Triumph Repair: runs rough, manifold vacuum, combustion chamber

Question
My 73 Spitfire runs rough after it is warm.  After warm, as I try and rev the engine it sputters, the only way to accelerate is to be very light on the gas.  Where should I start to look for trouble, leak in vac?  timing? elsewhere?

Answer
Hi Pat,
You just need to start testing what is necessary for an engine to run. An engine that sputters and don't want to run is missing one or more of the three items that are necessary to run, compression, fire or fuel. A compression tests to see if the combustion chamber is OK. You need to do this because that tells you the engine itself can do what it is suppose to. The conditions of each of the three are where you will find what is wrong. First the condition of the engine, Be sure to hold the throttle open and turn it over at least 4 or 5 revolutions. You want to see at least 125 PSI on all cylinders and maybe as high as 180 PSI. Do the compression test when the engine is warmed up.

When you test the fuel you want to first know if the fuel pressure is good at the time it is running bad. This means check fuel pressure to the carbs at the time it is running badly. Put a "T" in the fuel line close to the carburetors. You want to see from 1.5 to 3 PSI at the time it is running badly. (not more or less)

It is more difficult to check fire (spark) without a scope to look at but you can do something. The timing should not change from when it is cold to when it is warmed up but you can check it anyway. Then connect the timing light to the coil wire (not the plug wire) You will see a more rapid flash but when it is not running right look at the flash. If the problem is in the primary section you will be able it notice if the flashes are not smooth and steady when it is running bad.

It would not hurt to look at manifold vacuum when it is cold and note any difference when it is warmed up and not running right.

When an engine runs poorly when warmed up has a thousand possible faults so you need to narrow that down to a smaller and smaller area with these tests. Let me know what the results of each test is.
Howard