Backfiring Back Through the Carb

[cgrossl]

I have an 85 SWB Chevy, 2wd with a 305 and a TH350C. Over the last couple of weeks, it has developed a problem with backfiring back up through the carburetor. The engine is original with a Q-jet on it. It has about 135,000 miles on it. The Q-jet has been replaced at some point with Holley reman unit. I rebuilt the reman carb again last winter. It has been fine since then. The backfiring occurs mostly after the engine is really good and warm. It happens as I am squeezing on the gas pedal, like when pulling out or giving it more gas to go up a hill. Sometimes it is pretty bad and sometimes it doesn't do it at all. Timing, plugs, plug wires, etc. are all good. For some reason, I want to say that this is ignition related, but I'm not for sure. I've never seen a carb issue be this intermittent. Any suggestions?

[f.monroe]

I would think that since it is happening worse when it warms up , the plug wires are the problem . You might also check the timing to make sure it isnt too advanced but I bet the wires are the problem . They brake down with heat and start miss firing or cross firing . Frank

[piecesparts]

Have you had a moment where the engine coughed and bucked on you while driving it? I wonder if, at 135,000 miles your timing chain hasn't slipped a tooth or two and thrown your timing off. I have seen it happen and it is confusing when this occurs. The one thing that is a result of this, you may have a bent pushrod on a valve or two. The valves hit the pistons, when they are out of time. Another thing that could be causing it, would be the advance unit in your distributor is either hanging up or a broken spring on it. Those have a tendency to get pretty rusty inside the dist. cap.

[chevyjr73]

is it running bad at idle too? could just be a vacuum leak somewhere. if its running real bad at idle too you could have a valve problem

[cgrossl]

Yes, it is actually running worse at idle. I did check everything for vacuum leaks last year, but I guess that I will be checking again. Keep the ideas coming if anyone else has any.

[chevyjr73]

if its running real bad at idle too sounds like you have a bent or burnt valve.

[cgrossl]

I think that I may have been misunderstood. My truck is running worse at idle than it did before, meaning that it is idling a little too rough. Also, this condition is intermittent. Sometimes, it idles and runs fine. I think that rules out a burnt valve or the timing chain jumping a tooth. I'll check for vacuum leaks sometime this week. Keep the ideas coming though.

[margie's71]

you may want to check the valve springs, try to isolate the cylinder when the truck is acting up , and that will help. regardless of problem

[86 GMG]

could the power valve in the holly have a slight rip or tear in it that allows air or what-ever to pass through sometimes. i really dont know carbs that well in refrence to fixing them but i have had trouble with all types and holly was the worse for me, but know i run a demon (holly type carb) but i dont touch the darn thing all i do is call my buddy over to adjust it if needed.

[BIGCHEVYMAN]

might want to check the cam shaft. 135k is a long life for a small block camshaft in a truck of this era.

[piecesparts]

A lot of the early GM V-8s had problems with a cam lobe rounding off. It leaves that cylinder basically dead and if it is the exhaust lobe, it could cause some release throught the intake when it opens. That is due to the exhaust gases not going out the normal pathway.

[ragity]

Originally Posted by piecesparts

A lot of the early GM V-8s had problems with a cam lobe rounding off. It leaves that cylinder basically dead and if it is the exhaust lobe, it could cause some release throught the intake when it opens. That is due to the exhaust gases not going out the normal pathway.

i was having this problem with a 305 i had in a 79 jimmy and it was a cam lobe gone round on me as well would be the first thing i would check

[f.monroe]

How old are the plug wires ? I have seen this same problem many times and the wires were generally the problem. If you bought them with a lifetime warranty (like at Autozone or something ) I would take them back and try a new set . It is a cheap try . Otherwise remove the valve covers and check the action of the valvetrain . Frank

[magnethead]

Originally Posted by piecesparts

Have you had a moment where the engine coughed and bucked on you while driving it? I wonder if, at 135,000 miles your timing chain hasn't slipped a tooth or two and thrown your timing off.

our truck has 300,000 someodd miles on it with the origional stock engine/tranny. aint wore a cam lobe or jumped a tooth yet.

Originally Posted by f.monroe

How old are the plug wires ? I have seen this same problem many times and the wires were generally the problem. If you bought them with a lifetime warranty (like at Autozone or something ) I would take them back and try a new set . It is a cheap try . Otherwise remove the valve covers and check the action of the valvetrain . Frank

If you check the valvetrain, watch it with the oil. Just bump it around or have somebody with a flywheel rotator spin it around under the truck. Don't actually let it run/idle with the covers off, or 1. you'll have oil everywhere 2. you'll be depriving the parts of oil

[tkeduk8r]

wow... i know that the rounded cam lobe sounds bad, but being an auto teacher and have ing alotof experience with early 80 gm products.. that would be one of the first things i would check... they are infamous for that happening... take your valve covers off one bank at a time and start the engine, if the oil becomes a mess you can by little clips that snap on to the top of the pushrods and rockers to temorarily plug the oil passages... you should be able to see all the valve springs operating the same... if not there is your prollem... you can check for wiring issues by running your engine at nite with the engine compartment completely dark, and look for lighting between wires and your block or wire to wire... if its a timing issue, and youve adjusted NOTHING, then the fact nylon timing gears could have stripped and moved a tooth and need to be replaced... but if your going far enough to change your timing gears and chain, id do the cam swap while your at it too... you wouldnt believe what the right $150 camshaft can help even a 130,000 mile engine... any more questions let me know...
--Mike