I"m sure this has been discussed many times already BUT can someone shed some light as to why I have an off idle stumble on my Holly 4000.

The 1956  -  312 engine has an automatic trans. Already have a 57 tach drive distributor in the engine and have done Teds 4000 mod to upgrade the teapots circuit to use vaccum advance.

Float levels seem to want to be high in this carb . When set at float level specs it hesitates horribly. No vaccum leaks. Timing is at 6 initial.  I did bump it a bit as far as 10 with no help in the stumble.

Both the vaccum and centrifical advances are working.

It goes from a dead stop with no stumble. Its only when I am at 30 MPH or so and give it some gas pedal that it stumbles.

Dont know what else to check or recheck.

Suggestions.

Oldmics

Check the carb throat (hold the choke open and blip the throttle lever) for good pump squirts. If no, then inspect the accel. pump cup for damage (alcohol & gas stabilizer shortens their life).

6 VOLTS/POS. GRD. NW INDIANA

The carb is supposedly "brand new" from one of the major T Bird suppliers.
Supposedly run on a mule engine. HA !!!!!

I have already found sooo many things wrong with this rebuild its rediculious.Improper choke settings,missing parts,etc.

It has a good solid pump shot on acceleration,although that is one area that I have not yet inspected.I will go into the pump area and see what I can find.
I did have to move the pump lever to the other position as to get it to not die out on a stop to full throttle operation.

I will get the pump lever to an approximate mechanical position of 30 mph and see what the shot looks like from there.

It will go like hell from a dead take off - its just has that wierd stumble around 30 mph on slight acceleration.

Thanks for the suggestion.

Oldmics

I don't know anything about those carbs, but on other carbs I'd be looking at the butterfly position at light throttle cruise and the transfer slots before the primary really comes in. Hopefully, someone who knows these will know if that's a possibility. Part of this is where the secondary butterflies are set, and how much if any air they pass at idle. I wouldn't even post this if you hadn't said the whole rebuild was lacking.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Found the problem - well at least I have corrected the problem
.
The customer originally had brought me the new carb to just install on his machine.

His old Holly teapot was hesitating and running super rich. I never drove or examined the car with the original carb on it.

After installing the new carb and correcting all of the preformance issues WITH the exception of the 30 mph hesitation I have to mention that I had to use parts from his original carb to make the new one work as well as it did.

Everyone pretty much knows that a hesitation is usually a pump shot issue. I had not inspected the pump until Dan Tinder mentioned that concern. I knew that there was a new pump installed and did not really question that part of the rebuild.

And OF COURSE - that was the problem. The spring that was installed on the pump arm was too short and not allowing a full shot AFTER the half way mark.

Swiped the longer spring from the shitty donor carb and installed a new pump assembly , blew out the passage for good measure and WHALAAA its just fine!

So the moral is if it hesitates - go for the pump shot and spring length assembly.
Thanks for all of the help.

Oldmics

I recently had a bone stock 1956 Thundebird engine on the dyno and it had a stumble right off of idle.  Did the normal trouble shooting such as checking ignition timing, accelerator pump shot, the accelerator pump arm adjustment, air bleeds, fuel/float level, etc. and found nothing amiss.  I ended up rejetting the carburetor from #50 jets to #53 jets and that completely cured the stumble.  I attribute part if not all of this stumble to the ethanol laden fuels which makes the fuel mixture simply leaner.  This was confirmed with the wide band oxygen sensors that were mounted in the exhaust system while the engine was on the dyno.

Lorena, Texas (South of Waco)

That would pretty much dictate every 4000 teapot should certainty have its accel pump shaft set for the hole that produces the longest stroke, regardless of season/climate (unless most people in Florida or Death Valley have free access to pure gasoline)?

6 VOLTS/POS. GRD. NW INDIANA