MG Car Repair: MGB engine missing, at speed, fuel pressure gauge, fuel starvation

Question
Hi Howard,

I have a 1968 MGB with the GF motor, with the HS4 carburetors, now with 3,000 miles on a rebuild.  I have recently noticed when at speed (60-70 mph) the engine will miss, and continue to miss, if I try to accelerate. When it has happened I pushed in the clutch, then let it back out slowly, and it seemed to stop the miss.

The fuel filter is new, but the fuel pump may be the original piece.  One thought I have is fuel starvation; what is your technique for assessing the fuel pump?

Hopefully this symptom I am describing is not something that could have only a thousand possibilities, but that is why I am asking you! :-)

Thank you in advance for any and all information.

Regards,
Dan

Oops.  I just checked the dash pots, and they looked like they were low on oil (I couldn't see any, with the pistons removed).  The pistons though seemed to meet resistance when I pushed them in, but I filled the dash pots with motor oil anyway, to about 1/2 inch from the top.  Is that too much oil?

Answer
Hi Dan,
Any tests must be run while a problem is present or in the failed mode. So to test a fuel pump at 50-60 MPH you should put a "T" in the fuel line close to the carburetors and run a long hose to a fuel pressure gauge and run the hose out from under the hood and place the gauge under a wiper arm so you can monitor pressure at the speed you are having the problem. You should show 1.5 to 3 PSI at all times. This is a 100% test of fuel pressure. (be sure to put clamps on all the hose connections)

The test for ignition is easy by just putting a timing light on the COIL wire (not a plug wire) and tape the trigger down and put the timing light under a wiper arm. If it is a bright sunny day you may have to tape a piece of cardboard over the light on the windshield so you can see the flash of the timing light when you get to 50-60 MPH. The flash of the light will look like a steady light normally but any primary ignition or coil problem will show as a flicker to the steady light. This does not check cap, rotor, plug wires or plugs.

One more test is in order. Put a vacuum gauge on the intake manifold and run that out also so you can see manifold vacuum at 50-60 MPH. At no time should the vacuum go below 5 in. vacuum.

These are the three basic tests I run on a car that has a problem at speed and not at low RPM or in the shop.
Howard