chevy 305ci v8 idles fine but stumbles on acceleration

papacan · #1 May 11, 2008, 12:57 PM

It will idle okay,but will not respond to acceleration,stumbles or dies. Things that have been done: rebuilt carb.,2 barrel,replaced timing gear & chain,rebuilt distributor, so can you guys or gals help me out please! Thanks!

KeepItSimpleStupid · May 11, 2008, 03:30 PM

That's a classic accelerator or aux accelerator punp problem. Was there a diaphram operated accelerator pump (aux) with internal passages in the carb? You should see a squirt into the throat even with the engine off from the main accelerator pump.

The aux accelerator pump may not apply to your vehicle.

Another possibilty is the choke unloader diaphram. It should pull the choke plate a little open after it gets vacuum.

Also check for vacuum leaks.

Poor adjustment of the carb can also cause this. If you need the "propane enrichment" method of adjusting the carb, holler. Works like a champ. The problem is it's difficult to position the throttle plates into the idle system.

You'll need a source of propane, a regulating valve, an on/off valve and tubing to get the propane into the intake manifold. Usually the air cleaner vacuum hose works. You may have to sacrifice a torch head. The on/off valve must be downstream from the regulating valve.

papacan · May 12, 2008, 07:32 AM

Quote:

Originally Posted by KeepItSimpleStupid

That's a classic accelerator or aux accelerator punp problem. Was there a diaphram operated accelerator pump (aux) with internal passages in the carb? You should see a squirt into the throat even with the engine off from the main accelerator pump.

The aux accelerator pump may not apply to your vehicle.

Another possibilty is the choke unloader diaphram. It should pull the choke plate a little open after it gets vacuum.

Also check for vacuum leaks.

Poor adjustment of the carb can also cause this. If you need the "propane enrichment" method of adjusting the carb, holler. Works like a champ. The problem is it's difficult to position the throttle plates into the idle system.

You'll need a source of propane, a regulating valve, an on/off valve and tubing to get the propane into the intake manifold. Usually the air cleaner vacuum hose works. You may have to sacrifice a torch head. The on/off valve must be downstream from the regulating valve.

As I mentioned the carb. was pro-rebuilt & seems ok, I too was thinking vacuum leaks,I will try that. And how much fuel should the fuel pump squirt, when diconnected from the carb? thinking maybe a weak pump. Thanks for your help & concern!

ddollinger · May 12, 2008, 07:53 AM

Change your fuel filter/filters. A partially plugged fuel filter will allow enough fuel to flow through it to allow idle but not enough fuel under throttle. I can't count how many times I have seen people throw gobs of money at vehicles in rebuilt carbs, ignition components, etc to fix driveability issues to find out it was a $3 fuel filter. Even if it does not fix your issue you are only out a few dollars and they are supposed to be changed on a regular basis anyways.

Be aware that many vehicles have 2 fuel filters, one near the fuel tank and one near or on the carburator. Your repair manual will indicate where to find them.

KeepItSimpleStupid · May 12, 2008, 08:48 AM

Fuel pump would not necessarily have too much to do with it.

It's the ACCELERATOR PUMP. It pumps a burst of gas into the carb from the bowl. If it were bad, the car won't start. It should be a forcefull stream. Check with engine off.

Carb adjustment. Rebuilding the carb only gets things close.

PROPANE ENRICHMENT METHOD

Propane tank with regulator and on/off valve attached to intake manifold.

PURPOSE: To be able to turn on and/off propane flow without changing the idle speed. Only then are the throttle plates positioned properly.

METHOD:

Attach system to car with all valves off. Start car.

Open on/off valve. Adjust regulating valve for maximum RPM. To much propane and RPM will decrease. Turn off on/off valve. There should be little change in engine RPM when the on/off valve is turned off.

Adjust mixture and idle speed and repeat the above. I don't remember the exact sequence of adjustments. It's been at least 30 years since I've done the procedure, but the above paragraph is correct.

Your now in the idle system.

A really good indication that the carb is NOT ADJUSTED PROPERLY is if it's raining outside and the idle becomes rough and low.

Another possibility is the float adjustment. It was was easy on my Toyota. On American cars, it was hit or miss.

An accelerator pump problem and/or carb problem will show up as hesitation at idle and no load.

Fuel filter problems usually won't.



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