hey anyone out there ever had this prob? just put in a new 350 runs great but coming off idel i sometimes get a backfire out the carb when punching on it i was thinking carb adjustment untill today i was cruising the for lane taching about 3200 an punched it an now it back fired through the exaust. hers what im thinkin im runnin a comp cam an mallory HEI with a high fire box i noticed in the directions it said recomens non resistor type plugs right now im running ac delco recommended for my 69 4bolt truck motor they have the porcelon so im not sure rg45 i believe, another thought crossed my mind imporper valve timing so that would mean a loose or tight push rod because my cam is hydaulic flat tappet. has any of you ever had this problem do you think any of these things could be causing my problem? whats the best way to readjust the valves at this point?

 

If you're unsure of the valve adjustment, hot lash setting is probably the easiest way to check them, although it can be a bit messy. If you suspect valve timing, that's entirely in the timing chain installation.

Backfire (through the intake) on acceleration is frequently due to lean mixture, but can be due to advanced valve timing or improper ignition timing from excessive advance, spark scatter, and possibly inductive firing between cylinders with straight wires and non-resistor plugs. Since it has apparently now changed to afterfire (in the exhaust) I'd start by looking at the ignition thoroughly.

 

thanks for the info. i called the cam manufactor and they told me 8 to 12 degrees advance so i dialed in 12 wow the cam has 4 built in an i installed the cam 4 degrees advanced thinking that i would run between o an 4 now at 2500rpm i have 38 degrees of advance an stays there at 3000 so my total advance without vacume is 38 corect? i was surprised its not spark knocking and butt dyno says a bunch of hp there. so my test drive went like this first i drove soft then starting mashing the gas slowly at different rpms everything good then i came to a complete stop mashed the throttle boom out the carb or intake im close no more alternate backfires but im a little lost here am i still dealing with a timing issue or should i be looking closer at the carb? at this point all ive done is messed with the idol and mixture adjustments. the secondarys seem close seems pretty smooth transition could be opening a tad sooner.