well when I originally did the 4 brl swap, I used a holley 600cfm with adapters to a stock 4brl intake. When I went to tune it, I found the secondaries could not be adjusted like I wanted. If i put the lightest spring in that it would not bog with, it would have OK top end power. If I put a lighter spring in, it would bog like heck, but the top end was insane! So between that and having to use the adapters I decided to go with a q-jet. So I got a "performance" q-jet, from JET performance. it was the 75 and later design. I didn't do to much tuning on the q-jet for a while. I don't remember it ever really idleing to great. After I did a bunch of motor work to it, I couldn't get it to idle good at all. So I decided to swap my original holley back on to see if the carb was the prob. So I dusted it off and slapped it on. It idled good on startup, after 2 seconds of tweaking the idle adjustments it idled real smooth and vacuum went up and steady. Low end torque is also way up with the holley. I'm gonna play around with some secondary springs and see if I can get the top end up. Also pinging is down (I have to limit the vacuum advance, with the q-jet it was worse), and it has no run-on after I shut it off, the q-jet did.... I think the q-jet was running to lean... even though the jet I had in there was on the richer side of things... 0-60 run did 7.4. I think in better conditions it would be better. the road had some sand on it, but not much. Tryed an easy take-off, ended up spinning the tire into second gear, catching traction, kicking down, then taking off... I got a lead on a posi unit... gonna go with that and some 3.08s, should help out a bit... For the carb I'm either going to try to rebuild a holley 750 I have kicking around... or maybe look into and edlebrock... I've not been to happy with the q-jet at all... anyway... just thought I'd share my current buick trials...
there is definantly something wrong with the Q-jet.... but I'm not about to sell it... paid $360 for it.... I should have gotten a used one and have john osbourne do it up for me.... I'll keep it for a rainy day
I know how you feel, and was mostly kidding. That is a real drag, though. Have you contacted the manufacturer / rebuilder about it? Something is seroiusly wrong and you should be satisfied with your (expensive) purchase!! Good luck!
Basic mechanically question from a novice. What advise would you guys give to advance my limited knowledge regarding carbs. I took a 2 barrel apart awhile ago just to examine the thing but as of yet, I really haven't torn one apart to rebuild it. This is just something I'd like to start picking up and whatever you can suggest would help. By the way, are you judging low end torque and top end speed by simply running the car at consistant speed and take off, or do you diagnose what the engine is doing during idle etc....Thanks
72 skylark, The problems you were having with the 600cfm holley are from lack of carb. You need a 750 for a 350, especially a well put together one like you have. Stick with the holley, great performance, easy to tune and piece of cake to rebuild. You say you had a 750 kickin around that needs a rebuild; throw a trick kit in it, make sure you use new trottle shaft bushings, and my advice would be to install a quick-change secondary diaphragm. Hope that helps.
The problems you are having with the Q-Jet junning on when you shut it off is from the idle being set to high. If you put it in park and the idle is over 750 to 800 than the engine will have a tendancy to want to run on. I say this just in case you ever decide to use it again.
Biggest problem with the Q-jet is that it isn't set up right, and that problem is JET. Here in the So. Cal., area, JET is considered by just about everybody, to be a real serious industrial sized joke. Another carb issue that should be discussed is the throttle shaft bushings used in Q-jet rebuilding. The SOLID bushings some use are just plain junk. Think about it, they have to have a clearance, so they don't bind the throttle shaft when the carb is torqued onto the manirold correctly (you DO torque your carb at the correct 60 INCH pounds, don't you? 60 inch pounds is 5 foot pounds, BTW.). Since a Q-jet has areas between the ends of the throttle shaft bores and the actual "bearing" areas in the base plate that are already cut for apporocimately .030 clearances, it would make sense to add bushing materials THERE instead of butchering the carb all up with solid bushings. Since the throttle shafts also do not contact the base plate in the same areas that have this clearance, it makes sense that the shafts aren't work in those areas either. So...what to do? Well, we got .030 free to play with, and half of that is .015, which is the clearance between the base plate holes and the shaft, what do we stick in there? Solid copper busing, NO, bronzewall bushing, NO. What then? TEFLON. Sheets available from hobby stores, .014/.015 thick. Why teflon? It costs next to nothing, it cuts to size with common scissors, and it fits the area between the shaft bore and shaft in the base, without hassles. It also will deform to the torque offset of torquing the carb down, while not binding the shaft and/or allowing leakage. Next Q-jet base plate you have the throttle shafts out of, check it out. TEFLON sheet, cut to fit, the RIGHT way to rebush a throttle shaft.
JET does suck. I really don't think they did anything "performance" to this carb. Everything looked the same as a old carb I had. If I new better I would have bought a q-jet from edlebrock for the few extra bucks... But, the problem with q-jets is they were not designed with performance from the start. Maybe they do give a great compromise between milage and power, but I'd give up some gas for some power. I was a q-jet guy for a while... but I've jumped ship for performance designed carbs. I tricked out the holley 750 I had. Quick jet change fuel bowl kit, quick change secondary diapragm kit, accelerator cam kit.... bolted it on and it runs like a top, better than the q-jet ever did. I haven't had a chance to tune it at all. I had it on for a week or two, but unfortunatly I'm having some oil pressure problems, so the motors coming out for full rebuild.
Q-jet vs Holley For what it worth: When I bought my GS in 1982, it came with a Holley spreadbore replacement on the car and the the Q-jet came in a box (the Holley box!). Had the Holley rebuilt once and the car continues to run great (I don't think the neoprene float took well to the "carb cleaner" additive I put in the tank - won't do that again.) I have heard nothing but headache stories about re-building the Q-Jet. We could never get my Dad's Chevy 400 SB pickup to run right after having the q-jet rebuilt - finally bought a q-jet replacement. I am keeping the Q-Jet for "original equipment" status as I am sure someone out there could make it run right if I ever wanted to put it back on. The mechanics around here say "too many parts" when you ask them to put a rebuild kit in it.
