# Is it my carb or ignition



## kensoldgoat (Apr 28, 2013)

1969 GTO base model convertible 400/350h with q-jet and automatic. Fresh rebuild on the engine including rebuilt carb and rebuilt distributor. The vacuum line routing and ignition timing have been checked and verified. The car runs terribly then will suddenly run strong then back to running poorly. There is a lot of backfire through the exhaust. I am not sure where to look or focus my efforts. Any help is appreciated.


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## 1968gto421 (Mar 28, 2014)

I would check out initial timing and mechanical advance areas first including springs and weights. In my experience, backfiring is usually ignition although a sudden lean condition could provide the backfire. Greater minds than mine should chime in here.


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## 64GTOConvertible (Aug 28, 2016)

Points or HEI? (I'm a 64-67 guy, so forgive me if points were gone by 69)


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## 1968gto421 (Mar 28, 2014)

64GTOConvertible said:


> Points or HEI? (I'm a 64-67 guy, so forgive me if points were gone by 69)


Should be points as HEI was introduced on the '74 models.


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## PontiacJim (Dec 29, 2012)

"There is a lot of backfire through the exhaust."

I would look for an ignition problem. If gas vapors build up from not being fired while in the cylinders, the raw fuel mixture will enter the exhaust pipes and when another cylinder fires, will cause the back firing due to igniting the mixture while in the pipes. Used to run along in the car while foot still on the gas, shut off the ignition, then turn it back on to cause the loud backfire. Have heard of guys doing this and blowing off the muffler, especially a rusty/weak one.

Make sure you have all the necessary grounds on your engine as a bad ground could be the cause. I don't recall exactly what it was all these years past, but I recall an instance where there was an internal short/grounding taking place within the distributor, so check all the wires inside it.

Then next it is possible to be a lean condition (or even a flooding condition dumping too much raw fuel into the engine). The fuel pump could be going bad, the gas tank pick-up line has a "sock" on the end to act as a filter and this can get plugged or collapse, fuel filter, rubber fuel lines crack & split or collapse internally and can shut off fuel flow (make sure all rubber lines, front & rear, are replaced with modern ethanol friendly fuel line), the float could be set wrong or needle/seat sticking. Don't assume just because you have a rebuilt carb - that's no guarantee that it isn't a carb issue. 

So, many things to look into, but you have to do a trial and error approach, one step at a time, to eliminate what it isn't to find out what it is. :thumbsup:


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## geeteeohguy (Feb 2, 2008)

Make sure your ignition coil is not hooked up backwards. Make sure your condenser is good. Make sure your points are set at 30 degrees dwell, and the black wire that goes to the coil from the points is not chaffed. Make sure all your electrical connections at the igniton, distributor, engine grounds, and bulkhead connectors are good. IF all ok, on to the fuel system, as mentioned above. The sock thing happened to me on a cross country trip in the '80's with my '67. Talk about miserable................


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## 64GTOConvertible (Aug 28, 2016)

Most common areas have been addressed above. My issue is it comes and goes, which could mean just about anything. You gotta check the wires to the ignition switch and to the distributor. I'm thinking you're gonna find something chafed in there somewhere. 

That said, something floating in the gas could cause temporary blockages as well. Had that happen in my race car on the Great Race in 2003. Car would randomly just run bad. It finally quit on us in Jacksonville on the way home to NE PA. Fuel line was plugged. We blew it out from the top with compressed air and drove it home. Never went bad again. But you know whatever was floating around in the tank is still there!


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