The Stovebolt Site Forums The Shop Area Engine Shop Question about Engine backfiring

I have a 1950 Chevy pickup with a 350 engine / 4 barrel Carburetor / HEI distributor in it. The truck is not a daily driver. Lately when I start it, it’s backfiring through the exhaust. This is a random backfire and as the truck begins to warm up, it backfires less. It has headers on it and I have been told that if these leak they will cause it to backfire. I’ve made sure that the spark plug wires are not crossed. Any suggestions as to where I should start looking for a problem?

Thanks for any help,

Bobby

Does it also happen on decceleration?
Scott

Hi Scott,

Thanks for the reply.

No, not really (occasionally it has). It's mostly on accelerating or holding steady on the gas. The more RPM's the louder the backfire.

Check the valves on the passenger's side of the engine to see if you've got an exhaust valve on #4 or #6 that's only opening partway. There was a real problem with some engines in the late 70's to mid-80's wearing out exhaust valve lobes on the cam, and it almost always happened near the EGR valve passages. Backfiring was always one of the early symptoms of a flat cam. Leaky header flanges will create a problem also. It would be hard to run fast enough to give me a set of headers, much less make me spend my hard-earned money on junk like that!
Jerry

Yeah they had bad cam's back then,I replaced 3 of them on the same motor before I got a good one!!

Hi Jerry,

Thanks, I kind of suspected the cam, I was hoping for some better news . I totally agree with you about the headers. I found this truck sitting on the side of the road, with all the windows knocked out, fenders beat in (someone had been very mad) so I got a good deal on it as is. The headers were already on it. Now I'm trying to get it going as cheaply as possible.

Thank you again for your answer.

Merry Christmas,

The problem is actually with the exhaust valve guides that are close to the EGR passages in the cylinder head. The oils that were used during that time had a nasty habit of "coking" (forming hard carbon deposits) in a hot spot, and the EGR passages definitely got hot. I've actually had to drive pushrods out of a buildup of hard carbon in that area. The drag on the valve stems and pushrods from the carbon deposits would wear out the cam lobes.

When General Motors announced in the mid-1980's they would not honor the warranty on any engine that ran 10W-40 oil, they never would admit that one particular brand of oil was causing problems. However, unofficially, the initials Q-S kept cropping up in off-the-record discussions with the engine instructors at the GM regional training center in Memphis, Tennessee!

Switching to Valvoline or Kendall oil after replacing the cam and lifters solved a lot of problems.
Jerry

Most likely is the valves, but have you changed, and or gapped, the plugs lately? I had the same thing happen on my 292 I-6, and after putting new plugs in, it stopped backfiring completely. Sometimes best to start with the simple (cheap) stuff, and work your way up to the more expensive things!

+1

What Carb are you running? check to see if the choke is on or off. Next check plug wires, coould have burned one up with those rotten, no good, headers. (I like headers personally) Need to buy a good set of wires and plug wire boots. Check your timing.
When you broke in the cam did you follow break in instructions?

Is this a Comp Cam by any chance?

I have new Accel high temp plug wires and header plugs. 4barrel Q.jet carb. I think this is all o.k. I am still leaning toward the cam being the problem. I have driven this truck for awhile
and this problem has just started.I also wonder if the lifters
may be part of the problem.This thing sounds just like when you turn your key off going down the road and turn it back on.

One other possibility- - - - -pull the cap off the distributor and inspect the inside with a bright flashlight and a magnifying glass if possible. If you see something that looks like a very faint pencil line between a couple of terminals, you've found the problem. It's called a "carbon track", caused by the spark following condensed moisture inside the cap. Once the spark jumps a few times using the moisture as a conductor, a permanent low-resistance path gets established and replacing the cap is the only way to solve the problem. It's worth a look-see before more in-depth diagnosis happens.
Jerry

after you change the plugs before you start engine remove one of the plug wires and see if it runs different. keep doing this until you track down the missing cyl. you can also see which header tube is cooler than the rest,less fire less heat.

Thanks for all your suggestions and advice.

I replaced the distributor cap and rotor, it didn't stop the backfire so I'm going to try some of the other suggestions when the weather warms up here a little bit.

I'm still leaning toward a cam problem but it's just too cold right now for me to investigate that further.

So, thanks again for all the responses/advice, I meant to reply sooner but life got in the way.

Ok,so I finally got back to the truck. The problem was the plug wires. These were new wires so I really didnt even consider that to be the problem. Thanks for all your help. Bobby