Backfiring at idle

I've got 2 very different engines with the same problem. Both are straight 6's. They start fine, but when they're warmed-up they both backfire randomly out the exhaust while at idle.

#1 is a 225 cu 6 in a 1955 International dumptruck, stock

#2 is a well-worked 235 Chev; cam, pistons, 3 carbs

Both seem OK under light load (can't run either one under wot due to roadability and legal status)

Any ideas?

 

Valve adjustment???

 

What does a compression check show?

 

fouled plugs

 

old gas?

 

How old are the engines? You may have carbon build-up and they are pre-igniting. With engine running at a high idle, pour a small, steady stream of water down carburator. It will blow off any carbon.

 

Short of a leak down test. It sounds like when the engine is cold the cylinder is sealed off. But when warm and things expand, it sounds like an exhaust(s) valve is tight and stays open. That is a hot valve that cannot transfer heat. Usually that is how a valve can burn. Not enough heat transfer time. It is just a guess, but I will guess with Josh and suggest to at least check the clearances.

 

Had a similar problem, start with changing all the plugs with new ones properly gapped.

 

condenser

 

Try a diagnoses with a vacuum gauge and symptoms chart. Old method but it will tell you alot.

 

FIRING ORDER!
check the wires too
recheck and dont have them old plug wires to close to each other even if it is purdier that way

 

After sitting most of the winter, my 327 started doing the same thing-- adjusted valves, cleaned plugs, set timing etc.etc., no change---new gas and all was good again.

 

Thanks for all the replies

1)New gas in the Chev, old gas in the truck

2) Firing order OK in both

3) Shouldn't be carbon in the Chev - it's only got 3 hours running time

4) I'll do a compression check first

5) I was thinking maybe wire or dist cap, wires on both are old
(yeah, I should know better on a new engine)

 

Cracked coil?

 

stuck valve

 

Sounds very like the condensor - either a duff unit or a bad earth. Miht even be missing altogether.

 

Adjust the valves - they both need it anyway ... and you can eliminate "tight valves" from the list.