Random backfire through carb.
 

I have a 1979 GMC Sierra 15 with a 250 co 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 5he 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 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 replaced. I have not checked the timing yet.

Thank you for any assistance.

Re: Random backfire through carb.
 

79...79... can't remember. Do you have an idle stop solenoid on the carb? You mentioned rebuilding it, I know when I first tossed a new carb on you had to screw that guy in then adjust it both by turning the solenoid against the spring AND with the 1/8" hex head inside the solenoid too.

Before adjusting it, and just checking things for leaks, when that was off on its settings (was waiting for a laser tachometer to get here) it would backfire out the carb after a running a bit. Problem went away after setting the idle setting there to factory spec.

Might have nothing to do with your issue, just tossing it out there.

Re: Random backfire through carb.
 

Yes it does and I do not have it connected yet because the PO did not reconnect the wires to the choke or idle solenoid, but it was running fine till this weekend before the tune up and rebuild

Re: Random backfire through carb.
 

Double check all the suction lines in the fuel system. If the rubber is old, it can develop air leaks which can cause intermittent gas starvation. Change any fuel filters as well.

Re: Random backfire through carb.
 

Thank you for that info . I will look at them this weekend.

Re: Random backfire through carb.
 

Backfire through the carb means fuel is igniting in the intake manifold. That typically means an intake valve was open during combustion.

When that happens I look at timing, to make sure it isn't way off and igniting the fuel with the valve open.

If that is not the case, then I am going to start to suspecting an intake valve as the problem. Could be sticking in the guide and not closing properly, which is what this sounds like.

Re: Random backfire through carb.
 

I was afraid of something like that, I was told the truck sat for a number of years. Would that problem require the head to be rebuilt? I really don't think it is a timing issue because it fires right up and accelerates very well until it back fires, then shut it off and drives good for a short time.

Re: Random backfire through carb.
 

Could valve adjustment help with this problem???

Re: Random backfire through carb.
 

It could be this, or it could be the beginning of an intake valve and/or seat 'burning', and as the valve rotates (which is normal for a running engine) and the burned part of the valve matches up with the burned part of the seat, a pathway for ignited fuel/air mixture to enter the intake manifold explosively occurs.

If this happens too violently, or too many times, it's possible to bend the throttle shaft in the carburetor, causing even more problems.

Because your issue is sporadic, I'd put money on either a valve hanging up (i.e. getting stuck in the guide and not being able to close completely every time) or a burned valve and seat.

Nope, I highly doubt that a valve adjustment would do anything. If you have time, instead of running the valves I'd do a compression test and also a leak-down test.

Re: Random backfire through carb.
 

Thank you for the advice, now I have somewhere to start. I only drove it once like that with the back fire. I had it towed home and have only drove it a mile or so after I did the tune up and carb rebuild.

Re: Random backfire through carb.
 

Totally agree, thought of that and then forgot it.

Re: Random backfire through carb.
 

Most likely lean misfire or timing. Check throttle plate shaft for excess play in the bushings. Grab carb and try to move it around making sure all bolts are tight. Possible vacuum leak. Bad or wrong PCV valve (if you have one). Bad brake booster. Bad vacuum advance. Just a few things to ponder..

Re: Random backfire through carb.
 

Sorry, misread original post and was trying to delete mine.

Re: Random backfire through carb.
 

I already changed the plugs and still same.

Re: Random backfire through carb.
 

Will it idle ok, and backfires only when you try to accelerate? If so, check to see that the accelerator pump is adjusted correctly and with key off work the accelerator while looking down in the carb to confirm a good shot of fuel going in. No accelerator pump shot = violent backfire through the carb. The choke has to be working and correctly adjusted or it will cause this problem until the motor is pretty much warmed up, also.. I'm looking at a pic of the carb and I see a vacuum - operated choke pull-off. That has to be right, too. The backfires could have possibly blown the little rubber diaphram in it and caused it to not work and become a vacuum leak at the same time. All that stuff has to work together correctly to make the total package.
The compression test will tell you if an intake valve is burnt, but I honestly doubt that its the problem as this seemed to crop up after the carb rebuild.. I would definitely check the timing with a good advance light, the dist could have got loose and moved.. Lots of stuff to look at :)

Re: Random backfire through carb.
 

--------------UPDATE---------------

CHECKED:
CARB base - carb was not tightened down enough.
Vacuum - very steady and good vacuum
Choke and idle solenoid rewired (were not connected at all before)
Accelerator pump linkage adjusted and pump shot of fuel is going in
Timing was off - reset to 10 BTDC per emissions label


Have not checked yet:
Sparplug and compression, getting a compression gauge today.

Now I am getting a rapid popping the went up and down with rpm in 2nd and 3rd gear that will cause the engine to shut off, but it does restart easily with one press of the gas pedal. A couple of times it will go away for a minute then come back. I drove it almost a mile around the neighborhood, no faster than 40. The popping noise almost sounded like thumping and the vacuum lines did not come off this time. This only happens while driving, I can not get it to pop while sitting in the driveway.

Thank you all for the help so far.