Cranks but won't but backfires thru carb and exhaust.

[69fordkh]

So, I drove my 69 f-250 with a 390 fe around to warm every thing up and get it moving from sitting in the driveway. Parked it and after about a week went out to start it and it cranks and when it tries to start back fires thru the carb and thru the exhaust. The engine is a crate motor with with less than 10000 miles on it. It seems as the exhaust back fire could be caused by a overload of gas. Could that also cause a carb back fire? The vehicle sat for quite a while before I got it . It was running fine now this. The 2100 wbbl carb was new last fall. Is it possible to jump time after a short drive and shut down or maybe bent a push rod? Or is it just carb related? Please no derogatory response's.

[sparky72]

I would check the plugs, timing, coil, rotor, cap, and points (or electronic ignition if that’s what you have) before assuming a carb problem.

[69fordkh]

Thanks, rotor and cap are new reset points to .016. Coil falls within specs on both primary and secondary windings. Haven't pulled the plugs or got into the timing yet.

[Bigcalhoun]

I once got the timing off by 180°, hit the ignition and it backfired through the carburetor. Sounded like a grenade going off in the garage. Timing could be a potential culprit.

[cep62]

I realize this post is 4 months old and the O P hasn't been on in awhile.
But my bet is a hairline crack in the distributor cap.
I 've driven my truck and shut it off then it would act like that on the restart.
damp weather can help cause it too,
A close look will show a line that looks like a pencil mark between the terminals
that will cause the spark to fire early causing a backfire.

[Fraser]

To me, this sounds like bad gas. It starts fine the first time, but as it's burned it gums up the valves until they stick. result is bent push rods.
If you have old gas, it must be removed, adding a little fresh to the old is not good enough!

[1972hiboy]

I once got the timing off by 180°, hit the ignition and it backfired through the carburetor. Sounded like a grenade going off in the garage. Timing could be a potential culprit.

Ah yes, almost like a mechanics right of passage. As I read this I could almost feel the concussions of the technician induced explosions I had created in my younger age. Made me smile.

I vote for moisture got into the cap or bad condenser.

[Mach428]

Check that timing chain hasn’t jumped. Bring up number one cylinder to TDC and see where the rotor is pointing.