Triumph Repair: engine running flat, sheet metal screw, vacuum gauge

Question
Hello
I have a 1967 Mk3 spitfire. I have recently renewed points. condensor, cap, leads, arm, I timed it with a strobe. It appears to rev freely when stationary, but when on the road when you try to push on a little it starts missing an losing power (around 3500rpm ). If i floor the accelerator it all but cuts out, I have to lift off and coax the revs. back. Now I do not know wether it is a fuel or ignition problem.
Any help with how to isolate the fault between fuel or ignition would be very helpful
Thanks Gordon

Answer
Hi Gordon,
Read my answer to the 1973 Ser. III Jaguar V-12 in the Jaguar section. You may need to run the same tests but your symptom sounds more like a stopped up exhaust system. To test it you need to run a long hose out from under the hood from a port on the intake manifold and run it to a vacuum gauge. At 3500 RPM and at full throttle you should never see less than 5 in. Hg. on a gauge. If you slowly open the throttle from when it is cruising correctly and you see a steady loss of vacuum all the way to "0", You probably have a partially stopped up exhaust.

To be sure on cars that it is a lot of work to open up an exhaust I found that I could take a short piece of brake line about an inch or so long and drill a hole in the pipe up front anywhere that is easy to get to and the hole should be just under the size of the piece of brake line. I just drive the piece of line into the pipe and put a vacuum gauge on it (most vacuum gauges have a small section for checking fuel pressure) put the gauge under a wiper arm and drive the car. All of the British cars I have tested I found that they can not stand more than 1.5 PSI back pressure at full throttle and full load. If the exhaust is partially stopped up you will see an excess of 7 PSI at the time of failure. When finished you can pull the piece of brake line out and put a short large diameter sheet metal screw in the hole to seal it. If you are in a shop that has welding equipment you can just weld the hole closed.
Let me know,
Howard