Hello all, A couple of questions someone can hopefully answer. 1st, the motor is a 69 400 off a GS and came with the stock intake and a qjet from a 68 GS350. My chassis manual says the only difference between this carb and the correct #'d carb for this motor are the secondary jets. My question is, if this is true, can this carb be modified enough to work for a mild (400hp range), mainly street & show build? I've heard good things about John Osborne, would he be the best to handle this, and if so, how do I contact him? 2nd question has to do with the stock intake. I've been told that there really isn't enough of a difference, performance wise, between the Performer intake and the stock. Mainly just the weight savings. Anyhow, I've heard Jim Weise mention opening the plenum area and port matching to the stock heads being the way to go instead of the Performer. Can he, or someone else tell me if I'm thinking correctly, and possibly give me instructions for my machinist? By the way, the motor will be bored to 430 specs, stage 1 valves, 113 cam, TCI Breakaway converter connecting with a 3:42 posi rear. Sorry so winded, and thanks for looking!
The problem with the small block Qjet is the lack of air bypass. Big block carbs came with this feature. This allows a big cube motor to idle corrrectly. Especially if you run a larger then stock cam shaft. Also the secodary enrichment circuit is missing on some 350 Qjets. The best Qjets are the 800cfm units of the 71 and up 455s. I have also run one from a cadilac 500. It was awsome.
Q-jets Just to be correct, Q-jets don't have secondary jets - they use metering rods and the passages should be the same for small block and big block carbs. Here's another thought - my '68 GS400 had the correct part # Q-jet for a '68 350 on it when I bought the car 5 years ago. I thought somebody must have swapped it until I posted about it on here. I got several responses to that post saying that other '68 owners had the same experience - their cars came with 350 carbs! A couple of guys said that they had never seen a "correct" 400 carb on a '68 motor. I have never compared this carb to a correct one, but the air bleed ports look pretty small. I drilled 1/16" holes in the primary throttle plates at one time and I still have the carb on the car. Motor is now a 462 with 230-245 cam and some port work, and it runs pretty good. No idle problems, the choke, fast idle, secondary and the rest of the carb work fine and smooth. I may swap to an 800 cfm Q-jet some day to try it out, but there's nothing too hard to tune out of the 350 version.
I have a 68 gs 400 and it has the correct factory carb. If you are going to use this for a daily driver be careful about all the newer carbs because i have three of them and they all have different choke setups so they are not just a bolt on and go deal. You would have to do a little adapting of some kind or another. ED
a modified 350 q-jet will blow you away if it is done properly, don't worry about idle quality the right rebuilder can install the proper idle/transfer tubes for a big cam, and some factory carbs simply drilled the primary throttle plates with a #25-#30 drill to allow more air at idle. secondary enrichment can also be stepped up for performance. although the rochester carb book by doug roe is a little hard to digest, a couple of practice carbs and some patience and you can do all the work yourself. good luck
