Originally Posted by chevrolet1980
I got a 78 chevy crate engine,with stock quadrajet intake,modified cam,and the carb is off 77 olds.. I rebuilt the carb with new gaskets,float,float needle/seat,not shure what the original jets/metering rods were but i am runnin 51 rods,and 72 jets. the mixture screws dont do much for adjustment, one time it will sit there and idle fine and run rather good,and the next time it dont wanna idle for jack,and sometimes it is low or other time it is high. when i floor it from idle i get a puff of black smoke wich indicates it is rich.no matter how far in i turn the screws it dotn stop that puff of black smoke. i do have a vaccume leak wich i need to fine,but that would lean it out more so dont make sence why i got the black smoke when i floor it from idle.. also have the metering rod stop set at 2 turns out should i turn it in more? also the float is adjusted at 17 32'ds wich it idled at that setting one time and the other it didnt so i dotn think that the float level is the problem. well i cleaned the plugs it will idle now fair for about 20 sec sometimes longer then dies power piston is all the way down like it should when runnin but less black smoke now
The float level is too high at 17/32". That's over 1/2", the "normal" setting for a Q-jet is closer to 1/4", give or take. Your carb might be better at 7/16"- 15/32".
When using the "M" series rods, .075-.076" range jets will be a better choice for you, IMO.
The jetting as it now is, will probably be lean at WOT/low vacuum, high load conditions, it may be OK at cruise, moderate throttle.
The APT should be about 3 turns. Idle mixture screws should be about 1-1/2 to 2 turns out.