QuestionWe have a 65 Oldsmobile that is running rough and can't figure out how to properly adjust the carburetor. Any help would be great.
AnswerAlmost all the adjustments on the carburetor are set by the rebuilder during assembly. The only thing you can adjust without taking the carburetor apart are the various automotic choke settings (which do not affect the way the car runs except when it is just started on a cold morning) and the idle speed and mixture. Of course, first make sure that the automatic choke flap is totally open when the engine is warmed up (vertical) before doing anything else. If it isn't opening all the way, get some spray carburetor cleaner and spray all the moving parts in the automatic choke linkage to free it up.
The proper way to set the idle mixture is to screw the two idle mixture screws all the way in to the carburetor (GENTLY - don't force them!), and then back them each out 2 complete turns. That should be close enough that the car should run fine. You can then fine tune the two screws, 1/8 of a turn at a time, in or out, whichever makes it idle faster or more smoothly. Keep both screws set the same, however. If you find that one screw makes no difference, the carburetor has to be taken apart and cleaned out by someone who knows what he is doing, because there is an internal problem with it - probably a plugged idle passage. To identify the idle mixture screws, they are at the base of the carburetor, with the screw heads pointing toward the front of the engine, and each screw has a spring wrapped around it. If you take the screw all the way out, you will see that the tip of it has a very fine point - that is how you know you have the idle mixture screw.
The other adjustment you can make is the idle speed screw - this one is pretty obvious- it is on the accelerator linkage, and controls how far the throttle return spring can pull the linkage toward the totally closed position. It does not affect anything but the idle speed, so if your rough running isn't cured by adjusting the idle mixture, adjusting the speed isn't going to do much good.
If these adjustments don't improve the problem, you need to do some diagnostic work.
The first test you can do is is to search for a vacuum leak. The way to do that is to take the air cleaner off, then, with the engine idling in neutral, slowly slide a piece of metal across the top of the carburetor in such a way as to block off the air flow down into the top of the carburetor. Start with the metal to one side of the opening, then slowly slide it across so as to partly block the air flow. If at ANY point in this process, the engine speeds up - you have a vacuum leak. Take a stethoscope and listen of hissing all around the engine, especially at each of the vacuum hoses and at the base of the carburetor. If you don't have a stethoscope, a 4 foot length of garden hose will work as well - place one end of the hose to your good ear, block the other ear with a wad of tissue, and wave the other end of the hose around the engine (Watch out for the fan blades!!!). You'll be amazed at how you can localise sounds. When you think you have found a vacuum leak, try squirting a little WD-40 at it- if that makes the engine speed up, you've found your vacuum leak.
If you do not find a vacuum leak, the next thing you need to do is a compression test - if all 8 cylinders aren't even within 10% of the median, the car will not run right until the cause is identified and corrected, and this requires engine disassembly by a good mechanic.
I can sit here and type all night and probably waste my time, so I'll wait to hear back from you on how you make out with the above. If we haven't found the problem yet - I'll try again - just post a follow-up question.
Dick
