Suddenly, it's backfiring! Ideas?

On my way home from work yesterday about 2 miles from home, my car started to intermittedly backfire. I popped the hood and found no arching. This morning, I jumped on the freeway only to take it back home as the backfiring continued. Any ideas what it could be? The car has been running fine, the tune-up is fairly recent. I have an exhaust leak somewhere at the manifold if that means anything. Look at my signature for specifics on my set-up. Thanks in advance for your thoughts.

Distributor hold down loose?

I would think fuel/air problem, vacuum leak maybe.

Backfires when and where? Out the tailpipe when decelerating, or out the carb when accelerating?
One man's backfire could be another mans cough

It backfires under the hood.

I'll check a few of these things mentioned and report back.

drop a lobe on the cam? look at the rockers n see if 1 aint moveing much
doz it skip at idle?

Junk-Falcon":3jr8g8ca said:

drop a lobe on the cam? look at the rockers n see if 1 aint moveing much
doz it skip at idle?

Click to expand...

What he said... take the valve cover off and run it at idle. If you have a flat cam or busted valve spring the rocker won't move as much as the others. Mine was popping back through the intake out the carb. I could actually pull on on the valve spring retainer and make mine quit popping.

It seems to only (intermittently) do it under load. Today I drove the 22 miles to work with it again. The farther I got, the more it backfired. It even kept blowing my manual choke closed. After each back fire, I'd look down at the choke knob and it would be pulled out again.
I did check the distributor cap & Pertronix and all appears well. SInce it doesn't seem to be anything obvious, you might be right about the valves. That would really suck since I had the head completely rebuilt less than 5k ago. I do have a complete rebuilt valve assembly at home.

I would pull the valve cover and look for a broke valve spring or flat lobe on the cam.

If it still had points, I would suspect them. I don't think a Pertronix would cause it.

I don't know how old your cam chain is but one of the thing that can go wrong is the cam chain strechs as it ages. This retards the cam. When the strech get long enough it will start to back fire. Pop you distibutor cap and rotatate the crank back and forth to see how much play you have in the system. Do this both cold and warm.

I've had a sticky lifter for the past year; goes away after it warms. Hmmm.

I'll do a few more of the mentioned test/inspections. I'm having it towed home tonight when I'm through with work as I don't want to cause any more damage than I may have already. Thanks for replies.

Moisture in the distributor cap
dirt build up on the coil high voltage tower
carbon tracking in cap or on coil tower
wires cross firing because they are routed too close to each other
very lean condition
burned valve
bad cam / rocker / spring / timing chain.
bad ignition module

I think that sums up the possible causes in order of likelyhood.

Just on a wild hunch, check your choke set up. Make sure you choke plate and mechanisms aren't hanging up on anything like the accelerator linkages. I've just never see a backfire through a carb actually close the choke so hard that it pushs the handle out. I think something is getting caught while you are accelerating and is gradually closing the choke plate.

Well, I towed it home last night, all 22 miles as the driver explained to me how he used to get away with driving "high" back in the 80s. Wow, that was a long ride.

After I got it home, I pulled it into the garage and didn't touch it. I should have some time this weekend to get at it. I'm going to print this thread out and go down the list of everyone's thoughts, easiest stuff first. I'll report back.

Will be waiting.

I have had problems with a flat cam or worn out cam to do that. A worn or bad timing chain will do the same thing of which it is throwing it out of time. Most of the ideas are worth looking into. Check the gap between the petronics and make sure it has not moved.. Good luck

well? what'd you find?

and how would someone go about checking the timing chain? do you have to remove the cover?

There is a way to check it without taking off the cover. put a breaker bar on the nut on the crank. Take the valve cover off and rock the breaker bar back and forth. Watch the rocker arms to see how much you have to move it to have the rockers move. If it is quite a bit then your chain is more than likely bad, then you have to take everything on the front off and replace it.

I lifted the valve cover off and found nothing out of the ordinary, in fact, it was super clean and “tightâ€￾ looking. Distributor (pertronix) looks good as well. It's got to be something easy...

How far did the bar move when you rocked the bar back and forth less the socket moving? Try and turn the engine through and see how much the valve operate. That will check the cam. You have to check each one. If you have very little movement on one or more then the cam is probably flat on one or more lobes. If the timing chain is tight and the cam is moving all lifters properly then you can check your fuel for water. Water sometimes will cause the carb to pop when the water goes through it. To check if there might be water in the carb pop the top of the carb and look inside for dirt, water or anything that shouldn't be there. Try these and make sure none of your plug wires are not grounding out against something. The last thing I can think of is the petronics. Put a set of points back into the dist. and try it again and run the engine to see if that might have been a problem. Good luck.