The Stovebolt Site Forums The Shop Area Engine Shop Major Stumble at Throttle Tip-in

Hope I have better luck with this first post than I am having with my first engine rebuild.

I have a newly rebuilt 1957 235 that has a tip-in stumble so bad the engine will stop if I don't quickly back off the throttle after a hard acceleration. I see other posts with the problem but so far I haven't seen any solutions that have worked.

The engine is running on a test stand right now. I installed Fenton headers and the Fenton dual carb intake manifold with 2 B model, manual choke Rochesters. Carbs are rebuilt with no tags so probably have mixed parts installed. I am heating the intake manifold using the connection on the Fenton headers.

Accelerator pumps are throwing out good streams. I'm not sure that can be improved. Carb jets were reduced from 58s to 57s and a Colortune shows good blue flame throughout the range except when the throttle is blipped hard and then the Colortune goes to orange(rich).

Manually squirting raw fuel into the carbs on acceleration cures the stumble so it seems I am starving the carbs although the Colortune seems to indicate otherwise. Vacuum drops to zero when throttle is hit so how can carb get gas from any port if there is no vacuum for a split second? If this is the problem, what's the cure? Vacuum is steady 17 at idle.

I have a Mallory dual point dizzy and a Pertronix coil. Points, plugs, valves have been checked and rechecked--per the appropriate specs.

Any help would be greatly appreciated. I can't install the engine running as it is and this project is rapidly turning from a therapeutic exercise into a major stress inducer.

Thanx to all.

Hy billybobs, some of the four barrel carbs from the fifties used to exibit a stumble at throttle opening, it was because the vacuum in the manifold dropped to near zero, this caused the fuel to stop moving towards the cylinders it deposited itself on the floor and walls of the intake manifold. The "cure" was to increase the size of the accelerator pump, so that some fuel made it to the cylinders. You have already stated that "squirting raw fuel into the carbs on acceleration cures the stumble". so maybe we have confirmed lack of fuel to be the problem, maybe CarbKing will give us some suggestions on how to tune the stumble out of your setup. Does your distributor have vacuum advance? If it does your advance would retard to the static timing point as the vacuum drops to zero, do you think more initial advance would help any?

I had the same problem with model "B" I have only one carb but model B is known to have that problem I rebuilt a W1 carter and the engine runs great no stumble and the engine runs strong thru the RPM range.You could try only one carb at a time leave one closed and see if the stumble gets better ,than try the other carb and see what happens.Also you're vac should be around 21 inchs at idle and it will fall to 0 when the thottle is hit.Maybe theres a small vac leak,you could check with some carb cleaner.


Pete

Thanks for this reply. I added an extra oulet port for the accelerator pump and this helped. I also added some additional spring coils to the power valve spring so the power valve opens at a higher vacuum (I think) and that finally cured the problem. These are "weird" fixes it seems but this is what it finally took to get the carb to work. If this hadn't worked, the carb was headed for the trash.

The problem is the B model carbs- - - - -they're WAAAAAY too big for your application. When the carbs try to go from the idle circuit to midrange, the velocity in the venturis is so slow that you're getting nothing but air, no fuel. Unless you really sneak up on the throttle opening, your flat spot is the result of momentarily starving the engine for gas. The only solution without changing carburetors, and this is a "aspirin for a brain tumor" type of fix, is to give it a massive squirt of raw gas from the accelerator pumps and hope enough of it evaporates to hide the lean spot. The correct fix is to use carbs with very small venturis, so splitting the airflow between two carbs doesn't kill the velocity of the air through the venturis. You will still increase the maximum airflow at full throttle, (the only time where the dual carbs are any advantage in the first place), without destroying the off-idle and midrange performance in the process. Find a matched set of Carter carbs designed for the 216 engine, and build them up exactly equal in air and fuel flow. The RIGHT fix is to throw the dual-carb manifold as far as you can, and go back to what the GM engineers designed for the engine- - - - -one carburetor!
Jerry

A pair of the original "big" (2-15/16" flange) 235 carbs is too much. There is a slightly smaller venturi made for the Gen-3 230" "economy" motor, but not much better. There are even smaller B, but they have a different flange pattern.
If you want to work with these, you may find that the throttle disc is too far below the transition slot and the slot isn't exposed to vacuum at idle. A small chamfer can be filed or ground in the disc’s beveled edge next to the slot.
Very important: the initial spark advance greatly affects idle quality and off-idle response. Try bumping the distributor a bit to see if this helps (if it does, the advance curve will probably need some tinkering).

On my Model H Rochesters (3 of them) I had to increase the accelerator pump openings to cure my off idle bog. I just drilled a second hole in each carb, starting out with a .015" drill. I had to step it up a few more times till I cured it, somewhere around .022". The extra hole points down right in to the venturi.

Since the engine is so low compression, bump the timing up till runs better, even with low grade 87 octane, it should never ping. Mine runs better with more timing.

Joe

A great tech article on inliners.org on setting up multople Rochesters. http://www.inliners.org/tech/tech4.html

Billybobs and Joe H, where exactly did you drill to increase the hole from acclerator pump or add an extra hole? would you have pics of this? I too have a 261 with dual rochesters.

Sorry, I have no pix. I used a pin vise and a very small wire size bit (I can't remember the size) and put a smaller secondary hole right above the original hole. I tried to keep it small so it would have enough pressure to "squirt".

This provided part of the solution for me but I would advise extreme caution as I am sure it is a good way to ruin the Rochesters. Try everything else first. I was at the "end of my rope" and willing to try anything.

Prior comments about the Rochesters being too big are right on, I believe, but I am committed to the dual setup.

billybobs

I did just as Billybob did, drilled a second hole right under the factory squirter hole. I used a cutting torch head cleaning drill set from NAPA. It's a handful of really small drill bits and pin vise to hold them.
I was at the end of the line as well and had nothing to loose. If it doesn't work, slice off a sliver of lead solder and cram in th hole to plug it.

Joe