black smoke
 

having problem with tow rig. 350 chevy engine, single plane intake (i know its not perfered for towing) holley 600cfm with vac. secondaries manual choke, in a 74 gmc 4speed manual. Had same problem with 750 vac. secondary, this 600 carb came off a good running 454 about a year ago Smokes black while driving, could this be a power valve problem
? Vac. advance seems to be working, choke is off. any advise appreciated.
Ryan

could be the power valve if it smokes after a certain vacume is achieved. otherwise I would think going from a 454 to a 350 you will probably have to downsize the jets a little. Im thinking the BB needed a little more fuel.

what model carb is it?

I have a dual plane SBC wieland stealth mani sitting in my garage if you want one. I dont have any need for it right now.

the carb is 1850-2 and has factory 66 primary jetting in it, pv holds vac. but will i nstall a new one anyway and see if it helps.

350 cubic inch motor would create a higher vacume rate than a 454 due tot he restricted cubic size, thus the fuel is just being pulled out of the bolsters. try downjetting from 66 to possibly 60 then try it out.