76 f 100 with 360. replaced the ignition module yesterday and the truck ran good. repaced the mag pickup today. the original was rusty and stuck on the dist. shaft .cleaned everything up and installed the new one.. now the problem... most likley it was not advancing the timing befoe. now with the new mag. it will start and idle but backfires when you give it gas.. take off the vac hose from the dist. and no problem.. timing set at 18 BTDC. ANY IDEAS ...

 

It sounds like its not quite liking the advance of timing its getting.

 

Pull the distributor cap off, apply vacuum to the advance off and on, and make sure that the breaker plate inside the distributor springs back to the normal position when you remove vacuum. The top half of the breaker plate should move smoothly as you apply vacuum. It's very possible the top plate seperated from the bottom plate at the pivot point while you were working in there, as they're not very strong.

 

Thanks will give it a try tomorrow