MG Car Repair: 1975 MG Midget, rubber hoses, vacuum test

Question
Hi, my husband is restoring my 1975 MG Midget, it sat for the last 10 years.  It runs great when driving "easy" backfires when not.  Also sputters/backfires whenever you keep it at a steady speed, any steady speed.  One person told him its the carbuerator, another says its the timing. He replaced the fuel tank, fuel filter, oil filter, all rubber hoses, breaks, belts etc.  Double checked his timing but he said it seems fine.  This is the only real problem left.  Any suggestions/ideas to look at would be greatly appreciated as he has never worked on an MG before.  Thanks so much!
Kristine

Answer
Hi Kristine,
When a car was brought to me in the dealership with a complaint like that there is a procedure I have to do to locate the problem and that is, first I need to run a compression test (throttle open).

This must be done first. Then and only then if the compression is ok (130 to 170 PSI on all four cylinders) do I look at anything else.
Then I test the fuel pressure to the carburetor. (1.5 to 3 PSI) and run a manifold vacuum test. The vacuum should not drop below 5 in. hg. as you increase the RPM up to the RPM that you get the backfires.

If all that is ok and only if all that is ok you can set the timing to specs and with a timing light connected to the coil wire (not a plug wire like when you set timing) hold the trigger down and run the RPM up to the point of misfire and watch the flash of the light (at higher RPM it will look almost like a steady light) At the RPM that it starts to backfire or miss, if the problem is in the "primary" of the ignition system you will see "Flickers" in the steady light that seem to match the backfire or misfire. If the light stays steady then the problem is not in the primary of the ignition system.

If all this is OK and only if it is ok you can look at the carburetor. if you are in the US you have one Stromberg carburetor and you need to remove the top and look at the diaphragm to see if it has any holes or tears in it.

If you have not found the cause up to that point let me know the exact results of each test above and I will tell you the next tests.

Howard