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Hi all, I am trying to get my late dad’s 1970 GTO back running smoothly but I’m having a few issues. It’s a 1970 with a 400 and a manual transmission. I’ve replaced the plugs, plug wires, fuel pump, fuel filter and distributer. The car started right up and ran great for a few weeks then it started to sputter a bit when accelerating then it moved on to stalling at idle at times when at a red light, etc.. AND also started popping and backfiring at the Carb. This has made me scared to drive it. I don’t want to mess anything up. All you guys who know this car, please help with some suggestions. Timing seems to be okay. It was checked but it wasn’t rechecked after the fuel pump was replaced. I am hoping maybe just a rebuild on the carb will clear it right up but would love to hear what you guys have to say..
Thanks a million in advance.. 🙏🐐

Oh! 1 more thing. The tach goes into redline was too soon. Is this normal for this car?? Or do I just have a bad gauge.. I want to rev it nice and long in each gear cause it sounds amazing but I have to shift so quick between gears because the tach shows it going into the redline so quick. Sorry if this sounds stupid but I’m just a chick trying to keep my dads car looking and running good! CA23165C-A189-4738-A8F6-2D522E4D206D.jpeg
 

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Seeing the car seems to run fine, and stalls out at a light and sputtering, this typically means the carb is flooding on you. If you when it begins to sputter and you put it in neutral and give it some throttle, the engine will most likely pick back up because you are adding more air which helps with the rich fuel condition. The popping/back firing could also be related as unburned gas goes out the pipes.

That is the first check. So may need a carb rebuild. The ethanol gas can deteriorate rubber hoses - at the fuel pump and back at the tank, and the carb float needle which can let gas continue to flow and flood the carb rather than shut the flow down.

You may also have bad gas in the tank. If using ethanol, and the car has been sitting, ethanol gas deteriorates. It will only be good up to a couple months depending on climate. It absorbs moisture. Running something like Stabil additive will extend its life.

Could be a timing issue. If you still have a points distributor (you did not say what you replaced the distributor with), and if the gap/dwell is not set correctly, you might see some problems. So if you have not done this, I'd simply buy a new condensor and set of points. Try to get a good replacement set like original AC Delco if possible. If you swapped over to an electronic distributor, then no points worries.

BUT, my guess is that you did go with an electronic distributor. The original factory analog tach does not work with these. Your tach may read OK at lower RPM's then go crazy as the RPM's go up. Your options are to purchase an aftermarket tach that will work with the electronic distributor, or, send the original tach out to have a compatible circuit board installed in it that will work with the distributor. We just had a forum member go through this and another member let us know he does the electronic circuit board conversions.
 

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Jim..Thanks for the great info! Yes it is an electronic distributor. Like I said earlier, I did have 2-3 weeks of it running just fine when I got it running again. I went thru several tanks of gas over those weeks. I just put premium gas in it with a little stabil. Is that the wrong type of gas? I did everything to try and stop it from stalling including putting it in neutral with throttle.. sometimes it picked back up and sometimes it stalled me out on the road.

Timing could definitely be an issue since we could not really get it exactly where it needed to be when we tried checking it again, although I thought we got it close enough. I did check to see if the carb was delivering fuel and it was but I’m going to have it all rebuilt anyways just to be on the safe side. Could a rebuild on the carb possibly stop the backfiring and popping? It’s done it several times now and this worries me the most. I will only have it trailered if I have to bring it somewhere for any fixes. I don’t want to take any chances with the motor getting damaged. It backfired thru the carb and burned a nice hole in the air filter. But yesterday it backfired thru the back exhaust. And it was LOUD.. But it makes sense that it could be the carb flooding because there’s such a strong smell of gas coming from the exhaust when I backed it into the garage. And I noticed my clothes smelled of gas just from driving it to my moms down the street.

Thanks for the tach info.. I would never have known the Oem doesn’t work with an electronic distributor. This helps me out a ton.. There’s another tach in the garage, maybe my dad knew this already and that’s what that other tach was for. I’ll check it out. (y)

But any more info on the backfiring and popping issue will be much appreciated..
I’m guessing a carb rebuild is first on the list.. I want this car to run perfect.
 

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Yes, agree with the gang, evidence clearly of a rich mixture and your timing must be set correctly. A “backfire” is literally “a fire in the back..”....the back being the hot exhaust, when I burned fuel passes thru the cylinder it travels back to the hot exhaust and “Fires” or bangs or pops,...it is the fuel air explosion that should have happened inside the cylinder.

it can get there because the mixture is too rich, ( as when you described the strong smell of gas)....or it can get there because the spark fires at the wrong time, and does not ignite the mixture, or only partially ignites it, and a quantity of gas passes thru to the exhaust because of the inefficient timing.

so you can have one or the other or both. Overheating can cause problems as well. So rebuild that carb or have it reset at least, double check timing, and definitely do what LATECH said and disconnect that tach. you can just take the wire from the coil for the tach off, tape it off and try it.

you will get it! Keep trying, your Dad would be proud that you were trying to keep his Pontiac on the road!
 

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Jim..Thanks for the great info! Yes it is an electronic distributor. Like I said earlier, I did have 2-3 weeks of it running just fine when I got it running again. I went thru several tanks of gas over those weeks. I just put premium gas in it with a little stabil. Is that the wrong type of gas? I did everything to try and stop it from stalling including putting it in neutral with throttle.. sometimes it picked back up and sometimes it stalled me out on the road.

Timing could definitely be an issue since we could not really get it exactly where it needed to be when we tried checking it again, although I thought we got it close enough. I did check to see if the carb was delivering fuel and it was but I’m going to have it all rebuilt anyways just to be on the safe side. Could a rebuild on the carb possibly stop the backfiring and popping? It’s done it several times now and this worries me the most. I will only have it trailered if I have to bring it somewhere for any fixes. I don’t want to take any chances with the motor getting damaged. It backfired thru the carb and burned a nice hole in the air filter. But yesterday it backfired thru the back exhaust. And it was LOUD.. But it makes sense that it could be the carb flooding because there’s such a strong smell of gas coming from the exhaust when I backed it into the garage. And I noticed my clothes smelled of gas just from driving it to my moms down the street.

Thanks for the tach info.. I would never have known the Oem doesn’t work with an electronic distributor. This helps me out a ton.. There’s another tach in the garage, maybe my dad knew this already and that’s what that other tach was for. I’ll check it out. (y)

But any more info on the backfiring and popping issue will be much appreciated..
I’m guessing a carb rebuild is first on the list.. I want this car to run perfect.
If the engine is original and never rebuilt, it will have high compression. This could be a problem for you as you may get detonation. The premium is what you want and the stabil is a good additive to help keep gas fresh.

The carb probably should be rebuilt after sitting 15 years as my guess is everything has dried out and the needle and seat need replacing. All your symptoms would indicate a carb issue. So I would tackle that first.

You do not want to make too many changes/fixes at once as it will be harder to narrow down what the problem is, so step at a time.

If the carb is still the original Quadrajet, it has a number vertically on the side of the throttle linkage. The number will be 7040263 for the manual 350HP 400CI manual car unless your's is the Ram Air III engine. If you send it out for rebuild, take a couple photos to document the carb to make sure you get the same carb back. Just a CYA precaution so as to make sure you get back the original carb.

There could still be other issues that the car may have had prior to you getting it back up and running again. You did not say how many miles or if the engine has been rebuilt. If original, Pontiac used a nylon coated camshaft gear and the nylon teeth are prone to deterioration and was only good for about 60,000 miles. So a worn/sloppy timing gear/chain can pose a problem in timing the car as it will not remain steady. The play in the gear/chain can be checked, but if at the mileage I stated, I would not hesitate to just replace the timing gears and chain IF you know the engine had not been rebuilt or the gears/timing chain replaced. If unknown, then you can perform a test to check for worn gears/chain stretch.

Backfiring through the carb/exhaust could also be a sticking valve, but for right now, I would focus on the carb and then if that does nor cure the issue, move on to the next round of checks. (y)
 

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Don't buy an over the counter carb kit. The rubber in these cheap kits deteriorates with todays gas and you'll start having problems within a few weeks.
This vendor has one of the better kits.

 
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