Fuel System: throttle hesitation

[doublepaddle]

Need help here. Every time I step on the gas pedal I get a hesitation or flat spot where it sounds like it is going to stall out. No power and it bogs out. This only happens at very slow speed. When I double pump the pedal it is fine and I get great pick up. It runs fine at speed.

'57 Transtar 1/2 ton with 259 bored out to 289. New rebuild, 2 barrel carb rebuilt, distributor rebuilt, timing set correctly with Pertronics unit. Has a manual choke cable set-up that I use when starting cold.
Any ideas as to what is causing this and any recommended fixes?

[Flashback]

Sounds like acc pump adjustment, or dried up pump. Be sure pump plunger is moving the instant you move accelerator

[Corvanti]

this will probably get moved to the "tech" section. that's ok...

my first (easy) thought is too much fuel/not enough air thru the carb. maybe just an adjustment needed with the fuel/air mix screws.

our experts here probably have a better reason.

[R_David]

How long ago was the carb rebuilt? And who did it?

[cobraman428]

Accelerator plunger giving out. remove top of Carb and take out plunger. Older ones were leather. Newer ones are rubber. Take out plunger and lather up and soften with Vaseline. This is a temporary fix. Long term fix is to replace. Don't mess with adjustment screws or timing. Explore plunger idea right now. Good luck.

[jclary]

Sounds like acc pump adjustment, or dried up pump. Be sure pump plunger is moving the instant you move accelerator

Accelerator plunger giving out. remove top of Carb and take out plunger. Older ones were leather. Newer ones are rubber. Take out plunger and lather up and soften with Vaseline. This is a temporary fix. Long term fix is to replace. Don't mess with adjustment screws or timing. Explore plunger idea right now. Good luck.

I agree with these two. Also, depending on which carb you are running...some accelerator pumps have adjustable linkage. Make sure the connecting rod on your pump is in the linkage hole that gives it a straight linear movement as it travels up and down. Wrongly adjusted and the pump will bind. Good luck as you work through the problem. Be sure to come back and post what solves it.

[Corvanti]

well, like i said, "wait for the experts"! never mind my earlier post. the carb flow (rochester), or after that, timing changes which worked on that problem on a couple non-Stude's i had...

i reckon i should stick to recommending possible "easy fixes" i've only done with the 3 Studebakers i've owned.

again, i'm sorry for my earlier info!!!

[avantilover]

I recently had the Rochester carby in My1966 rebuilt and it occasionally hesitates. The auto club mechanic recently said its a leather seal and needs some use and will be fine. So it seems to be.

[PackardV8]

Is the carburetor the original 259" Stromberg WW?

1. Could be the float level is set too low or the fuel pump isn't putting out enough pressure. Low float levels can cause a bog.

2. As has been suggested, that you can doublepump it indicates accelerator pump shot is lacking.

3. Sometimes a carb kit just isn't enough. If the throttle bores are worn; not unusual with an old Stromberg, the air leak throws off the idle circuit and thus the transition to the main circuit. Difficult as it is to part with the money, a "remanufacture" with bushed throttle bores and the full meal deal is sometimes the solution.

259 bored out to 289

Could that be "stroked out to 289"? Boring a 259" to 289" would require a 3.76" bore; a .1975" overbore. Probably has been done, but not by me.

jack vines

[GinettaG12P]

Take off the air cleaner and look into the carburetor. When you open the throttle, you should see a squirt of gas going into the carburetor throat. No squirt or feeble squirt means that your accelerator pump isn't functioning properly.

[doublepaddle]

So here is where i stand now. I have plenty of gas to the carb. after checking the squirt. Maybe too much. I lowered the pressure valve inline with the electric fuel pump. It helped quite a bit. Now hesitates sometimes. Seems that the manual choke is in play here. By moving the manual choke cable slightly I can dial in and lessen the hesitation. Very small amount of movement but there is a small amount of choke needed to run without the hesitation. Slight higher idle speed at rest.

[PackardV8]

By moving the manual choke cable slightly I can dial in and lessen the hesitation. Very small amount of movement but there is a small amount of choke needed to run without the hesitation.

Closing the choke both richens the mixture and increases the vacuum which pulls in the main jet flow.

IIRC, you increased the displacement from 259" to 289", thus increasing demand. Is this the original 259" Stromberg WW carb?

One other factor could be an intake manifold, carb throttle shaft leak or power brake or distributor vacuum leak. Any of these could lean out the mixture.

jack vines

[doublepaddle]

Yes this this the original carb. Stromberg 6-120. Special to V8 plus auto trans (factory).

[Mike Van Veghten]

Once you have your carburetor problems worked out, a few more degrees of ignition timing will also help your hesitation situation.
Stock timing values will get the engine running...a few (4 to 8 degrees) more degrees will help it run much better.

Mike

[jackb]

...still sounds like the acc. pump ....