Car stalling from off idle to throttle - Pontiac GTO Forum
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post #1 of 18 (permalink) Old 08-22-2018, 11:33 PM Thread Starter
 
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Car stalling from off idle to throttle

Hi guys,

New to the forum. Just rebuilt my Pontiac 400 on my 67 goat. I am having a problem when idling at a stop light. When the light turns green and I give it gas, it stalls out and almost dies. If I feather the throttle off the line, it seems to be fine. It mainly happens when the car is cold but still happens a bit when warmed up.

I checked the accelerator pump and it is getting a good squirt at first throttle. Carb was rebuilt recently (did it before the rebuild also). Timing is good and getting spark. It almost feels like the car is losing all power. Dash goes dim and almost shuts down but then comes back to life. It only does it from idle to throttle. If im already driving and give it gas and floor it, its fine.

Any ideas what it can be?

Thanks!
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post #2 of 18 (permalink) Old 08-23-2018, 07:40 AM
 
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If you place the car in neutral, let it idle and then give it gas, does it bog / die? If not, most likely your idle is just set too low.

What is the idle RPM set to? Before going to crazy with diagnosis, I would raise idle RPM, and see if this solves your problem.


When you set idle, how did you do so? Did you maximize vacuum (if so, how much do you have)?

If timing is good then it is most likely a fuel issue - either too rich or too lean. From my limited experience, if the car feels like it bogs as you roll the throttle, then you are running to rich. The best way to tell is to log AFRs and see.

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #3 of 18 (permalink) Old 08-23-2018, 03:25 PM Thread Starter
 
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In neutral, if i rev it up it is fine. I did put a mild cam in on the rebuild. RPMs are around 800 so I will try to bump it up a bit.

Vacuum in Drive at idle is at about 16-17. In Nuetral and Park vacuum is at 20.

I pulled the plugs last night after about 300 miles on the new motor. They look perfect. Not rich or lean.

I will raise the RPM's a bit at idle and see if it helps. Also, going to make sure timing is true at TDC and reset timing just in case.
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post #4 of 18 (permalink) Old 08-23-2018, 04:26 PM
 
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Make sure your choke butterfly is opening fully and not loose or fluttering, since you say it mostly does it when cold, the air on acceleration may be pushing the choke closed and causing a rich condition when your squirter feeds it.

Since i (most of us) only drive in good warm weather and warm it up before hitting the road i have mine locked full open on the dual quads.

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post #5 of 18 (permalink) Old 08-23-2018, 06:51 PM
 
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Originally Posted by Charlie Ashby View Post
In neutral, if i rev it up it is fine. I did put a mild cam in on the rebuild. RPMs are around 800 so I will try to bump it up a bit.

Vacuum in Drive at idle is at about 16-17. In Nuetral and Park vacuum is at 20.

I pulled the plugs last night after about 300 miles on the new motor. They look perfect. Not rich or lean.

I will raise the RPM's a bit at idle and see if it helps. Also, going to make sure timing is true at TDC and reset timing just in case.
Well I think raising the RPM to ~900 / 1000 should do the trick...

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #6 of 18 (permalink) Old 08-26-2018, 12:40 PM Thread Starter
 
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Tried and no luck. Starting think it is something with the ignition. Almost feels like the car loses all power. Lights DIM, etc.
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post #7 of 18 (permalink) Old 08-26-2018, 01:29 PM
 
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Originally Posted by Charlie Ashby View Post
Tried and no luck. Starting think it is something with the ignition. Almost feels like the car loses all power. Lights DIM, etc.
If that were the case, why would it be fine when in neutral and rev'd up ? The only thing having the car in Drive does is load the motor.

How is your vacuum advance configured ?

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #8 of 18 (permalink) Old 08-26-2018, 01:52 PM
 
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Lights might dim as the RPM's fall low when just about ready to stall - somewhat normal.

What gas are you using? Stock compression?

If you have the original balancer, it is possible the outer hub has slipped on the rubber ring and you may be getting a bad reading at the balancer when timing it.

Pull the vacuum line off the vacuum advance and plug it so you are not sucking and air.

Advance your timing to about 14 degrees and try it. If better, go 16 degrees, if better try 18 degrees.

You timing may be retarded or vacuum advance/mechanical weights in the distributor are not working correctly so it dies out on you.

You should have about 10-12 degrees of Initial with vacuum advance disconnected. Then when you hook it up, you should see your Initial jump up. Vacuum advance will add 20 degrees at the crank when fully activated. You have plenty of vacuum, so you should see near 32 degrees on the balancer once you re-connect the vacuum advance while it is idling. If you blip the gas, it will drop because vacuum is being lowered.

You did not say if you have points or electronic? If points, verify the dwell setting of 30 degrees. Points may have gone out of whack.

If electronic, does it require full 12 volts to power it? Most do and the factory distributor wiring has a built in resistor wire that only allows 8-9 volts through. Not enough voltage can have a big effect on the electronic styles if it does not get the full 12 volts.
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post #9 of 18 (permalink) Old 08-29-2018, 12:40 AM Thread Starter
 
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91 octane here in Cali. Stock compression and havnt had any issues with pinging.

I pulled out my dead stop and reset the timing. It was off by 3 degrees on the balancer mark. So I marked the true zero degree mark and then advanced the initial timing to 12 degrees. It seemed to help the issue a lot but it is still slightly there. Not nearly as bad though.

I have the factory Delco distributor with the factory wiring going to it.

I had the timing set at about 9 degrees, which was really 6 degrees. After advancing it to a true 12 degrees, the motor has awaken and it is a beast. Still not pinging, even in this 85 degree heat.
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post #10 of 18 (permalink) Old 08-29-2018, 06:41 AM
 
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Quote:
Originally Posted by Charlie Ashby View Post
91 octane here in Cali. Stock compression and havnt had any issues with pinging.

I pulled out my dead stop and reset the timing. It was off by 3 degrees on the balancer mark. So I marked the true zero degree mark and then advanced the initial timing to 12 degrees. It seemed to help the issue a lot but it is still slightly there. Not nearly as bad though.

I have the factory Delco distributor with the factory wiring going to it.

I had the timing set at about 9 degrees, which was really 6 degrees. After advancing it to a true 12 degrees, the motor has awaken and it is a beast. Still not pinging, even in this 85 degree heat.

Good work. Sounds like you got it. Again, key here is detonation or "pinging" under hard acceleration. Very damaging if you don't correct it, but it sounds like you are OK.

Timing and your distributor advance curve is important in getting these engines to really perform to their maximum.

If you experience any more problems, just let us know and we'll try and sort it out.
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