High Idle Why?

[xirdneh]

Car idles around 1000 to 1100 RPM and backing idle screw off all the way does nothing.

1. Check throttle cable adjustment (two 12mm nuts). Is it too tight? There has to be a little slack. If that does not lower idle move on to #2.

2. Check Throttle Positioner at front of carburetor. With engine running does it have and hold vacuum? If not, figure out if it’s bad or why it has no vacuum. If you want to disconnect it just back off the adjustment screw till there is a gap or remove the screw. If that does not lower the idle then move on to #3.

3. Remove all vacuum lines from carb and plug their ends. Then cap openings at the carb. The idea is to isolate carburetor from vacuum lines. Start the engine, if the carb is good the engine will idle below 700 RPM. If it still idles high, rebuild the carb or get another one. If it RPM cycles up and down then move on to #4. Also see Note #3

4. With all vacuum lines hooked back up Check the primary cut solenoid. That’s the one on the back of the carb. It could be a two wire or single wire. Here is what I do. I cut the solenoid wire (or wires) about 3 inches above the big green plug and install blade connections. Then I rig a 12 volt light bulb to the black wire so that it lights up when juice is being sent to the solenoid. Start the engine and watch the light, it should burn bright and steady. If it fades in and out in sync with the idle cycling then something is wrong with the solenoid set-up. Try connecting the solenoid straight to the battery, if engine stops cycling at least you know the solenoid is good.

Note: 1. How to find out if a solenoid is working. (usually if it is not working car will not idle at all, no matter how high the RPM) Follow solenoid wire to big green electric plug and disconnect plug. With engine not running rig jumpers of some kind to connect solenoid directly to battery and listen for click. If solenoid does not click remove it and replace. Sometimes you can get them working again by rapping them on a table. There should be a small “O” ring on the end.

Note: 2. The secondary solenoid on the driver side of carb has nothing to do with idling.

Note 3: It is possible that a leak in the intake manifold or leaking intake manifold gasket could be the problem but I have never encountered that. I keep a good carb around so I can swap out what I think is a bad carb. If by chance I install a good carb and still get a high idle then I would test for manifold related leaks. This is how I would do it. Remove valve cover and back off all of the valve adjusters till loose. That would close all of the valves. Next I would install a plate over the holes where the carb sits. That plate would have a hose connection screwed into it that could be hooked up to a vacuum pump of some sort. There would be another connector for a vacuum gage. Then I would see if I could hold some vacuum. Luckily I have never had to do that because so far it has always been the carburetor.

I have re-built and/or cleaned at least 20 carburetors and only 3 out of 5 are successfull. By successfull I mean they IDLE correctly. I wish I knew just exactly what needs to be tweaked or cleaned in those carbs to get them to idle right.

[marlinh]

Excellent write up. You have given me some good tips. Thanks.