[dhcarguy]

my car is a 65 GTO with tripower, 4speed, Lunati 230dur./460lift cam, stock#77 heads with heavy springs and HS roller rockers , it's a 66 389 block, .30 over. Pertronix coil and module in stock dist. (not sure you need all that info).
The problem is,The car starts up as always and runs great until after about 25-30 minutes when good and hot (180- 190 degrees) . then it starts to stumble. I first thought it was fuel related. I changed the 7 yr. old fuel filter. Next day took for a ride and it still did the same thing. I then replaced the 12 yr. old ignitor II coil. Took for a ride , same problem. Now I am thinking on changing the module in the dist. (which is above my paygrade lol). when in my driveway after this happens ,I can tach up the engine to 5000rpm's without a hic up.
What could be my problem??

[johnta1]

I'd see if you have an accelerator shot when it happens again.
(pull over, turn engine off and take air cleaner off)

Not sure how easy it would be to check float level when it happens but could be low.
Fuel percolation may be a problem also.

Does it stumble continuously or just once?
When cruising? Taking off?

Might even try advancing the timing a couple degrees to see what it does?

Another hard to diagnose case over internet.

[punkin]

I had something similar a number of years ago...when warm and under load it would mis-fire. After many different attempts, the final solution was the spark plug wires.

Once the car was warmed up and acting up, I pulled it into the garage, turned the lights off, opened the hood and had someone stand on the brakes and give it a bit of gas (in drive) and when under load I saw some little dancing pixies arching off the plug wires.

If you're out of ideas...this might be something worth considering.


Good luck to you.

[Jim Scites]

You can also check plug wires by turning the lights off and with a spray bottle mist some water over the plug wires. You should see fireworks if the wires are bad.

[Navy Horn 16]

Disconnect (and plug) your vacuum advance and see how it acts.

[JC455]

*IF* it is the module, there's a Pertronix module that ends in LS- Lobe Sensing- that you might consider. It uses the old points lobes to get its signal, so there are no magnets to adjust, no air gaps to play with, and no bolt-on wheels hanging from your dist. shaft.
Just an FYI.

[Schurkey]

Originally Posted by dhcarguy

The car starts up as always and runs great until after about 25-30 minutes when good and hot (180- 190 degrees) . then it starts to stumble. I first thought it was fuel related. I changed the 7 yr. old fuel filter. Next day took for a ride and it still did the same thing. I then replaced the 12 yr. old ignitor II coil. Took for a ride , same problem. Now I am thinking on changing the module in the dist. (which is above my paygrade lol). when in my driveway after this happens ,I can tach up the engine to 5000rpm's without a hic up.
What could be my problem??

STOP REPLACING PARTS.

Diagnosis first, parts replacement later. This could be any one of dozens of possibilities. You've gotten mostly good advice so far. Check everything already suggested, plus:

What is your fuel pressure fully-warm?
Will your ignition system fire a spark-tester when fully warm? If your ignition system uses a ballast resistor, you need a spark tester for "points" ignition. If there's no ballast resistor or resistor wire, you need a spark tester calibrated for "HEI" ignitions.
Is your heat riser valve open?
Is the temp gauge correct? (i.e., is the engine really running 180--190?)
Is the fuel tank properly vented? Any change if you run with the fuel fill cap loose or off?




I wouldn't walk across the street for a free "Pertronix" kit; but that's a personal bias. I feel approximately the same about Mallory Unilites. Give me HEI every time.

[Gary H]

I'd lean toward a fuel vapor lock problem. Try non ethanol gas if it's available in a high enough octane in your area. If not high enough, mix in a little race gas to raise it. Process of elimination, but I've run into the same thing with several cars and todays ethanol gas.

[Old Man Taylor]

I also suspect fuel vapor lock. The ethanol gas makes that situation much worse. I found that out the hard way on a Car Craft cruise into our local mountains.

[FrankieT/A]

Worn spark plugs? Gap too wide?

[Greg Reid]

The trouble with diagnosing this kind of problem is that fuel problems and ignition problems both show up when things get hot. You have to find out which one it is.
The last time I had an issue like this, I did just what John suggested... I drove it up and down my street until it started the symptoms...as soon as it started to stumble, I pulled over and quick as a could pulled the breather lid off and gave it a couple of shots. There was no fuel. It only happened when it got hot.
Bottom line is, find out for sure if it's ignition related or fuel related. Then you can isolate it.

[dhcarguy]

Thanks everyone, looks like I will be busy next week. I will do the easiest stuff first. I don't think it is ethanol, because I use 1 gal. of 112 Sunoco to 3 gals. of 93 with ethanol treatment. Next I will pull the plugs, check and gap(.40). Do the spark plug wire test. I 'll go on from there.
This is going to take me awhile, I did pull the carbs and rebuilt them about 4 years ago. I can at least do that myself.

Thanks again,Dave

[steve25]

Things you can Bank on are that a cold motor requires much much more fuel to run properly then one at even only 160 degrees, as the air to fuel ratios are close to nearly reversed!

Even though it may stumble under load if it does not mis under load above 4000 rpm then I do not think it's ingition related.

I would think that as posted a stumble or flat spot in the throttle when hot is avapor lock condition, or something in the that middle Carb like the power piston , or the power valve itself hanging up when the Carb heats up and expands and changes shape to some degree!

One good throuble shooting aid you have is that it's got 3 carbs, so what you can do when is hot is to take some mechanicics wire and tie up the throttle linkage so that it's not progressive any more ( make it 1 to 1) if then when you drive the car the issue changes for the better then you have pinned down that it's something going on in that center Carb for sure!

[Bluesugar]

Where do you find "112 Sunoco"!!...

[mgarblik]

Originally Posted by Navy Horn 16

Disconnect (and plug) your vacuum advance and see how it acts.

I think this is a good diagnostic idea to try. Don't want to put words in Navy Horn's mouth because he didn't elaborate. But what this does is: 1. Helps isolate a pick-up coil wiring problem by eliminating the movement of the plate it attaches to. As the coil ages, the two wires can get tiny breaks in the conductor from being pulled and stretched for 50+ years. If the problem goes away, it needs a new pick-up coil.

[steve25]

Yes it is a good simple and good thing to try, but he is not running a HEI set up that fails those dam small pickup coil wires, just sayin!

[dhcarguy]

Originally Posted by Bluesugar

Where do you find "112 Sunoco"!!...

There is a heating and cooling business near me that has I think is 98 unleaded. They also have 112 leaded. The owners are into racing.

[dhcarguy]

Originally Posted by steve25

Things you can Bank on are that a cold motor requires much much more fuel to run properly then one at even only 160 degrees, as the air to fuel ratios are close to nearly reversed!

Even though it may stumble under load if it does not mis under load above 4000 rpm then I do not think it's ingition related.

I would think that as posted a stumble or flat spot in the throttle when hot is avapor lock condition, or something in the that middle Carb like the power piston , or the power valve itself hanging up when the Carb heats up and expands and changes shape to some degree!

One good throuble shooting aid you have is that it's got 3 carbs, so what you can do when is hot is to take some mechanicics wire and tie up the throttle linkage so that it's not progressive any more ( make it 1 to 1) if then when you drive the car the issue changes for the better then you have pinned down that it's something going on in that center Carb for sure!

Thanks Steve,
I now am leaning to something in the center carb. I just may pull them all and just run on my spare center carb to see for sure. I made block off plates for the ends years ago.
I’m in no hurry. But will post results

[Schurkey]

Originally Posted by mgarblik

If the problem goes away, it needs a new pick-up coil.

...or the tune has way too much advance at part-throttle.

[amcmike]

Disconnect and plug the vacuum advance @ the carb. Check the initial timing, and the total timing (with centrifugal). If all that looks correct, try driving it without the vacuum advance.