C-20 Mileage.

[67shortie]

Try bumping your initial timing to 12-15*, but keep your total timing to around 36* all in by 2500-3000rpm. If it detonates back it off until it doesn't and keep it there.

Also, I don't know if this needs to be said or not,but make sure you have the vacuum advance hooked up and working (not when settting your initial timing though, you will be way off if you do) and have it hooked up to a ported source on your carb. On a Holley this will be the port on the upper part of the pass side metering block.

Doing this will bring your idle speed down and keep your throttle blades out of the transition slots at idle keeping extra fuel from being pulled in without need.

Quadrajets ARE good carbs , but as mentioned can be finiky if not set up with the right factory combo. Jets, metering rods, power piston springs, main feed well resrictor sizing and float settings has allot to do with the carb being happy on any engine. There are quite a few of different setups from the factory, and as Longhornman said mass rebuilders don't care to much about this and put in whatever they have laying around and those carbs have more of a chance of running badly right out of the box than good....

Every little bit will help, pay attention to every detail and you will be rewarded.