71 Rochester Monojet MMV Carburetor tuning

[trevarthan]

I've got a 71 Rochester Monojet MMV Carburetor on a C10 with a 250 L6. Just recently rebuilt it. When I bought it from the original owner, he said he thought something was wrong with it. After rebuilding and tuning, I think I see what he means.

If I tune the idle mixture correctly, the truck runs and doesn't stall, but it hesitates badly when cold and noticeably (but not terribly) when warmed up. Pulling the plugs, they're dark with black deposits, so it's clearly running rich.

If I tune the idle mixture a half turn lean, the truck runs fine. No hesitation when cold or warm. Unfortunately, it stalls randomly at idle, when pulling up to stop lights and such.

From what I've read, the mixture screw mostly controls the idle mixture. It affects the mixture under throttle, but not a whole lot.

This has me thinking my main jet is too large. Am I correct?

If I change the main jet, do I also need to change the metering rod?

I'm in Tennessee, so pretty low elevation, but sorry, I didn't know I needed to take a picture of the main jet when I was rebuilding the carb, so I'm not sure what the number is on it. I'll have to remove it to find out.

Alternatively, could I also lean the mixture screw and raise the idle speed?

Thanks.