1950-1966 [Subscribe to Daily Digest] |
If you can unscrew the mixture control screw almost all the way (i.e. full rich) with little or no change in idle quality, it strongly suggests you've got an air leak somewhere. Ditto the need to drive with the choke partway on. When extra air leaks into the intake tract without going through the carburetor, this naturally causes the fuel mixture to go very lean (which, as you note, also can cause hot running.)
Normally the adjustment of the idle mixture screw is fairly sensitive -- at idle, very little air gets past the almost-closed throttle plate, so small adjustments of the screw have a significant effect on fuel mixture. As you turn the screw through its range of travel, from all the way in (too lean) to all the way out (too rich) the idle speed should start out much too slow, then gradually smooth out and speed up as you approach the correct setting, then slow down again. However, if a lot of extra air is bypassing the carburetor, the part affected by the mixture screw is only a small percentage of the total, and changing the adjustment will have little effect. To keep the engine idling at all, you'll have to drastically richen the mixture using the choke -- which is what it sounds as if you're doing.
So, now you've got to work your way through the top of the engine and try to track down possible sources of air leaks. Sometimes you can hear a high-pitched hissing by listening carefully, although it can be hard to detect if the engine is a bit noisy. Failing the audible method, you have to investigate visually and physically.
The vacuum advance line to the carburetor is one obvious possibility: pull off the rubber tube at the carburetor and apply vacuum to it (it's much more pleasant to use a pump, but lung-power will do in a pinch) and see if it holds; if not, the diaphragm in the distributor vacuum chamber or the rubber tube itself is at fault. The larger vacuum line from the intermediate plate (under the carburetor) to the power brake booster is another possibility; this one is really too big for the "suck" test, so I'd suggest just pulling the hose off the intermediate plate, plugging the plate's vacuum port with a rubber cap, and seeing if the engine now responds properly to the idle mixture screw. (Don't try driving the car this way unless you've got a very strong leg, as disconnecting the hose disables the brakes' power assist.)
The carburetor gaskets -- between the carb and intermediate plate, and between the intermediate plate and the manifold -- are also possible leak sources, as is the join between the two halves of the carburetor body. Oh, yes, and of course make sure that any extra vacuum inlet tubes sticking out of the carburetor are correctly plugged. If none of these turns up the trouble, you have to face the possibility of a cracked intake manifold... although I think the other leak sources are more likely.
Of course, it still could be something other than a vacuum leak, but the business about the idle mixture screw and the choke sounds like a "smoking gun" to me. If you'd like to gather more evidence before you plunge into the game of spot-that-leak, you could take the car to a garage that has an exhaust gas analyzer and see whether the fuel mixture is seriously weak, but I'll bet that you'd find that it is.
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