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Rolf, I've had a similar experience (which I'll describe in a sec) but think you'll save time in the long run if you work through all the basics to get your idle back around 1000 where it belongs (after the engine has warmed up enough to open the automatic choke, if yours still has an automatic choke) rather than using the guess-and-fix method. I'm not clear on whether you mean you can't get the engine to idle below 4000 (you can set the speed lower but it won't keep running at that speed) or that it just "wants" to idle at 4000 (you can't set the speed any slower) but either way, here's what I'd do:
-- Start with the purely mechanical stuff: Does the throttle linkage move smoothly through its range? When it's closed all the way, does the idle speed screw rest against its stop? Do the automatic choke linkage and fast idle cam work freely? You need to check all this to make sure it's mechanically possible for the throttle to get back to a position where it can idle at 1000. Assuming that's OK...
-- Look for air leaks: Is the carb securely mounted? Are the various vacuum hoses OK? PVS valve (thermostatically controlled vacuum switch) uncracked? Quick check on all the hoses is to disconnect them from the carb and plug the pipes on the carb that they connected to (you can get rubber caps at an auto supply store for this.) If plugging the carb's vacuum ports fixes the idle, then you've got a leaky vacuum line somewhere. If that's all OK...
-- This is slightly out of the normal troubleshooting order, but might as well check the ignition timing because after this things get harder. I believe the consensus we've reached is that you should set the timing to 6 degrees BTDC at *cranking* speed (to make sure the distributor's mechanical advance doesn't get into the act) with the vacuum hose to the distributor disconnected and plugged. Get somebody to crank the engine while you handle the timing light. (Again, I've got a weird story about this coming up.) If that's OK...
-- Well, unfortunately, my next guesses would involve problems inside the carb, which would involve taking it off and rebuilding it. That's why I suggested checking the timing first! If the carb is OK, the vacuum system is OK and the ignition timing is OK and the car STILL won't idle below 4000, you've got a fairly exotic problem and I'm out of my depth troubleshooting it.
Okay, here's my weird story -- it's a lot like yours. I got my '74 Sonett all back together after installing a 2-barrel Solex carb; seemed to run great in around-town testing, so went for a long drive. About 40 miles up the road, the car got in a snit where it wouldn't run below about 2500 rpm; if I'd take my foot off the gas it would just quit dead, so I had to keep "blipping" it all the time to keep it running. I drove it home and parked it, and the next time I started it up it would idle sort of OK... until I went on a long drive again, when the same problem would come back.
I checked the carb's idle jet (easily accessible on a Solex without taking it off the car) and it was OK, so I convinced myself the problem was deeper in the carb and that it would have to come off and get rebuilt (groan.)
Fortunately, though, I decided to check the timing first and found it was WAY off! Put it back where it belonged and suddenly the idle problem was solved!
What I figured must have happened was this: When I put the distributor back in the car after swapping carbs, I hadn't quite tightened the hold-down bolt enough. When I first tried the car it was still in the right place... but as I drove it, the force of the spinning distributor shaft kept shifting the timing. When I'd park the car and then restart it from cold, the PVS system (which changes the distributor advance depending on engine temperature) masked the effect just enough to let the engine idle (barely) until it warmed up enough for the PVS to kick in.
Still, this is an unusually weird problem, so if I were you I'd check the more likely stuff first. I hear that they teach medical students, "When you hear hoofbeats, think horses, not zebras" and that's good advice under the hood, too!
, Tue, 13 Jun 2000 20:09:01
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