Random carb backfire

[Richardy]

I have a 1979 GMC Sierra 15 with a 250 ci 6 cyl.with a Rochester varajet ii. I have had this truck for three weeks now and have driven it twice to work which is over and hour away one way with no problem, as well as driving it locally almost daily during this time. This past weekend I drove it to work, an hour away, I ride a bus during the week, without any problems but halfway back it started backfiring through the carburetor so bad it blew off the vacuum lines. I attempted to drive it another 5 miles down the interstate while having to stop every 2 to 3 mies to replace the vacuum lines, in between it would run fine then backfire. I have replaced the spark plugs, wires, rotor, distributor cap, as well as completely rebuilding the carb and replacing the floats and making sure they were at the correct height. This is not a constant backfire , it only happens after driving a few minutes then I stop , shut it off , check everything and when it starts back up it runs fine for a few miles, highways speeds or in town after replacing everything. I have not had a chance to verify the timing but it fires right up and runs really strong until the backfire, then shut off, fire up and runs s good for another short distance. Also the choke and idle solenoid are not connected because the P.O. either cut them off or moved them around because I have not found the idle solenoid pigtail , the choke wire was melted on the exhaust pipe.

I have searched the forums and could not really find anything like this. Please help I am completely lost and looking for a direction to go from here. My internet searches have stated to check everything I have already replaced.

Thank you for any assistance.

 

[Charlie]

 

[Richardy]

Thank you

 

[Richardy]

Could valve adjustment help this problem?

 

[Charlie]

I would check timing first since you stated you haven't checked it. You stated you replaced dist cap. Did you replace rotor and points or do you have an HEI? With points, you should adjust dwell too.

 

[Richardy]

I have an HEI on it. And the rotor was replaced. I have also been told that it may be a burnt intake valve.

 

[Snoots]

Pull the plugs and give each one a look. One of them will likely look worse than the others. I wouldn't think it's a valve because that is apt to be more regular than the occasional backfire. But what do I know?
I'd do the plug thing and try to narrow it down.

 

[Richardy]

Sounds like a good place to start . Would it be an idea to check compression?

 

[1987 GMC Jimmy]

Maybe on the bank that you find an unhealthy plug on to see if a valve is bad, but it won’t be necessary for the whole thing unless multiple plugs look sick. I’d also check for a stable timing mark when you’re doing the timing to make sure the chain isn’t stretched. It may just be running really lean either because of a too far advanced timing or a fuel mixture issue.

 

[Richardy]

Thank you I will add that to my to do list this weekend.

 

[Richardy]

The inline 6 250ci does not have a timing chain, it is gear driven. Just found out looking for a timing chain, lol. So back to checking plugs this weekend.

 

[1987 GMC Jimmy]

Oh, whoops. Totally skated over what engine it had. I’m sorry there, man.

 

[Richardy]

No problem.

 

[HotRodPC]

Pull the plugs and give each one a look. One of them will likely look worse than the others. I wouldn't think it's a valve because that is apt to be more regular than the occasional backfire. But what do I know?
I'd do the plug thing and try to narrow it down.

I'm with Snoots here. When I first started reading, bent push rod, broken rocker, flat cam lobe or something like that. Being it's intermittent that can't be it. How long has it been since you replaced the distributor cap? Sounds crazy, but you have a crazy problem so play along for a minute. As that button on the top of the cap wears which is carbon, the dust stays in the cap. A little sweat or moisture in the cap, most of the dust has piled up in one area or even sticking to the cap, you can have arc jumping all over the damn place in the cap like a lightening storm imitating a timing problem. You could also have a cracked dizzy cap and the arc will follow the crack intermittently. So I'd just pop the cap off taking a look. Look close for cracks. While there, take a screwdriver and knock the big chunks of corrosion off the terminal posts. Spraying it out with WD-40 wont hurt it either if you want to use that for cleaning it out. Don't need to be perfectly dry to put it back on. Spark will fire just fine right through WD-40 as if it wasn't even there.

 

[Snoots]

Sounds like a good place to start . Would it be an idea to check compression?

You're quite right. If you are going to pull the plugs you might as well.
Get a piece of cardboard and punch some holes in it to hold the plugs.
Keep them in order and write down what you find (including compression) for each cylinder.