The Stovebolt Site Forums The Shop Area Engine Shop Backfire Issue?

I've searched on here, and found many articles about timing, vacuum, etc about similar issues, including a very specific one about popping sounds from the engine on a 235 (same as mine), but none are quite exactly what I am hearing and seeing (I don't think). When the engine is cold, or under load at all, it runs smoothly. Once it is warmed up, or even close to normal op temp, I am getting a popping sound that I can hear both at the carb/manifold area and in the tailpipe. Almost like a backfire, but not nearly as loud as I have ever heard in any other vehicles. Is it just a baby backfire? It's every couple of seconds irregularly. It only happens at idle, and usually only after I have slowed down. If I put the clutch in at speed for example, it will not do it until I have almost come to a stop, which seems odd to me. When it happens, I have sometimes seen a little smoke around the intake. The heat riser valve jumps as well, but since I can't make the noise happen by fiddling with it, I assume that is a symptom and not the cause. Any suggestions on this? Or should I just go through the normal timing and vacuum checks and adjustments and hope it goes away? This is a '59 3/4 ton with a 235 BTW.

Worn bushings in the distributor possibly.

Do you have solid lifters? If so, you may have too little clearance. When it warms up, the gap gets smaller and you blow compression by the valves. If this is the case, make sure you adjust them, because this is what burns valves. Also, is your gas fresh. Stale gas will cause valves to stick. Have the valves been reworked lately? If there is not enough clearance in the stems, the valves will stick when they warm up.

I'll have to check on the lifters. The gas is fresh. The engine had an overhaul by the previous owner. Not sure what all work was done during that, but other than this small issue, it runs and starts amazingly well, so I assume it was not total BS. That would have been the last time the valves had anything done to them (or they were possibly replaced). About 2-3 years ago.

Valve adjustment needs to be a "preventive maintenance" procedure, done at least once a year unless you've got a trailer queen that never sees much driving time. We used to include a valve adjustment as part of the semi-annual tuneup on the daily drivers 50 years ago, along with replacing points, condenser, and spark plugs and adjusting the carburetor. Electronic ignition, hydraulic lifters, and fuel injection have pretty much eliminated tuneups before the 50,000 mile mark or so. The oldtimers still need some TLC, however!
Jerry

A lean carb will do the same thing ,I had the same problem when I frist started My rebuilt 216 with a Mod. B carb rebuilt that again and it got better but did not go away until I rebuilt and installed the Carter W1, if the engine ran fine before look at rebuilding the carb and check the gas filters also .And like they said before check the valve adjustment.

Pete

I'm agreeing with the value opinion

Mine dose that as well. I think it is the carb. I installed new valve springs and .060 shims and adjusted the valves and the issue still exists. I rebuilt the carb and it did not fix the issue. The only thing else it could be is a vlave seat bed. But ill figure that out later.

Thanks for the suggestions so far. I'm going to look at all of this on the weekend. I assume if it was the valves, the fact that it goes away at higher RPMs is due to the timing advance, right? The carb being too lean would also go away (or be masked anyway) as the throttle opens wider, right?

its very possible. When I first got my truck it had not ran in 12 years. When I tryed to turn it over it hit the valve. Turned out there was a valve stuck in the open position. So I replaced the valve springs, Vlave seals and cleaned up the stuck valve. But most likely I will replace them later on. Not gonna heart it so Ill leave it for now. You could have a bad valve seal causing smoke. Vlave springs are $1.70 each on www.rockauto.com Cheapest you will find anywere.

Messed with the fuel mixture (the easiest thing to try), and it had no effect. On to the valves. I spot checked the two that I could when the engine stopped in its random position, and the feeler slipped in what felt too easily, so even if that's not the cause, they do apparently need some adjusting. One problem - Should the crank be nearly impossible to turn by hand?? Is there some trick to it?

Pull the spark plugs. It turns a lot easier when you're not fighting compression.
Jerry

Of course!

I had a similar situation on a '39 with 216.

Turned out the hot box was just a little burned thru, it was affecting the mixture etc.

Think and start simple. Don't over diagnose right away. You will be ticked off if you rip it down and in the end find something really simple. Do you simple tuneup adjustments and check the distributor shaft to see if it has excessive wobble. Check you plugs and wires. Inspect the valve springs and rocker arms real good when adjusting them. Just be aware of all the surroundings when you are working on the simple things and you'll figure it out.

Thanks for the advice guys. Keeping my fingers crossed that it's simple! I found the L6 235 troubleshooting pages in the FSM buried in the section on the V8 (not sure why), and there is actually a section for popping sounds. The possible causes listed include everything you guys have suggested, plus (gulp!) the head gasket. I'm REALLY hoping not to have to tear this down far enough to check that! I'll be taking Builder's advice and starting with the simpler stuff first.

Okay so I started with the plugs, wires, distributor, since that was easy. No difference. Then I went to the carb adjustments again, regardless of mixture it didn't go away, but upping the idle speed helped - which is really just masking it. Onto the valves. As I was doing them, some were way off, so I was encouraged. Once I buttoned it all back up, it started up right away and was noticeably smoother, but once it got warm, the sound came back (albeit quieter). So I started messing with the throttle and observing, and I did notice I have a pretty bad exhaust leak on either #5 or #6 right where they meet, or somewhere under there and it's shooting up. Didn't notice it before, but it puffs smoke when I hit the throttle. So this could point to the same thing Dusty saw right? I need to figure out my next step. I'm pretty sure the head gasket is not the culprit based on the compression and lack of mixing fluids. I need to replace the exhaust anyway, so maybe I'll start there. I could also go with a carb rebuild...

Well I think you may be on to some thing. I know for a fact my exhaust manifold leaks and my exhaust in rusted to hell. So if it works let me know. Ill have to do the same thing.

If you have a head gasket leak, you should be able to detect it by blipping the throttle and holing your hand near the head gasket to feel it. From what you described, your carb is OK. I wouldn't go there yet. Try to find an exhaust gasket ands start there to see if it helps. When the truck is running and making it's sound, watch the distributor look around at the linkage for the advance to see if it moves. If you see it move, see if you can isolate it to make it stationary and see how it runs. Keep us up to date.
Builder

If your exhaust gasket has been leaking for any length of them, there is a good chance that the manifold is warped or eroded. I make it a point on 50+ year old engines to put the two manifolds together and take them to a machine shop and have them trued on a belt sander. This will stop your exhaust leaks and vacuum leaks as well.

Really starting to look like the exhaust. I drove it to work today, not just around the block, so it was more of a real life test than this weekend, and it is happening a little less, and is much much quieter now after tightening the bolts. I'm going to get a new gasket as Builder suggests (they're cheap) and if that doesn't work, I will try the machining suggestion from crenwelge.

Builder, just FYI the vacuum advance does not jump or move when it makes the sound, and I didn't feel anything around the edge of the heads when I blipped the throttle.

Again, thanks to ALL of you guys for the help! I love this forum.

Just an update - it was the exhaust gasket. When I took off the manifold, the rear section was bent, and the alignment sleeve on the intake had been crushed. Looks like whoever had it off last forced it on incorrectly when they put it back. Not only is the sound gone, the engine is so quiet now I can hardly hear it inside the cab at idle!