1968 Pontiac GTO Vehicle is sputtering once outside tempera

Tiny
HAPLOGROUP
  • MEMBER
  • 1968 PONTIAC GTO
Engine Performance problem
1968 Pontiac GTO V8 Two Wheel Drive Automatic

Hello, Ive worked on this car for many years but I am not very good at carburetor problems and I think this is what I have.

This vehicle has a 455 with a 750 Holly double pumper. The car runs great in cooler temperatures but whenever it gets warm outside I have problems. The car starts to stutter at low RPM's. After I get past 3000 or so RPM's and then come to a stop the car just stalls. It starts back up if I have the pedal on the floor and I rev it, but it starts stuttering again. Especially on hot days I can't drive the car because it just starts missing and backfiring etc.

Please any help you can give would be appreciated.

Like I said when its cool out the car runs like a champ.

PS I still have the original engine in my garage, lol!

Thank you.
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Saturday, May 2nd, 2009 AT 1:58 AM

7 Replies

Tiny
MERLIN2021
  • EXPERT
Does it have the original ignition system? Points, or did you install MSD type?
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Saturday, May 2nd, 2009 AT 7:17 AM
Tiny
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I'm sorry, I have electronic ignition. Hy-Fire.

Thanks.

I live in Vegas, so this is a real issue as its hot here all the time now.
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Saturday, May 2nd, 2009 AT 8:26 PM
Tiny
MERLIN2021
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When it starts to sputter, have a look at the choke, is it fully open? If not open it by hand and rev it, does it now run better? Temperature is the key, it may need a new choke thermostat. Also possibly an adjustment.
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Sunday, May 3rd, 2009 AT 7:48 AM
Tiny
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I really appreciate you're help and thank you!

Because of the weather here do I really need the choke? Can I just take it off or disable it? Would that solve the issue if the adjustments on the carburetor are correct?

Thanks.
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Monday, May 4th, 2009 AT 1:53 AM
Tiny
MERLIN2021
  • EXPERT
Choke aids in cold starts, and if all is working opens fully as the engine warms, DO NOT remove it! Choke thermostats are real cheap, and easy to adjust when engine is cold, you just tighten it until there is VERY light tension on the butterfly, in closed position. They do sell a conversion to hand operated choke if that would interest you.
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Tuesday, May 5th, 2009 AT 1:22 PM
Tiny
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Thank you so much and I will be making a donation.

To clarify you think the thermostat in the choke is not working properly and I will try and fix this. Because I am not familiar with how to find the thermostat on the Holly 750 I will look at the manual. Is there anything else other than the thermostat on the carburetor that could be the problem? Could there be other jets on the carberator that need adjustment? Like I said this issue only happens when the temperature gets warm.

Thanks.
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Wednesday, May 6th, 2009 AT 5:12 AM
Tiny
MERLIN2021
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Jets are only cleaned or replaced, with bigger or smaller ones for more or less power, but a can of spray carb cleaner isnt a bad idea either! Spray around in the throat and outside areas, Cleaner the better!
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Wednesday, May 6th, 2009 AT 3:31 PM

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