MG Car Repair: 1976 mgb running rough, fuel pressure gauge, vacuum gauge

Question
I have had this car for a year. It has 61,000 original miles. Ran well initially. Problems starting and keeping it running this year. Fuel pump sounded like it was running erratically. Changed pump and put new in-line filter. New plugs gapped (the old ones looked warn on visual inspection). Now it starts well, but as we run at higher revs the engine sputters, we lose power and eventually stalls. It will idle well and can rev out of gear just fine. Thanks for your help. Harold

Answer
HI Harold,
When an engine has a problem at speed and not at low RPM, you need to look at two items. When you are at speed, the fuel use is high and the exhaust flow is very high so a problem in either will cause problems. To know for sure if it is a fuel supply problem you need to test the fuel pressure at the time of failure (at speed) Put a "T" in the fuel line at the carburetor and run a long hose out form under the hood an place a fuel pressure gauge under a wiper blade so you can see the pressure when you are at speed and getting the problem. Most of the time an ignition problem will cause a lot of misfires and back fires and you didn't say that.

If that is not a problem you need to test for exhaust back pressure. The correct way is to take about a 2 inch long piece of brake line and drill a hole in the exhaust pipe just a little smaller than the OD of the piece of line and the hole must be as close to the exhaust manifold as you can get. Then attach a long hose to your vacuum gauge and place it under the wiper arm so you can see back pressure at speed. You must not have more than 1.5 PSI at high speed. This is a 100% sure test.

A third test is to do the same with manifold vacuum. Most street cars should not have less than 5" HG on a vacuum gauge at full throttle at about 3500 RPM. This test is only about a 50% sign of a stopped up exhaust.

Most vacuum gauges have a small section for fuel pressure so this one gauge can be used for all of the above tests and the gauge is not expensive and can even be purchased at Wall Mart or such.
Howard