So I have a freshly rebuilt carburetor on my friend's 442 and it seems to have a vacuum leak. Running super lean. With my hand over the carb, the idle and vacuum reading raises. Looked for a leak everywhere. When I direct a red straw stream of WD-40 (only thing at arms reach) at the primary throttle shaft on the drivers side, the idle and vacuum go up. The throttle shaft has visible play in it. The rebuilder says the throttle shafts were re-bushed and the play is normal. He says the leak isn't necessarily from the throttle shaft itself. Now, the guy probably has forgotten more about carbs that I know, but what he said just didn't sound right. Can someone explain this to me?
There are several types of bushings used these days and some are junk. Some are plastic and wrap around the shaft. Bronze bushings seem to be the best and when properly installed should form a nice seal to the shaft. It has to have enough tolerance to move freely, but should not have any vacuum leak that is as significant as you mention. Some of these 50 year old base plates have some significant wear to them after years of use.
So you think the throttle shaft is definitely leaking right? After I walked out, he had me second guessing my diagnosis. btw Mark, my friend Greg sent you his 69 442 4 speed carb for restoration. He showed it to me when he got it back. It looked awesome.
Thanks Jason! We have worked hard on plating and body color to get the right look. I think the carb you described is leaking at the shaft. Could possibly be the base gasket in same location, but shafts are common. And many people try easy fixes.
Some re-manufactured Q-Jets have the primary shaft with bushings installed, but they are usually the cheap brass sleeves which aren't very good. The best solution is to install the bronze pre-oiled bushings using the kit sold here: https://quadrajetparts.com/service-installation-primary-throttle-shaft-bushings-p-361.html I've done a good 1/2 dozen carbs with this kit and had excellent results.
Installing bronze bushings (done correctly) will reduce side play at the primary shaft to less than .001". The factory clearance was a LOT greater than that, I've measured new in the box NOS stuff at close to .008". At a minimum I would install bronze bushings and get shaft play to a minimum, but there may be other reasons why the engine speeds up when the choke flap is tipped in or you place your hand over the choke housing......Cliff
