MG Car Repair: 77 Midget only revs to 2500 rpms and spits and sputters, vacuum gauge, manifold vacuum

Question
QUESTION: Hi I have a 77 Midget and it used to run great but Now It has sat for a year or two.  Now the car will only rev up to 2500 rpms no matter how much gas I give it.   The car starts fine and idles just fine but after I hit the gas it sounds like it bogs and spits and sputters.  There's alot of intake noise coming out of the carb also.  I just installed a new fuel pump and fuel filter.      I was wondering if the problem was a fuel issue, like the carbs need to be rebuilt or if it is a spark/ electronic issue?

Thanks

ANSWER: Hi Killian,
It sounds like a partial stopped up exhaust system. On some cars the exhaust is easy to undo a flange and drop the exhaust enough to run the test, But on cars that I suspected a stopped up exhaust that were difficult to drop the exhaust down for a test I found I could take a short piece of brake line (about 2 in long) and drill a hole just a little smaller than the line and then drive the little piece of line into the exhaust (usually close to the front of the car) and put a long rubber hose on to the brake line with a gauge (a fuel pressure section of a vacuum gauge worked well) and start the car and rev the engine and watch the gauge. If the gauge goes higher than 1 1/2 PSI on a high rev it is stopped up. GM claims some of their cars can handle up to 3 PSI but I never found any British car that can handle over 1 1/2 PSI at high RPM.
You can run a preliminary test with a vacuum gauge on the intake manifold. Idle vacuum should be high and as you slowly increase RPM the vacuum should drop but should never go below 5 hg on the gauge. A stopped up exhaust will go to zero at the point the RPM will not go higher. This is a preliminary test because other things can cause manifold vacuum to go to zero. Nothing but a stopped up exhaust will cause excess pressure in the exhaust system.
Howard

---------- FOLLOW-UP ----------

QUESTION: Well I disconnected the exhaust and it still did the same thing...    There's a foreign auto place in town here and he told my  brother that there's a voltage regulater that converts the 12volts to 6volts  and if that is bad then it will keep burning out the cap and rotor.   So if I put a cap and rotor on it he says it will drive normal for a little bit then it will burn out the cap and rotor again....  And that's my problem right now..    Is that true at all?


Thanks,
~Killian

Answer
The terminology may have gotten lost in the transfer from one person to another. There is a resistor in the circuit that does change the 12v to a lower voltage to operate a 6v coil but when they fail, it is usually a disconnect not a problem with caps and rotors.

You need to find what is causing the failure at the higher RPM. To be able to see if it is fuel or fire you need to put a timing light on the coil wire (not a plug wire) and hold the trigger down on the timing light and rev the engine to the point that it stops going higher and watch the light. Any problem with the spark will show in the flash. If that is ok put a fuel pressure gauge on a "T" fitting and place the "T" in the fuel line at the carburetor. Then watch fuel pressure as you rev it to the point that it stops climbing. Fuel pressure should stay at from 1.5 to 3 PSI all the time. If that is correct you need to open up the carburetor to see if the diaphragm has a hole or the float level is wrong or any other problem in the carburetor.
Howard