backfire through carb, increase or decrease decrease accel pump
 

Hi all,

buddy with a 396 chevy. backfires through carb when quickly revving up the engine in neutral. it's way better if initial timing is increased to 25 degrees, but i don't want to run the engine like that.
I was thinking maybe it's just getting completely wrong amount of fuel. so with 16 degrees in idle it backfires badly. (268 duration cam. rather mild for that engine)
the carb is a 770 street avenger. the pump is 50cc and yellow cam (all stock).
This (in my opinion) is the biggest you can actually go. is it worth trying to reduce (let's say just use a pink cam). or should we increase the nozzle size?

more or less? :-)

ohhhh forgot to mention. when he revs it a few times (quickly) i get sick behind his car with black smoke. so my guess is it's too much.

I'm mostly wondering: "is too much pump causing backfire through carb" ? so far I always thouht this is ignition only problem