need help get my stroker to achieve its potential - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 06-10-2017, 11:10 PM Thread Starter
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need help get my stroker to achieve its potential

I have a 67 GTO, owned since '79, had engine rebuilt in 2013. After many discussions, I decided to go with a stroker kit from Tin Indian. Had the engine professionally built in Wisconsin where it was at the time and went with KRE Heads and Edlebrock hi rise and the original quadrajet. Car ran fine overall but the quadrajet was unable to feed the engine and it would starve out at 4,000 RPMS.
I moved to Colorado in 2015 and was determined to solve the problem so I could fully enjoy my car again. The first I did was to replace the carb with a Holley 950 DF and remove the sock in the tank. It ran ok at first but has ongoing bouts with sputtering and backfiring and dying, especially in hot weather. We figured it was due to the lack of sufficient fuel pressure so the following work was done: went from 3/8" to 1/2" fuel line tank to pump and pump to carb, replaced fuel sending unit with 1/2" RoBBmc unit along with RobbMc mechanical Fuel pump for 550 HP Pontiac, added a 100 micron fuel filter at the tank. During all of this, I had problems with my Pertronix distributor so replaced it with Proform HEI distributor.
The car now should be working fine by all accounts and opinions. But I had it out last weekend for a car show and early AM ran fine, except acted up twice briefly and stopped. On the way home in the afternoon, it acted up all the way home and almost did not get it there....sputtering, backfiring, hard to keep running at low/mid rpms. Acts like it can't get gas. I have no confidence to take it anywhere, am at a total wits end as to what could be the problem.
I welcome any thoughts and suggestions as to what to do and in the meantime its going to stay in the garage until I can solve this or find a speed shop to give it to as we have tried everything.
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post #2 of 4 (permalink) Old 06-11-2017, 11:13 AM
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With all that you have done, it does not sound like a fuel delivery problem, but still could be. I assume you have checked your timing. Have heard where the timing has moved and that's all the problem it was.

First check is of course the fuel cap/fuel tank. Your car should have come from the factory with the fuel tank with a vent tube that is visible in the trunk. The cap should be the un-vented cap when using the vented tank. Has the tank ever been changed out? Could be the wrong tank, ie a non-vented tank with the original non-vented gas cap which is causing a vacuum and not allowing gas to flow. Just for fun, when it happens again, pull over, loosen the gas cap or take it off so you don't lose it and drive it to see if the problem stops. Don't do this will a near full tank of gas and don't accelerate hard as the gas will dump, just drive it easy to see if anything changes. Could be that simple.

Do you have a fuel pressure gauge mounted that might let you know that the fuel pressure is dropping or going away when this problem happens? Might be a good investment to use in diagnosing a fuel delivery problem. You can remove it later if you don't want to keep it.

Do you have a return line going back to the tank? You could have fuel percolation problem which can be a real problem with the alcohol in the fuel and when the car is hot. Adding one could be helpful. Factory AC cars had these installed. I see RobbMc offers this as PN 1010 Vapor Return Option on the 550HP pump.

A phenolic spacer under the carb could also help as it acts as a heat insulator if the carb is getting too hot and the fuel is boiling inside.

Does your distributor have a 12volt power wire? The factory used a resistor wire for the original points type ignition. Using the factory wiring will not deliver the 12volts your electronic distributor needs and it will operate poorly and exhibit problems as you describe. You might want to make up a length of wire with alligator clips on the end of them that will reach from the Positive + side of the battery to the Positive + side of the coil. When it starts acting up, pull over and attach your test wire. CAUTION: the car will not turn off using the key with this wire attached, so you have to pull it off in order for the engine to stop. If the car runs better and problems go away when you do this, it is a power supply to the coil. Could even be a bad ignition key switch, broken wire, bad ground, or something shorting out if a wire has rubbed through. I always suggest a battery disconnect switch at the battery if you have an electrical short and wires begin melting/smoking and a good fire exstinguisher that is handy just in case with these older cars.

Have you checked the condition of the battery/alternator/voltage regulator? Low voltage can cause all kinds of problems. Make sure your battery is good and the alt. is putting out the needed volts. Local auto parts stores can usually check this if you can't. Also, if you added things to the car like big amp stereo's, electric fans, or other power drawing opetions, the factory rated amperage of the alternator may not be enough. A local guy was having this problem. Charging system checked out fine as did his battery. But his battery would go dead after crusin' for the night with lights on and the big stereo system. AMP ratings were too low for all the accessories he had going on with all his lights running at night.

Check all wires to the distib. & coil. Is the coil matched to the distrib.? Often, a hotter type coil is needed to work with the electronic ignition. These are usually recommended by the distrib. maker. Run the car in the dark at night and look for any electrical arcing or grounding.

Have you checked the advance curve of the distributor? This is very important. Could be that something is not operating as it should or is binding/sticking. Do you have a vacuum advance on the distributor?

Just a few suggestions to get you going and in a direction of narrowing down the problem. Right now it could be several things from carb, fuel, ignition, wiring, valve adjustment, etc.. So hard to figure out at this point without more input.
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post #3 of 4 (permalink) Old 06-11-2017, 11:44 AM Thread Starter
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Thank you for taking the time to reply back.
Answers to your questions:
-My car has the original tank with the vent in the trunk and non vented cap
-I put a liquid gauge on the fuel rail and it shows 8# at startup and goes to 4-5 after warm up, just don't know when on the road
-No return line back to tank at this time, but open to trying it
-I have the spacer under the carb
-When I had problems with the Pertronix, we hard wired 12 volts straight to the distributor as we had drops to 9 volts. This hard wire is in place now for the Proform and have the kill switch under the dash.
-During all of this, I also replaced battery and voltage regulator so all good there
-The new HEI has its own coil and I replaced all plug wires to match
-We had set the timing, but vacuum questions don't have answer for.

Do you think a fuel regulator should be inline ahead of the fuel rail? If I had problems at full throttle is would point to fuel problem, but seems to happen randomly at low/mid range so just does not seem right. Maybe I try the return line and use the 3/8" line I left in place?

Thanks again for taking the time to give your input PontiacJim.

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post #4 of 4 (permalink) Old 06-11-2017, 04:12 PM
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May seem strange but after going through a total overhaul of the ignition system, plugs wires point cap rotor etc. I accidentally found my problem. My hand leaned on the throttle linkage making the car run smoothly again. The octane boost I was using caused my throttle body to stick open. I cleaned it all up and use 100 octane instead now. Problem solved. Sometimes it's simpler than you think.
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