/8"

Total of 8 sizes.

WOW I had only found 5 of the 8.
So I guess theres
XXS XS
S M L
XL
XXL XXXL
And they all look the same externally.
I better re-measure my collection to be sure.
Thanks for the info!

I have read where the 1-5/8 venturi was the 261 carb.

Preacher:
Was the 1-5/8 referenced here or somewhere else?
Note the poster here says (maybe) 1-5/8 but throttle bore not venturi. AFAIK the 261 ran the 1-15/32 x 1-11/16 carb. I cant find a carb with 1-5/8 venturi, seems it would have a very poor signal:
http://vcca.org/forum/ubbthreads.php/topics/102703/

Panic:
Do U know application for the 1-3/16 or 1-9/32 carbs?
I cant find any, then again data is pretty sparse sometimes.

Here's what I have so far, as long as we are on this topic:
t=1-1/2 v=1-1/16
235 truck economaster 60-62

t=1-1/2 v=1-7/32
216 all 50-53

t=1-9/16 v=1-3/16
cant find any

t=1-9/16 v=1-9/32
cant find any

t=1-9/16 v=1-11/32
235 52-56 for sure, maybe 50-62
pont 6cyl 50-52
194-230 62-67

t=1-11/16 v=1-15/32
261 all 54-63

t=1-3/4 v=1-9/16
230hipo 64
250 66-67
292 63-67

I'm fairly sure of the above but of course edits welcome!
Still cant find applications for those 3 new sizes

It was some other site and some other time (so many places to go on the internet) it may have been on H.A.M.B. they got the volume over there.

The majority of 261s saw their service in Dump trucks and school buses (I dont think they were even available in half tons), where they were worked to death and were used at full throttle for much of their working lives, so thats probably why they could use such a large venturi to throttle butterfly relationship, maybe they did not have too much of a "good signal" to draw fuel from the bowl with at lower rpms, but they didn't live at that rpm area long with their "granny low" gearboxes and astrnomical rear gear ratios. (maybe even 6.17:1)

Thats a good list of sizes, didn't know that there was that many different models. My biggest complaint against the Model "B"s was their sick little nasally "whistle they made at part throttle--I once made an air cleaner out of a big grapefruit juice can that sat horizontally, and wired it down to the airhorn of my '54s model "B", it sounded bad at WOT- kind of a roar, my neighbor had a '55 235 (non-hotrodder), he liked so well that he bought it from me for $5.00 back in '61 to put it on his car! Just call me TLowe!