The Stovebolt Site Forums The Shop Area Engine Shop Backfiring from tailpipe

Hi Eveybody, I am looking for ideas on what to check for the backfiring that just developed. Last fall when I parked "Shasta" my 52 with a 56 235 in her, she ran fine. I would go out every week or two an run her up to operating temp. When I went to drive her a couple of weeks ago she had developed a nasty backfire. After reading past posts I have a couple of ideas, but any and all suggestions are appreciated. Ok here is the situation. She starts easily, starts to warm up, then starts popping. Then she frequently dies. After sitting for awhile she starts, pops, and if I keep her running once up to operating temp quits popping. I did find the exaust to tailpipe loose. I tightened it but that did not help. She also has a new Carter YF carb, plugs, points, and condenser. Thanks for the help.

Backfire out exhaust?
Popping? you talking out the top of carb?
Or there is a steady popping out exhaust?

Underdog, thank you for the reply. It is a popping out the exaust. It is not a steady popping, and only occures as the engine starts warming up. Once warm it quits popping. As soon as it warms up here I plan on checking the dist, and the points. To see if I can narrow down the cause. Jimmie.

A cold engine needs more fuel so if it's running lean it could have a tendency to pop when cold and would go away as the engine warms. You could have a passageway in the carb blocked by a piece of crud.

Could a stuck manifold heat valve cause this? How do you free a stuck one... or should you leave it as it is? I wouldn't want to risk getting carbon and rust pulled into the engine, along with possible intake leak later. Opinions?

bump to top A few answers...

https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Board=1&Number=64915&Searchpage=1&Main=10061&Words=manifold+%2Bheat+%2Bvalve+%2Bstuck&topic=0&Search=true#Post64915
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Board=1&Number=56929&Searchpage=1&Main=8923&Words=manifold+%2Bheat+%2Bvalve+%2Bstuck&topic=0&Search=true#Post56929
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Board=1&Number=2438&Searchpage=1&Main=406&Words=manifold+%2Bheat+%2Bvalve+%2Bstuck&topic=0&Search=true#Post2438

Does this popping/backfiring only happen at idle/fast idle speed? Nothing during acceleration or deceleration? Does it happen if you run the engine until warm, then let it cool for an hour or so, then run it again?

Ignition timing might be involved, but that shouldn't change with engine temperature. There is a possible connection, so check that, and make sure the idle speed is correct after setting the timing. Carburetor ice will cause stalling at idle and low speed, but I can't see that causing the exhaust noise you have. If the heat riser valve is stuck open, it is possible to have carburetor ice causing stalling during warm-up, especially after an over-night cold soak, so that possibility is worth checking into. Does this condition happen predictably, regardless of air temperature and humidity? If so, carburetor ice is less likely, but still possible. The only way to determine with certainty if your carburetor is icing is to run the engine with the air cleaner off until the engine stalls. Then look into the carburetor throat to see if you see frost around the throttle valve. If you do, you have to get the heat riser working to help prevent its formation. You have to be quick with that inspection, because the ice will melt pretty quickly. But water droplets on the throttle valve are strong evidence that ice was there.

As for loosening the heat riser valve, soaking the shaft with penetrating oil of your choice, and tapping on it fore and aft will eventually loosen it. But remember, you aren't trying to drive the thing through the manifold- only loosen it. It takes patience and perseverance. When it starts loosening, try turning by hand to help get it free. When free, it should move freely by hand and the weight should return to the raised position- when cold that is.

Harvester

Thanks from me. Good answer. Good questions.

Hi Guys,
I thought I would update this thread. Once again I would like to thank everybody that responded to my query. After posting this I worked on the truck and "fixed" it so well that it wouldn't even run. After checking everything I don't know haow many times. It comes down to the fact that I had the timing and points so screwed up that she wouldn't run. After a couple of weeks away from the truck, I set everything back to square one. Low and behold she fired up and I managed to fine tune the timing and went for a drive this afternoon. Once again everybody my heart felt thanks, Jimmie.