Pulled the dist. and replaced the stator pickup coil because engine had been stalling during warm up. Reinstalled dist. and timed it at idle with vacuum advance pulled and plugged. NOW- Engine starts and idles, but sputters and backfires when the slightest throttle is applied. If I disconnect the dist. vacuum module line, engine revs fine with throttle applied. What the hell did I do wrong??

Stock '77 F100 302

 

Are you certain the distributor and rotor was reinstalled in the same position? Sounds a little bit like it's "off a tooth".

 

That is very possible, but I got it to time with the timing light, although, I noticed the vacuum module is in a different position now.

 

Possible there's a vacuum leak? Make sure the vac advance is working as it should.

 

No vacuum leaks and vac advance module is new. Engine runs smooth under throttle (off idle) when vacuum line is disconnected from distributor vac module. Engine won't run (backfires) when vacuum is applied to the distributor with throttle.

 

OK got it to smooth out by backing out the vacuum module adjustment about 6 turns. Apparently it was pulling too much timing advance when vacuum was applied.