MG Car Repair: MGB rough idling, combustion chambers, stromberg carburetor

Question
I have not touched the car and it was running beautifully until it began running rough at idle.  It seems to be getting worse.  The idle falls to 500 RPM, it is running rich with slight black smoke and sputters as accelerated.  However once I get it to higher speed 45+ it runs fine.  Seems weird to me.  It is a '77 MGB with the ZS carb.  Starts easily but quickly the RPMs drop. I did fill up with regular unleaded a few weeks ago but switched back to premium for the last fill up.  Any ideas?

Answer
Hi Hugh,

It does sound like a carburetor problem but you can't assume anything.

First run a compression test to confirm that the combustion chambers are ok (125 PSI to 170 PSI on all cylinders with little difference between cylinders)

After you do that and not until you do that, You can look at the carburetor. The Stromberg carburetor is a "constant velocity" carburetor meaning that when it is working correctly the speed of the air going across the jet is the same no matter what speed the engine is running. Only the volume of the air differs. When you see black smoke out the tail pipe, that is an indication of an over rich mixture. This can be caused by several things. Your carburetor has a automatic choke which can be at fault. Also the vacuum operated piston in the carburetor could be not raising enough thus causing the air speed across the jet to be too high. It could have a float problem causing it to be flooding.

You need to look at the most likely first and that is a problem with the top diaphragm first. Remove the four screws on the top and remove the piston and diaphragm and inspect the diaphragm for tears or holes and be careful not to damage the metering needle. Note that the needle is spring loaded toward the engine. Now turn on the ignition and listen for the clicking of the fuel pump while looking down into the carburetor at the jet that the needle was in. While the fuel pump is on, you must not see any fuel coming up and out of the jet. If you do have fuel running up out of the jet, you have a float problem and you must remove the carburetor and correct the float needle and seat and set the float level.

Let me know what you find.

Howard