I give up and I don't have the time right now to figure this out so I appreciate any idea's from the list.
I have a persistant irregular backfire just at easy cruising speed. Accelerate or let off and it go'es away.
Just changed engines to a worn used without spectacular compression. Aldon distributor static timed at 8. HS2 carbs set up correctly I think.
Any idea????
Tnx,KA.

How persistent? Is it like a machine gun or just an occasional pop?

Strange that you have this during smooth running, but it goes away when you let off. Usually it's the other way around.

Steve: Just the occasional pop, very agravating. Seems to increase when load is the least. I.E. descending slight grade.
Throttle shafts are tight and no easily visible intake leaks.
The engine that I replaced was backfiring due to a blown head gasket. Maybe theres a blown gasket in the intake tract that I hav'nt found yet. Cleaned the distributor cap. Compression is even on all four but low.
KA

Hi KA,
I had a similar problem on a 1973 1275. It was the headgasket. It was sucking antifreeze, but there wasn't any white smoke.

Try pulling off your vacum line from the manifold and suck into it to see if there is a pin hole in the vacum advance, it should hold negitive pressure without changing or easing up. Had somthing similar, when I did it there was neg pessure but it bled off, it turned out that the plate inside developed a crack and was allowing air in throwing my timing off.
Is this the same distributor?
Dan

I agree that it sounds to me like your timing if a bit too far advanced. Back it off 3 or 4 degrees and see how it does.
Bill