The Stovebolt Site Forums The Shop Area Engine Shop he's alive!!!

officially have george henry back on the road! insured, licensed and already back to work. only one persistant problem. just off idle pulling away from a light or after every shift, there is a little stumble before he picks up. have gone through the carb a couple times (well at least 5 times) with the most recent change being replacing the accelerator pump thingy ma bob. definitely improved the situation but still a little hesitation.

any thoughts???

I would suggest checing the timing before doing anyhting else to the carburetor. Try advancing it a few degrees, that should help.

Most of the time when you suspect a carburator, it's an ingnition issue.

already did the timing check. prior to starting (static) and after with a timing light while running.

not ruling out a timing issue but unless i am going to advance the timing just as a test to see if it improves the situation, i am confident that is covered.

i really believe it is a fuel delivery issue from the accelerator pump.

I'm assuming that you're running a Rochester 'B' stock carb is that correct p.j.?
Did you have that problem before you rebuilt the engine?

This symptom has been hashed over hundreds of times and I have yet to find anyone that has an answer to it. I've run into it myself and after a couple of years and a half dozen 'B's finally threw in the towel and solved it with a Carter W-1. I've talked with dozens and dozens of people who are well versed in automobile theory and excellent wrench twisters, that have run the 'B, and I keep hearing the same story over and over. That is the “they can’t get the stumble out of it” and then there is always, “just ignore the factory specs and jack the timing up till it pings or won‘t start any more.”
I disagree with the timing issue and believe that these engines when stock and in good mechanical condition should run just fine with the stock settings. You don’t get much more basic than these Stovebolt engines and the ignition system is pretty simple to set correctly. One thing that I would say about the distributor is that during a rebuild all to often guys simply wash, paint and put in a new set of points, condenser, cap and rotor then call it rebuilt. They don’t completely disassemble and service what’s under the breaker plate. As with the carburetor a careful complete disassembly, cleaning, lubrication of the weights and check of the springs is necessary for it to for it to operate the way it was meant to. And don’t forget to check out the Vacuum diaphragm for proper operation.

With that said, what’s my solution? Like all of the experts, I of course not being one of them, simply don’t have a logical concrete answer. About all I can suggest is what you have already done, CAREFULLY and THUROUGHLY rebuild your ’B’, make sure your float level is right and the check balls are in where they belong . Make sure that the high speed circuit is working correctly paying special attention that the power piston is free and that the plunger is not bent or binding at the main well and that it works VERY smoothly and that the spring is correct. Because this is a vacuum operated device that operates with a very small vacuum signal and is probably the weakest link in this carburetors chain of events.

I had a theory that the fuel was the problem because it was diluted with alcohol. Alcohol as a fuel does not have the same energy that gasoline does so you need to burn about 1/3 more to get the same performance from an engine burning straight alcohol. I’ve experimented with avgas, non-alcoholic racing fuels, I’ve even driven out of state and brought back five gallons of regular auto fuel that did not contain alcohol and none of this changed the way the engine ran with the ’B’ carburetors on it. Switch to the W-1 and problem gone.

So I guess some will ask, “why did that jerk bother posting this reply if he doesn’t have an answer”?
Well, cus I suppose that ’m just trying to give ya some moral support p.j. and let you know that it might not be you or your tuning skills that are at fault, it just might be that you’re dealing with a carburetion device that was a poor design from the get go.

Denny Graham
Sandwich, IL

Denny, Is there a W-1 that specifically fits a 1962, 235? I always have that stumbling problem, no matter what I do to try and get rid of it. Sounds like trying a W-1 would be my best bet.

Denny is right I went thru the same thing with my 216 tell I got a W1 on her and she ran great after went to Kansas city and back got 16 MPG and NO trouble with the later 235s You.re going to have to find a YF carter the W1 is to small I just installed a 55/235 in My 52-3100 and found a YF4480s carter rebuilt it and am installing on the engine soon. There is a down side to the YF the pump is made out of a material that gets hard after a year or so with modern gas ,but I don't really care if I have to rebuild the carb every year or so ,as long as it makes my truck run good whats an afternoon working on my truck I'm doing that anyway.

There are alot of different YFs that You can use also Carter-Weber options search around and read up theres ALOT of info out there.

Pete

Agree with Denny and others, Carter YF or a W-1 will solve the problem. I went through this as well and was amazed at how good things got when the B went away.

This is interesting to me, as I have a choice of a Carter or Rochester for the 261 engine I'm working on for my '59 Apache 3200. (I have 3 Rochesters and 1 Carter, all single bbl.).
I wondered if there was a preference of the two from experienced Stovebolters, and before I even got to the point of asking, I found this thread.
P.J., Denny and others, thanks for the input.

Jerry

thanks for the support denny (and others).

yes i have the rochester "b" carb and it ran fine prior to the breakdown/rebuild. it was never very good on fuel mileage but that was not the reason for driving the truck in first place. it is definitely running lean because running with just a bit of choke reduces the stumble a bit more.

i will look into the carter carb but since funding a new carb is not an option right now, i will just have to tweek here and there to get it running the best i can.

I think most of us found that by fattening up the mixture by adding a 1/2" of choke while you're accelerating worked. But it's a flashing red light indicating that for some reason the 'B' is running lean. I got pretty good at it the first year, pull the choke, run it thru the gears and push it in when she smooths out at cruising speed. Still, that taint right!
I've got the short block back for the 216, head was done 8k miles ago so my test bed will be a fresh up to spec 216. When I get it back together and broke in I'm gonna do some more experimenting with the 'B'. I still think that the 'B' needs a little more fuel from the accelerator pump if your using alcoholic fuel. So that is going to be the circuit that I'm gonna focus on when I get a chance to get back to it.
DG

I beleive you might be on to something, Denny. After all, these old carbs were not designed with what passes for "gasoline" thse days. I will add that my suggestion on the timing was based on personal experience with a similar problem. With a stock engine/oe type distributor, I would only suggest no more than 5 degrees advance, which well within the range of the "octane selector" on the vacuum advance. I had to go 15 degrees on mine, but my enging is not stock, and the distributor I'm running doesn't have a vacuum advance. I also have never run Rochester on mine, only Carter YF and, currently, Holley 1904.

not sure how much alcohol is in the gas i am putting in the tank but you could be right denny. is there any way to change jets on the accelerator pump to compensate for the "lean" condition the new type of gas may be creating?

i know here in arizona we go to a "winter" fuel and since we are in the dead of winter around here...i think it was a chilly 60 degrees today...they are for sure only offering the grain fuels.

one thing i do know...i didn't realize how much i missed the sound of my old chevy burbling down the road.

I searched Stovebolt Page for an answer to my hesitation problem after I rebuilt the Rochester B on my '54 3100. I found a number of posts that suggested raising the float level. I raised mine two or three 32nd's. It substantially reduced the hesitation - but didn't totally eliminate it. I haven't played with the timing yet.

- Lonnie

Sorry guys, but I can’t buy the bit about the float level. Changing the float level an 1/8" up or down in a Rochester ’B’ carburetor is not going to make a bit of difference in the hesitation of these engines. Whether the truck heading up-hill or down-hill that carburetor was designed to handle a 30° incline without running short on fuel. And if you were accelerating up a 30° incline and you experienced a hesitation because of the float level it would not simply be a stumble but rather a sigh of a dying engine and all the choke in the world is not going to deliver more fuel.

Bill I agree with ya about the octane selector. It was included in the design to compensate for variations in the fuel grade. Back in the day when fuels were not as consistent as they are today, if you got a dose of lower than rated octane fuel you would experience pre-detonation and compensate by retarding the timing. The higher octane fuels with their slower burning flame front would require an advance in the timing to get the power out of them. But once again as you point out, the +/- 5° adjustment would cover all the fuels available for this engine.
I understand the 15° advance in your case. When running a non-stock configuration you can throw the factory specs out the window and you become the design engineer.

p.j. I don’t have the time to study the pump circuit right at this moment. But the only removable jets are the main and power jets. The one common thread in these complaints is that the hesitation usually shows up after the carburetors are rebuilt. Suggesting that the rebuilder did something wrong. But since 99% of the kits are Standard Brands, Hygrade 123A kits could it be possible that the by-pass passage is not the same as the original accelerator pumps? If more fuel is by passed this could result in a lean mixture as the pump is operated. I would love to find an original GM accelerator pump for autopsy. I certainly have a bunch of the after market pumps out of carburetors that have been rebuilt and I would guess that the vast majority of the ’B’s that we run into have been rebuilt at one time or another. I’d be very surprised to find a 60 year old carburetor that had never been rebuilt and was still operating properly.

Denny Graham
Sandwich, IL

Denny -

I agree. Seems screwy that float level could stop hesitation. Here is info on my carb rebuild. Perhaps we can find a common problem with other rebuilds.

Disclaimer: My Rochester B is only the second carb I have ever rebuilt. (First was a Carter AFB in 1966 on my 1963 300HP 327). So I am no expert and operator error could definitely be a factor.

One of the Stovebolt posts that I read when researching my problem said that the inlet for the accelerator pump well is high and that low float level may lower the fuel in the float bowl below the inlet. I dunno, but it makes sense. If this is true, then raising the float level will fill the well and allow the pump to push a full load of fuel into the air stream to prevent hesitation on sudden hard acceleration.

Raising the float level did releave most of the hesitation on my '54. Hesitation was so bad that it seemed to loose all power when the throttle was opened wide at idle or when moving. It did not hesitate before the rebuild. I raised the float level twice. The hesitation lessened each time. I may be able to raise the floats even more, but I stopped while I was ahead.

This is the only varience that I know about on the rebuild.

The old float needle seat did not have washer under it. The instructions called for a fiber washer under the seat. None came in the kit, so I made one from an extra air horn gasket. The float needle therefore sat higher than previously. This required an adjustment to the floats.

At casual glance, it looked like the float needle seat, the float needle, and the accelerator pump were identical to the ones that I removed.

Here is a 1976 Pop Science Carb Troubleshooting article on Google Books that states,
"Hesitation ... Main causes are low fuel level in the float bowl, insufficient accelerator-pump travel, worn or malfunctioning accelerator pump ..."

Another entry on Google Books, Rochester Carbs: Tune, Rebuild, or Modify, notes that low float level will cause a lean running carb. That may be a cause of having to pull the choke cable 1/2" on acceleration.

Hope I didn't just add to the confusion!

- Lonnie

Since the lean situation seems to be in the power circuit not idle, acceleration or circuit used for crusing(light load),I would study the passages taking the fuel from main jet and power valve to the main discharge nozzle.this circuit has a joint (probably gasketed) where the main well tube is attached to the upper body-a leak in venturi signal here will result in a lessened fuel delivery.
Then study the power valve and it's operating piston and spring-when accelerating lowers manifold vacuum signal the spring extends the rod,opening the power valve and introducing more gas AROUND the main jet into the main well tube--NOW that power jet passage to the main well tube has a calibrated restriction-it seems that could be a source of trouble causing leaning of the mixture when the power valve is needed to enrichen for acceleration.
The reason mixture is made richer or leaner when fuel level is changed is that the level in the bowl is the level in the main delivery circuit and when higher, then the same venturi vacuum signal lifts/delivers more fuel than at a lower level.
Just my thoughts from studying the Rochester b drawing and 50+ years involvement with carburator problems.

So in light of whatI can see in th drawings and your description of the response of the engine I would:
1) make sure the fit of the main well tube to the upper body is smooth and sealed well.
2) locate the restriction in the passage from the power valve to the main well tube and assure absolute cleanliness
3) remove from the upper body and clean the power valve piston and its bore to assure free movement of the piston when the vacuum drops
4) locate and clean the passages both above and below the power valve piston and look at the sealing of them through the gasket.
5) the spring above the power piston is what pushes it down to open the power valve when throttle is opened and vacuum drops.if it is missing/brokenor not free to operate your lean condition when calling for power will exist
6)the piston rod must hit the power valve check ball when the spring extends it or no enrichening
7) clean the power valve itself and try to prove it allows flow when open and seals off when not actuated
Study this power circuit and think about it's function--in my opinion the trouble is to be found there.
Good luck and keep your eyes open for other possibilities.

enginenut,

thanks for the info. not sure i can stop driving the truck long enough at the moment to go through the carb again, but it is at least something i can check out when it is less in demand.

my belief is that it is somewhere in the accelerator pump (power piston) that is limiting the amount of fuel being dispensed.

check, check, and re-check. will let you know what i find!

But PJ, the accelerator pump and the power piston(power circuit) do not have the same function and the difference can be discerned.The acc pump only delivers a shot of fuel and an influence on the mixture strength momentarily when the throttle is MOVING toward increased opening.When that movement stops the delivery stops.
The power circuit adds fuel to the main jet delivery when manifold vacuum is lowered below a set point as in heavier load and increased throttle opening an continues to do so as long as the lowered vacuum condition exists.
So if the engine only stumbles when throttle opened but pulls good after the load/speed is increased look to the acc pump for weak or shortened delivery.
If, like on a gradually increasing grade with throttle being opened gradually, the mixture seems lean and the choke can help it but is needed continually the problem is probably found in the power circuit.
The Rochester is a sound and sufficient carburetor and will perform when it is right.