I put a quadrajet on my 350 as the engine is stock and the previous owner had installed a Holley with a square bore/ spreadbore adapter. The Holley actually worked quite well but was a little soft when the secondaries came on. The quadrajet I have now came off a running engine and was told it ran great (as they all say). It is a stock carb off a stock off a 78 buick 350. The carb performs well everywhere except idle where it is rough. It appears to be lean as I can partially cover the the primaries & it smooths out. I set the air screws with a vacuum gauge and achieved 16in of vacuum. There are no vacuum leaks except one potential one which leads to my question. If i spray a little brake cleans around the throttle shaft i can change the idle for the better. The shaft does have slight play but not what I would consider excessive enough to cause such a rough idle. I did not install a gauge to measure the play but I realize any play is cause for potential problems. The odd thing is the side without the throttle cable or spring (passenger side) seems to have more play and affects idle most when sprayed. I have no problem installing bushings in the throttle plate if needed but thought I would check with the experts first. Also the throttle closes fine & the plates do not bind in any way due to the play in the throttle shaft. Thanks.
Classic vacuum leak. Are you sure it is the shaft? If so, putting bushings in the base plate is your best option.
Yea, if you have the tools, put the bushings in. I put bushings on primary side on all of my builds. And some on the secondary side, but not nearly as common.
I guess I knew the answer, just wanted to hear it from others The base gasket is new and it only changes rpm when sprayed right at the throttle shaft. The idle is also a little unstable as well as rough. If I am taking the base plate off might as well buy a kit and put that in. I never seem to get away with buying a used carb, always end up tearing into it!
If you're going to kit it, you might as well pull the idle tubes and open them to .035 or so. It can only help you especially if you've got a little extra air getting in.
WARNING! There have been a few folks advocating on the Forums to use a standard 3/8 drill bit and drill press to bore the baseplate for new bushings, this is a BAD idea as it does NOT keep the drill bit centered in the bore or keep it from going out of round during the drilling operation and making the hole too large/off center. In other words a half-ars method recommended by folks who certainly have little if any experience at this deal, and are better off keeping their day job and staying off the Forums, at least when it comes to giving advice in an area that they really know little to nothing about. Considering how many throttle bodies Ive had sent here that were ruined using that method, its just a good idea to do it once and do it right, rather than risk the possibility of destroying a part that is difficult and in some cases impossible to replace. Anyhow, the factory tolerance on the primary shaft was pretty loose for my liking right to start with. Ive measured brand new NOS q-jets at .008-.010 side play on the primary shaft right out of the box. This allows some unfiltered air to enter the engine and within 50,000 miles or so most throttle bodies are worn out past .015, Ive seen some twice that number. The installation of bronze bushings, done correctly is a permanent repair for that issue and will set the side play at less than .001, and should be part of every complete/correct rebuild done on a q-jet carburetor.IMHO.Cliff
Cliff, what about secondary shaft? I consider this as another source of vacuum leaks, but when i am thinking about it i am alittle concerned that leak free secondary shaft can couse somehing like secondary npzzle drip, or am i to overthinking this and all shafts has to be sealed?o No: Peace,
It's fine if the secondaries have a little play there. Most will be about .008" or so, as it gives the huge throttle plates enough room to settle back into the bores and seal up. They are designed to seal tightly each time they close. If you get them too tight, they can be difficult to get correctly centered and indexed for full closing. If you get a leak back there, they can "nozzle drip" and make idle tuning nearly impossible......Cliff
