Posted by Paul (in NY) on Thursday, August 17, 2006 at 9:47AM :
In Reply to: 251tune-up help posted by mclark on Wednesday, August 16, 2006 at 11:03PM :
Is the backfire you hear a sharp report like a firearm, or a muffled cough? If you are hearing a muffled cough when blipping the gas, before it picks up in Rpm, try this.
Pull out the choke a little ways, blip the gas pedal. If it still coughs, pull the choke a little further. You should not have to go beyond 1/3 pull of the choke. If pulling the choke gives you normal quick pickup of RPM’s and no cough, you have a vacuum leak.
The vacuum leak could be around the base of the carb, worn butterfly shaft holes in the carb body, carb gaskets, intake manifold.
Otherwise, some ignition system issue.
Paul