MG Car Repair: MGB stalling issues, british sports cars, stale gas

Question
I've had my 1977 MGB for 11 years since my mom gave it to me. She bought it brand new and it has stayed in the family. A few years ago when I was living in a different state my father took it to have the oil changed and they used synthetic oil which somehow blew the engine. It now has a donor engine from another 1977 MGB but it is having a few problems. The last time I drove it was a couple of years ago and it really seemed to be lacking power. I just pulled it out of the garage today and got it started. At first it was idling at what seemed a very low RPM and eventually died. I got it started up again and adjusted the throttle control to idle around 1000-1100 RPM. I let it run for awhile, about 15-20 min which it did and sounded good but eventually slowed down to around 750 RPM and then just stalled out. Can you tell me what my problem might be? I am planning on doing a rebuild to the engine next summer but I would like to drive it around a little bit without the worry of it stalling out on me. I am really confused why it would run for upwards of 20 min just fine and then decide to stall itself out. I thought that when I adjusted the throttle that it would've fixed the problem (figuring it was a problem with the installers of the new engine not knowing what they were doing) but since that was not the case now I am clueless.

Answer
Hi Justin,

Your symptom does not pinpoint the problem so some tests and some work is required to find the problem. From what you say it has been setting for a while and probably has stale gas in it. You need to drain the fuel and put fresh gas in it and remove the fuel line at the carburetor and run that into a can or such to clean out the pump and lines by running some of the new fuel through it.

Many old British Sports cars are stored and then brought out to run and many were brought to me in the dealerships so there was a procedure I had to go through to be sure of getting them back on the road.

First I needed to run a compression test (throttle open) to be 100% sure the combustion chamber was ok as it is a waste of time looking at fire or fuel if the compression was not up to par. (125 to 165 PSI on all cylinders with very little difference between cylinders. A Compression gauge is not expensive.

If and only If that is ok do you bother to go on to ignition.
First remove the spark plugs and either clean them or replace them and note what they look like when you remove them. With clean or new plugs connect each plug wire to a plug and lay all the plugs with the wires attached on a metal part of the engine and have someone try to start the engine. Watch the sparks at each plug (not is bright sun light). The spark should be thick and blue in color. A thread like orange or yellow spark is a weak spark and can cause a no start or a hard start. If you have have good blue sparks then set the timing to specs and put the plugs in. This is a rough home method of testing spark, we had a scope that told us everything about the spark but most people don't have that equipment available.

Next and always last is fuel. A simple first test of the fuel supply to the carburetor is to turn the key on and listen to the pump (if you have a SU type pump) It should "click" rapidly for a second or two and then slow to a very slow "click" or even stop clicking. This is an indication that the pump has filled the carburetor and has built up pressure 1.5 PSI to 3 PSI.
If the engine does not start just spray a little starting fluid (available at any auto parts store) into the intake and if it starts right away you know for sure it is a fuel related problem (carburetor, fuel lines or fuel pump)

The tests must go in the sequence of compression first, fire second and fuel last.

Let me know the results.

Howard