The Stovebolt Site Forums The Shop Area Engine Shop Help chasing an exhaust backfire when engine gets hot

Details: 1950 3100, 216 engine, running a Carter YF carb, aside from the Carter carb, all else is stock.

I recently starting chasing down what seems to be a fairly quick onset of a backfire.

I noticed it at first on deceleration. I tracked that down to being a little too rich on the carb and leaky exhaust manifold donut gasket.

In the process, replaced worn points, reset timing, points gap, cleaned and regapped all plugs, readjusted the carb and it seemed to go away.

I have it running smooth as can be, until the engine gets really fully warmed up and the backfiring seems to return.

It's consistently not backfiring when it's first started and just getting warmed up. Runs fine for five minutes or so with choke wide open, then starts popping under a little load at maybe 2500 RPM (guessing, didn't check it yet).

I realize diagnosing such in post can be difficult, but I've read (and learned) much from the experts here so I'm hoping someone can help get me focused on the next steps more quickly.

Thanks in advance.

well, you are in the right place... someone should be along with more questions and suggestions.

-Woog

Maybe too rich of a mixture??????? Or a weak return spring not shutting the throttle soon enough,

Could be exhaust manifold to block leak. When it gets hot, the manifold expands exposing a leak. The leak allows fresh air in that ignites from heat. Could also be a weak exhaust valve spring or leaking exhaust valve seat allowing fresh fuel / air mixture into the exhaust.

I might suggest adjusting the valves. One may be just a touch tight and when the engine gets warm it doesn’t seal. Just throwing out an idea.

Check the operation of the flapper valve (bi-metallic spring) which deflects heat away from the bottom of the intake once the engine heats up.
What did the spark plugs look like before you cleaned them? Where they covered in soot as though it was running very rich?
YF carbs have a diaphragm for an accelerator pump which are compromised by the blend of fuel available today. They start to develop a pinhole which causes a rich condition. As they fail further, it becomes extremely rich. I have had this happen 2 times with mine. I was able to drive the truck home by pulling the throttle knob out to keep the throttle plate open far enough to burn the excess fuel so that it would continue to run.
Carl

Carl,

I too was going to suggest the heat riser. I had a similar issue that cleared up once I got it working correctly. It was an easy fix, but caused me s lot of issues.

Many thanks for the quick suggestions.

@WE b OLD: I guess it could still be running rich, but when I first started chasing it, I leaned out the mixture as much as I could. Prior to that the plugs were all a little sooty. I haven't thought about a stronger return spring. Will investigate that.

@Joe H and justhorsenround: Checking valve adjustment was next on my list of things to try, just haven't had time to work it. Never done it, so any advice here beyond what I've read already would be appreciated too.

@52Carl and ndkid275: I'm not familiar with the function of the heat riser valve two of you reference. I'll pull out the shop manual and do some reading, but if you know a good way to test functionality and fix if broken, please advise. Carb was rebuilt approximately a year ago and it's never had ethanol gas in it, but it's possible that the pump has a pinhole, also something to check.

Again, many thanks. This site is invaluable for those of us who have worked on vehicles for many years, but new to these specific trucks.

I assume that you changed the condenser with the points? I had a similar problem with mine and it was the condenser that was six months old. I hooked up a timing light and pointed it into the windshield. Running the engine in the garage I could see the spark fail as it bucked and died.

Interesting. I did change the condenser when I changed and reset the points.

Based on the above, I think I'm going to (in this order):

1). Check function of heat riser, free up if stuck
2). See if I can lean out the carb a little more
3). Check/replace throttle return spring
4). Replace accelerator pump, in case it has a leak
5). Check/adjust valves

Again, many thanks for the advice.

http://www.1954advance-design.com/Documents/Adjustment%20of%20valve%20lash.htm

Everyone does it different but I think this is the easiest way for dumb guys like me.

Chris

Is the pop from the intake or exhaust? Either way, a pop under load is coming from a sticky valve, or one that's beginning to burn a little. Since it's only happening with the engine fully warmed up, I'm betting on carbon deposits on one or more valve stems. The only effective fix is a valve job, but you might do a temporary fix by using a lightweight oil additive like Marvel Mystery Oil or KROIL to the crankcase. Use half a pint of MML, or about 4 ounces of KROIL to an oil change. The oldtime fix for a sticky valve was Casite Tune-Up added to the oil, but I'm not sure if that's even being made any more. If it is, it's probably a poor imitation of what was available 40 years ago.
Jerry

I have routinely experienced exhaust popping upon deceleration caused by a loose exhaust system, make sure all clamps and bolts are tight.

Ed

Thanks for the additional ideas, suggestions and advice. The experiences of others is invaluable.

I'm still working this as I have time.

So far, I've discovered the heat riser is stuck (not sure if it's stuck open, closed or somewhere in between) and the spring is missing. I'm currently coating the shaft with Kroil and tapping the ends in hopes of getting it freed up.

Some heat from a Propane torch will help the KROIL do its magic, also. The counterweight is supposed to roll the flap to the "open" position. I've found some heat riser springs on Ebay for a Jeep F-head engine that are almost identical to stovebolt springs, at less than half the price the resto-ripoff places charge.

https://www.ebay.com/itm/Jeep-Willys-GPW-MB-CJ2A-CJ3A-M38-L134-Heat-Riser-Spring-G503-G740/182196172425?hash=item2a6bbcf689:g:e6cAAOSweWVXfBiW

Jerry

A failing coil shows up more when it gets hot