1973 ford f100 muffler backfire

hi mi dad just picked up a 1973 F100 with the 360(5.9) standard we got it running again after beeing parkd for like 5 years now the issue is whenever i leave the vaccum advance hooked up it backfires trough the muffler and really loud louder than a shotgun blast i remove the vaccum advance and leave it blocked the truck runns really good checked timing and it was set at 20btdc so i moved it to 6btdc lik e its supposed to but same difference any ideas suggestions appreciated

Points and timing go hand in hand, Check the dwell angle, or new points gapped to specs.

I know the pro’s will pick this up but in the mean time…do you know if the vacuum advance is working …

well we replaced the points and gapped them at around 19 not exactly sure and as far as the vacuum advance working i tested creating a vacuum with the distributor cap off and it moved with the vacuum and returned back to placewhen released. can the points make it back fire that bad? and if so wouldnt they do it all the time not just when the time its being advanced? thanks for the quick reply by the way

You might want to check for a stretched timing chain

To do this, remove the distributor cap. While someone turns the engine by hand at the crank bolt watch the rotor in the distributor. Have them stop and then turn the engine in the opposite direction. If the engine can be turned more than 5? before the rotor in the distributor begins to move, the timing chain is stretched and could be causing the back fire.


run with waterboy…i believe hes on the right trail. Re-enforcements are on the way

Does it idle smoothly? If it is running poorly there may be a broken valve spring or sticking valve. If it seems to have a miss lift each plug wire off the distributor cap while listening for a change in the idle. If the idle worsens the cylinder was OK. If pulling the wire seems to have no effect the cylinder was missing and you should check the compression on that cylinder. If weak, pull the valve cover and inspect the spring and look for excess slack on the rocker indicating the valve is sticking.

Also. If there is no slack in the push rod/rocker when the cylinder is at TDC the lifter could be frozen.

The timing was so far advanced because the carb is not providing vacuum for the advance. That’s why it ran ok at 20, but is a backfiring dog at 6. probably gonna have to rebuild carb. Is it a two barrell? And if I remember correctly, gap for Ford V8 points is .017 to .020.

Check the idle speed. It shouldn’t be much higher than 600 RPM. If that doesn’t help, richen the idle mixture about a quarter of a turn and see if that helps. Backfires may happen if the mixture is a little lean. They can happen for the opposite reason too but keep trying.

Test the compression because you may have a burned exhaust valve.