Posted by Paul Cook in Kempner, Texas on Thursday, August 17, 2006 at 10:54AM :
In Reply to: 251tune-up help posted by mclark on Wednesday, August 16, 2006 at 11:03PM :
A rebuilt carb does not mean a properly set up or adjusted carb.
The momentary backfire can indicate fuel starvation caused when carb butterfly opens quickly ("blipped") which feeds more air and the mixture goes lean. The accelerating pump gives an extra "squirt" of fuel to keep the mixture about normal until increased airflow through the venturi pulls enough fuel that the normal carb function is resumed. This is the transition from the idle system to the high speed system
I'd check the accelerating pump linkage. On most carbs, there are several settings that determine the amount of the "squirt". Also, the pump piston can be damaged when being installed. Check it as well.