Engine Hesistation/Stumble 1964 327/300

[Bruce Boatner]

Re: Engine Hesistation/Stumble 1964 327/300

Mike,

The car has hesitated since the first day I had it. I have a strong hunch the problem is related to the carb, because after I rebuilt the carb the problem was MUCH better (it was really bad before). The car still hesitates, but the pucker factor is much less when turning onto a road with oncoming traffic.

I tried advancing the timing. Pinged like a baby's rattle but still hesitated. I never thought about the timing chain, but the former owner said the entire top end was rebuilt about 1000 miles ago. I tend to believe him because the engine runs smooth as silk above 1500 rpm and not too bad below that except for the hesitation issue.

I appreciate your input. This has been a real head scratcher so far.

Bruce

[Clem Z.]

check the power piston springs

make sure that you have the proper springs under the metering rod power pistons because if the springs are too light the carb will be slow at going to the rich stop causing a hesitation

[Bruce Boatner]

Re: Engine acc. pump/ Hesistation

Frank,

Thanks for the advice-. I will definitely try adjusting the pump and see what happens. I hope this works! If I understand you correctly, I should INCREASE the distance between the top of the pump rod and plunger base by 2 ribs right? Is quite a bit of pressure needed to make the adjustment, or should I expect the plunger to move relatively easy?

Does anyone know a source for oversized pumps just in case?

[Bob R.]

Re: Engine acc. pump/ Hesistation

After reading the entire list of suggested fixes and your responses I see where the top end of the engine was redone recently. If a more aggressive cam was installed your car will not idle at the original spec rpm. My 63 300 HP engine with powerglide should also idle at 450 rpm. After rebuilding the engine and putting in 202 valves and a more aggressive cam the rebuilder changed the vacuum advance weights and told me to set the idle at 700 rpm. It runs excellent at 700. At lower rpm it stalls and hesitates.

[Bruce Boatner]

Re: Engine acc. pump/ Hesistation

Bob,

Thanks for your input. Unfortunately, the engine hesitation problem still occurs at a higher idle rpm. While playing the automatic choke, I drove the car around with idle at about 1500 rpm and it still hesitated.

Wish a had a spare AFB lying around to slap on and see if any change occured so I could at least rule out/isolate to the carb. I'm sure with everyone's input and some more trial and error I will eventually solve the problem. Until then, I won't be winning many redlight to redlight races.

Bruce

[Brian Monticello]

Re: Engine acc. pump/ Hesistation

Bruce,
While you have the AFB off, you might want to take a looka at the accelerator pump check valve. If it allows back-flow your total pump shot volume will be reduced.

brian

[Brian Monticello]

Re: Engine acc. pump/ Hesistation

Bruce,
While you have the AFB off, you might want to take a looka at the accelerator pump check valve. If it allows back-flow your total pump shot volume will be reduced.

brian

[Bruce Boatner]

Re: Engine acc. pump/ Hesistation

Brian,

Probably a stupid question, but does the check valve involve a ball bearing? When I rebuilt the carb, I was unable to easily free a ball bearing (cannot remember where is was located exactly. I was not sure what the function of the ball bearing was, let alone if it should move freely, so I just let sleeping dogs lie.

What function does the check valve/ball bearing perform in the AFB? Would a stuck? ball bearing be a likely suspect of my hesitation problem?

Thanks!

Bruce

[Brian Monticello]

Re: Engine acc. pump/ Hesistation

Bruce,
The check valve lets the pump bore fill with gasoline as the diaphragm moves up. AS the accelerator is pressed and the diaphragm moves down, the ball bearing is supposed ot move towards the bowl and ensure the fuel flows to the pump nozzle - not back into the bowl.

If that ball bearing does not move freely there is a good chance that most of your pump shot is squirting back into the bowl.

You can check this by filling the pump bore with fluid (I used water), blocking the nozzle or pump passgae, and pushing the plunger.
You may need to to put your finger over the check valve to feel for flow.

In other words, as soon as the pressure in the pump bore increases, the valve should close. Under normal circumstances, the valve is open.

There is also a check valve under the nozzle. It moves up to allow fuel to squirt but closes so the pump draws in fuel instead of air - may want to check that too.

I attached a picture of an AFB for others - the pump check valve is in the left bowl at the base of the pump bore.

Brian

[Clem Z.]

another thing to check

do you have either a square brass pointed needle or a square brass retangle with a small ball bearing under it below the pump squirter between the front 2 barrels? this is the check valve to prevent the fuel in the pump circut from emptying out between pump shots.

[Frank H.]

Re: Engine acc. pump/ Hesistation

Bruce
The carb with oversized accelerator pump was a napa rebuilt,so they may have them.
I should mention the excessive wear probably means you have dirty fuel/tank condition to deal with at some point or this condition will return.