I’ve always had a carburetor on my car but I just started to get into the tuning aspects of them I have a Speed Demon 650 carburetor on a stock 305 it was running pig rich! So I jetted it down to a 66 primary and a 70 secondary. I also changed to a Holley 25 squirter. But now I get this nasty low rpm lag and it will even backfire gas up through the carb. It did this also with the larger jets that were in it just it had no low rpm lag and it only backfired through the carb when it was cold now it does it all the time! What size jets and squirter should I be running on a stock 305 with headers and a Performer EGR intake?

 

Usually, backfiring thru the carb is an indication LEAN condition. Check your float levels, then try upgrading your front jets, one step at a time.

 

I'm thinking Barry is right on this. Backfire through the carb is usually lean. When you say it was doing it before when cold makes sense since if your choke is not doing its job (or maybe your Demon has no choke at all) the carb will be lean until warmed up...and blam through the carb. Now that you jetted down, it is lean all the time. It is easy to mistake carb tuning issues as being rich when it is really lean and vice versa without the aid of an 02 sensor or something similar. As Barry mentioned, check the float level first. And again to echo Barry's advice, "one step at a time". If I have learned anything from carb tuning, it is to be patient and only make one change at a time.

 

Also check your accelerator pump and make sure that you have enough fuel shooting. If not you may need to increase the squirter and the cam to get optimum acceleration mixture. This will usually be apparent if you have a stumble or backfire when you snap the throttle quickly.