WOT - Backfire ??

Hey Guy's after 4.5 years, I finally drove my truck. Although it's only a 400 I'm very pleased.
Have an issue tho.. Driving normal it's great under hard accel or short WOT it will pop out the exhaust, big puff of black and buck a little like the tranny is trying to shift then it goes on.
It's a 750 holley (off of my 440) with vacc. secondaries, auto tranny. Timing seems right on easy starts after it's been driven and warmed up. I set it at about 12* stock 400 with a comp cams xe256h cam. Any ideas about which way to go - Thanks

check your total advance. I'd proabaly start at 36* @ 2500rpm. Also you really should tune the carb. Check the vaccum at idle and under load. turn the mixture screws all the way in and then out 1/4 turn. Adjust until you get the highest vacuum. Then re-set your timing full advance w/o the vacuume advance. Then re-set the carb again. It will take a few turns of the wrench.

Thanks -
On this dizzy, I don't have a vacuum advance, I think it came from a 78 or so fury or .... looked like a cop car...

Take your time, and walk away when you need to. It beats throwing wrenches down the street....

Quote:

---

Thanks -
On this dizzy, I don't have a vacuum advance,

---

has somebody taken the advance pot off and is the timing locked? its it a lean burn distributor?
i have seen a advance pot taken off and the block off plate screwed on. only prolem was the advance wasnt locked out so at full throttle the weights would just fly around causing some very strange backfires.