This question is probably too vague but I'll give it a shot. I realized that I have a different cam in the engine. Unfortunately I do not know what it is. I'd assume not a racing cam but could be mild to a higher performance type. The Q-Jet that's rebuilt is still having trouble with it. The mechanic suggested a Holly 600. Any thoughts? Told you it was pretty vague. 1968 GS 400.
The Q-jet can handle it just fine, BUT, it must be built for the engine. The original Q-jet for your engine was a 750 CFM unit. A Holley 600 is not the answer. See the engine in my signature? I run a Q-jet on the street. It was built, and recalibrated by my engine builder. It idles and runs great, and gets 14-15 MPG.
q-jets : mark in texas or ken at everyday performance in conn . either will ask ya some questions and can give ya an idea on what it will take I'm sure . ken is down below in the vendor section , both have always had good reviews . larry is correct - and if that was a "generic" rebuild it will NOT be tailored to your application . a lot of variations on those carbs . 400v8 correct ? what intake do ya have ? if a spreadbore style ( q-jet ) then a standard holley will need an adapter and if its an aftermarket intake with a holley pattern then same deal - adapter needed for q-jet . spreadbore holleys are 650 cfm , same problem as a 600 - too small . actually they will probably run ok with adjustments but will run outa steam way early . don't buy a carb for the deal then have to make too big adjustments - do your homework and get the carb closest to what you need and make small adjustments . if you just have to use a holley I would recommend a holley #3310 . 750 cfm manual choke , vac secondaries . solid basic been around a long time carb . and there is an electric choke version also . the reason I say #3310 is because it is almost the same price as the basic holley 600 ( $315-$345 750 - $300 600 ) looks better and is better suited cfm wise . and with a little searching at swaps e-bay and CL etc I've picked up great lookin 3310's for $65-$125 . stay away from the ones with no choke or look like they've been apart a bunch of times . that way if you are not happy with it - sell it - no big loss . kits are $20-$40 depending , a ton of parts for 'em , easy to figure out and take care of .
Buick 400 (even a Buick 350) needs 750 cfm. As stated, a properly calibrated Q jet will get you there. Holley 600 is not the answer, just a profit center for the parts store.
