MG Car Repair: 79 Midget Zenith carb problems, stromberg carburetor, vacuum leaks

Question
Howard - my problem is my idle is at 2000 rpm and I cannot get it lower.  I think the problem is with the auto choke.  I have rebuilt the carb as best I can with a kit.  The choke seems to be working backwards, meaning, I can start the car by holding the gas pedal down and letting it run for about 20 seconds.  It idles at about 800 rpm.  But, once the engine warms up, it goes to 2000, which is the opposite of what I would expect from a properly functioning choke.  What can I do?  HELP.


Answer
Hi Bill,
First the "Choke" on a Stromberg Carburetor is not really a choke. It just adds extra fuel for cold starting. It does have a external fast idle screw though. So you may or may not even have a choke problem. However, for any engine to run at 2000 RPM requires a lot of air so no matter how much fuel is added no engine can run at 2K unless it has excess air.

First look at the two throttle screws on the throttle shaft linkage. The one down low is the choke fast idle screw and the other is the idle screw. If the choke system is stuck "ON" it can hold the throttle open to get 2K idle. The two screws are easily identified as they each have a spring on the screw itself. If neither screw is holding the linkage and throttle open when warmed up and running at 2K, go to the other side of the carb and back close to the throttle shaft is a unit (decel valve) remove it and examine the diaphragm as most get hard and stick open allowing air to bypass the throttle plate. Clean the face of the valve and turn the adjusting screw so as to apply excess pressure to the valve thus closing off the bypass valve. This is a test of that valve as the problem.

You should also look for any vacuum leaks in the intake manifold. The standard test for a manifold leak is to get it running at idle and warm and very slowly start to restrict the air that goes into the carb. If it at any time it smoothes the engine out or makes the RPM go even higher, then you have a intake manifold leak. If it is only a carb problem you will only slow the engine down until it quits by very slowly restricting the incoming air. If you find a intake leak don't rule out the possibility that you can have both problems.

Howard