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1968/72 Pontiac Firebird belched fire and died

I ask this question on behalf of my father who is to stubborn to ask any one. So my dad has a “68” firebird with a 400 motor. however it is more of a 72 since the motor, trany, etc, is from the 72. I would like to fix it for him so I can move it and park my Daytona where it currently is. There are two major problems as far as I can tell.

Problem 1: it does not start. You turn the key and get a click but nothing else. I have changed the starter, check the battery and connections, and even looked at the Neutral safety switch. But I can’t get it to turn over. It will start up sometimes out of the blue but there is no pattern to when it works.

Problem 2: this is the most recent problem. So my dad had it at storage and after repeated attempts all day we finally got it running. So we pull it out and start driving. It was running rough. It sounded like it was struggling to stay running. Then we got it up to 45 mph and all hell broke loose. It started backfiring, dying out, and final came a loud boom. I was looking out the window when the big boom came and I could see fire burst out from under the car. We pull over, leave it running (my dad didn’t want to shut it off because he didn’t know if he could start it again) and while was still running horrible it was not on fire. We attempted to limp it back to storage but it finally died and we needed a tow. My dad though it was the carburetor. That maybe true but I just need a second opinion.

The car is a bit of a mess so these could be two separate problems or possibly related. If anyone has any ideas or may know the problem, I would greatly appreciate your advice.
Thanks,
Mark I

The starting problem sounds a bad connection somewhere.

The back firing, rough running might be caused by a jumped timing chain.

Tester

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I can imagine how cobbled up the battery cable ends are so like @Tester my first/best guess is bad connections. But on the bad running I’ll guess that the plug wires are either in terrible condition or they are crossed. Is the engine a Pontiac 6.6L?

How old is the gas in the tank?

It’s is the 6.6L motor. The connections look ok on the battery and the lights and that still work, I’m fairly certain it’s not them but I can double check. The motor was changed when we got the car and the wiring look like it was done by a college kid after a couple shots. It’s a bit of a mess in there.

The gas in the tank during the great explosion was about 4 months olds but we did put about two gallons of new gas in it when we first started it.

My dad was convinced it was the carburetor. But I’m for some reason doubtful. The timing chain does sound a bit more likely to me.

If the engine has not been re-built but is still original, the timing gears were plastic and tended to strip at around 50K miles. Yours is probably brittle as glass so one run and done!

There may still be a carb problem. sunk float, over-rich, stuck float belching fuel out the vent.

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My dad has had the car for about 25 years now and he has taken it well passed 50k in millage. So I will probably check the carb since it the easiest, but the timing gears are starting to sound like the problem.

To check for a jumped/stretched timing chain, mark where #1 tower on the distributor cap is on the distributor, and remove the distributor cap.

Turn the engine over by hand at the crank bolt until #1 cylinder is at TDC.

Does the rotor in the distributor point anywhere near the mark you made?

If not, the chain jumped time.

Now the turn the engine over by hand in the opposite direction.

If the crank can be turned 5 degrees or more in the opposite direction before the rotor in the distributor starts to rotate, that’s why the chain jumped time.

Tester

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That’s a good part, but not all of your problem. Putting two gallons of fresh gas on top of who knows how many gallons of varnish produces more gallons of varnish. Plus I agree with the others about timing chain issues.

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Thanks I’ll give that a try. I’ll also start with a fresh tank of gas. Now I just need to solve the starting problem.

If your timing chain is sufficiently out of time, you risk pistons slamming into the valves if you even try to start it.

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I’ll keep that in mind and check before I start it. I think I’ve narrowed the starting problem to the ignition switch I tried bypassing it and it seemed to crank.

If you simply cranked it you could have bent some valves.

For the fails to crank, measure the voltage at both electrical terminals at the starter motor, probing terminal to starter case, during attempted cranking (key in start). Both should measure at least 10.5 volts. What do you get?

The backfiring and general ornery behavior sounds like either a big vacuum leak has developed, or a carburetor problem. Be sure to check the condition of the pcv and egr valves.

Ok I’ll check the EGR and the PCV. I’ll also double check the connections.

A couple of years ago my similar era Ford truck started running really poorly. It happened all of a sudden. Rebuilding the carburetor solved all the issues.

I got carb out and I’m going to rebuild it a hope that’s it.

Take a close look at the linkage before beginning to dismantle the carburetor. In fact it would be worth the effort to put some marks on the linkage pieces and take a good picture as reference.

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