how large of a carb can perform well on stock 273 commando ?

65Val said
650 is a bit too much for a 273. Also, is it 4 speed? If not, get rid of the dbl pump.

67Dart273 said
ANY small fairly stock engine should have a vacuum secondary carb and not a DP or other mechanical secondary design. That's one reason the old venerable Holley "780" was so popular, the vacuum secondaries let you throw it onto about anything.

The AFBs and AVSs are also much more forgiving in that regard

I would think a 600 AFB would be about the top of the max

This is terrible advise.

First off you did no tuning on the eninge or carb so to blame it on the carb is just a scapegoat.

2nd a vac secondary carb is nothing but a mask for a problem(s) and thus creates one or them. Only run vac secondaries on a auto car? WTF? Learn how to tune an engine. If it doesn't work right with a mech sec carb you don't have it dialed in...period. I don't know any engine builders that recomend a vac sec carb. My engine builder doesn't even stock them. They are not a better carb they are more driver friendly as you control when the secondaries kick in.

Put the holley back on. Set the timing and then get your idle speed and idle mixture screws set. Get a vaccum reading. Now you are ready to begin digging deeper into the carb.