MG Car Repair: carb adjustment, john deer tractor, air flow control

Question
I have a 1960 mga with dual su carbs. they were rebuilt 1 1/2 years ago. I ran the car for a couple of months and they were fine. I then started restoration and the carbs sat on the shelf for over a year. i have reinstalled them and am having trouble with the rear carb. i can turn the jet adjusting nut far both ways and when i push the piston lifting pin the engine slows.( too lean). the front carb responds well to adjustment. any thoughts? also, just to make sure, turning the jet adjustment nut in is richer?

Answer
Hi Gary,
You need to back up a little and confirm that the throttle plates are in sync. If you don't have a "Uni-syn" gauge you can use a piece of vacuum hose or such to listed to the hiss in each carburetor with it's air cleaner off. The SU is a Air Flow control constant velocity carburetor so it is very important that the intake air going into each carb be the same before any jet adjustment can be done. It is also very important that the compression be up to par on all four cylinders.

Another point on the SU used on MGAs is that each time you adjust the 5/8 nut upward the jet itself must be lifted hard as the small springs on the lever can not always be counted on to raise the jet to meet the adjusting nut.

Also, I have always had much better results by this method which is not what the manuals say. When you raise the pin on the front carburetor you are testing the REAR carburetor not the front carb.

When you lift the pin on the front carb you are testing the rear carb. If the RPM raises and stays fast the rear carb is too rich and if the RPM drops off and the engine dies the rear carb is either too lean or the throttle plates are not in sync. When the adjustment is correct the RPM will drop off and the engine will sound like it is an old John Deer tractor running.

Turning the adjustment nut "IN" which it "UP" leans the mixture and turning the nut "OUT" (DOWN) which is rich.

Let me know how you do.
Howard