I have an unusual set up on my '71 Skylark convertible. I have a 350 Buick motor but my heads are dated '73. It was originally a 2 bbl Rochester but I wanted a 4 bbl. It took me a while to find an 4 bbl. intake that would work with the '73 heads but I was able to source one here on the forum, it has the hot air passages in it. I ran an Edelbrock 750 on it for a while but it ran poorly especially during warm up so I looked for a Rochester Q-jet. I needed one with a hot air choke and I ended up with an 800 cfm unit that I again sourced from our members. After all of the searching, the hot air choke carb. I got was not a perfect match, so I wound up putting an electric choke conversion on it. It runs well but I keep hearing from the members here that it is not "right". So what carburetor should I be using? And if you are going to ask me for numbers from the intake, heads or carb. you are going to have to tell me where they are located. Thanks! Lee
Seems to me you have a good carburetor on the car now. Does it run well? If so, I say leave it alone.
If you have a 71 engine with 73 heads and a carb from another engine then you're obviously not looking to keep it all original (matching the numbers). I personally believe that the Q-Jet you have will be fine for you. When a QJet is built properly, it's hard to beat. If your choke works right, idles smooth, secondaries kick in smoothly, and your fuel economy is on par....why change it?
That looks like a 75-79 Buick Q-Jet, which can actually be pretty desirable. They had a lot of 2nd gen modifications that made them better carbs than the earlier models. The one watch out is if it's a 75 carb with the freako expanding APT bellows or whatever they are. If it's a 76 or up model, it'll have a faint circle in front of the airhorn for a knockout plug above the float well. I have two carbs like that and like them alot. If it's one of those and it's built well, you've already found your answer. Whats the carb number on the drivers side?
I was able to get some numbers off of the carb. but they are faint in some places. 7045240----9 and under that 2834? not sure of the last two digits, they are hard to read.
That would make it a 75 Buick 455 Carb - not the greatest, but by no means bad. It just has no external access to adjust the part throttle A/F mixture, everything else is a great design. Like I said, if it's built right with good parts, there's no reason to want anything else. At least, thats how I look at it.
Well I do appreciate the positive feedback from you guys on this one. I had heard many nay-sayers and was beginning to get a complex about it. The carb has always performed well, all I need to do is fine tune the choke adjustment. I do have one other question though, in the picture I posted earlier you can see a screw adjustment for the (vacuum) secondaries. I mistakenly messed with it thinking it was a part of the choke pull off. What is the correct adjustment procedure?
Dont worry about them, if you like it,it's good!! That carb is plenty good for a mild 350, and if it runs right, no reason to get rid of it. That carb -as it sits- on the right motor will do 12 second quarter mile or maybe better. It'll be fine for yours, especially when it's dialed in right. Given, the 76-79 carbs offer some additional tuning capability but the 75 isnt a BAD carb at all. Now a quick question - what screw adjustment for the vacuum secondaries did you turn? The screw/spring combination on the vacuum canister arm shown above the choke housing? The reason I ask is if you adjusted anything visable in the picture, it wasnt the secondary air valve tension. What I think you turned was the adjustment for the choke pull off, and that has alot to do with the way the choke performs too. Your best bet is to get a hold of a 75 shop manual and follow the choke setting procedures closely. if you cant lay your hands on one, there is a copy of a 73 manual online here: http://boattail-riviera-by-buick.com/library/documents/documents.php The 73 manual details the settings for a different (divorced choke) carb, but the principals are the same and you'll figure out what equates to what on a hot air carb, they're not that different. Also, if you're into understanding all the systems of that carb in depth, this book is the best you'll find and indispensable: http://www.amazon.com/Rebuild-Modif...&qid=1363433102&sr=8-1&keywords=cliff+ruggles Good luck!
Rhett, Thanks for straightening me out. Yes, I guess it was the choke pull off then, it is the screw on the arm in the picture above the choke housing. I'll do some research and fine tune it. Thanks to all who responded here. Lee
Lee, That is the primary choke pull off. On most carburetors, the choke pull off is adjusted by bending the link, but you have one with that screw adjustment. The primary choke pull off does double duty. On a cold start, it opens the choke blade AGAINST thermostatic spring pressure, so that the engine runs smoothly at fast idle without loading up. If the choke pull off didn't do it's thing, the engine would run too rich because the choke blade would be closed too much. Any year chassis manual will give you an adjustment procedure, but it usually involves some special gauge to measure the distance the choke blade is opened after the engine starts. I would just adjust it so the engine runs smoothly after it starts cold. With the engine off and cold, open the throttle so the choke blade closes. Then operate the pull off link by hand, or with a mity vac. You can then see the effect the adjustment has on choke blade opening. I guess something like 1/4" would be optimal. The primary pull off also links the secondary air valves, to slow their opening, but I don't believe that is effected by the adjustment as it has a separate link.
Well that would explain a lot. I had done some reading and mistakenly went ahead and bent the rod for the secondary pull off as my choke pull off was not working at all. It looks like I misdiagnosed or at least used the wrong adjustment to get my choke to pull off. My choke problem as it stands is this - it is all or nothing. When I start the car the choke engages, but as soon as I blip the throttle to slow the idle down, the choke engagement (fast idle cam) drops out completely. The steps on the fast idle cam are not getting a chance to engage. Will adjusting the primary choke pull off (screw adjustment) restore the fast idle cam operation? Do I need to readjust the secondary pull off to allow the primary pull off to engage first? I went on the boat tail forum and was not able to enlarge the pages to make them legible. I found an Autozone article describing the 1975 Rochester 4 bbl. (Cadillac 1967-1989) choke operation but it is not very detailed, it is a bit vague. Thanks again, in advance for your help! Lee
It sounds to me as if the primary pull off is pulling the choke blade too much. Try starting the engine cold with the air cleaner off, so that you can see the action of the pull off. Try adjusting the screw so that the pull off opens the choke blade JUST ENOUGH to let the engine run smoothly. It is either that, or you do not have enough tension in the thermostatic spring. Try adjusting the choke housing richer.
Thanks Larry. Crummy weather here again so this is going to have to wait a few more days before I can get to it.
Well I had things pretty much all bent out of whack. I had over adjusted the rod on my secondary choke pull off to the extent that it would not let the choke close 100%. I re-set the secondary pull off to allow full movement of the choke. I re-set the electric choke coil spring to close the choke all the way and then manually operated the primary choke pull off to check and adjust the action there. Once I got the primary choke pull off to open the choke approximately 1/4" against the choke coil spring tension I figured I was in the ball park so I got brave and turned the key. Guess what? It works! The engine raced a bit for a few seconds and I was able to watch the fast idle cam drop out by increments as the choke spring warmed up. A minute or so later the motor was warm, the choke was fully open and the motor was running smoothly at idle speed. I also found that I no longer have to pump the hell out of the gas pedal to get the motor to start when cold. The fully closed choke provides lots of suction and pulls the fuel mixture very nicely. I also added a ground wire to the choke spring housing to ensure a good path to ground for the electric choke, before doing this I noticed small sparks arcing on the choke linkages. Now the choke spring warms up quickly and the motor does not race excessively. Thanks again for the guidance! Lee
Yeah, that's what I figured. That secondary pull off is kinda useless. They added it for better choke control (translation emissions). As you have discovered, you don't need it.
