Triumph Repair: 80 spitfire weber DGV vapor lock, fuel pressure gauge, weber dgv

Question
I have a 1980 Triumph spitfire with a Weber DGV 32/36 with electric choke and pacesetter headers.  It has been working fine for about five years and has recently started to give me problems.  It runs great cold but when it comes up to temp it has a tendency to run rough and if you shut it down and wait five minutes it won't restart until it cools down. This sounds to me like vapor lock, but why would it start that now after years of working fine.  It also doesn't do this all the time.  It has been suggested to me that it might be a fuel pump problem or a clogged fuel line. I have not checked the fuel pressure.

Answer
Hi Dave,
It could be vapor lock as you say and the only way to tell is to put a "T" in the fuel line just before the carburetor and connect a fuel pressure gauge to the "T" and monitor fuel pressure after a hot engine and an attempt to restart. Fuel pressure should be from 1.5 to 3 PSI at the time of failure. With a poor running when hot engine you should run a long hose from the "T" to the gauge and place the gauge under a wiper arm so you can monitor pressure when driving at the poor running condition. This way it tells you if it is fuel supply causing the problem.

If the engine runs rough at idle when hot you can do this, Slowly restrict the intake air to the carb with you hand a little at a time. If at any time during the slow restriction the engine either smoothes out or the RPM raises then you have a lean condition either because of a carburetor problem or an intake manifold leak somewhere. If that is ok and not the problem rev the engine a few times to clear it out and let it idle again and have someone at the back of the car watch the exhaust pipe closely as you rev the engine a few times from idle. If they see any puffs of black smoke than the carb could be too rich or the auto choke is partially on. It is a common symptom of a rich mixture to run great cold but run poorly (especially at low RPM) when hot. Another clue is to look at the color of the spark plugs after it has been running for a while. Any signs of flat black color is another indication of a rich mixture.

All of this is on a fuel problem and you can't be sure that is the problem yet. When a car came to me in the dealership with your type of complaint I had to first check the engine itself by a compression test and then adjust the valves and run the compression test again. then I would set the timing and when I was satisfied that was all ok then I would look at fuel with the above tests.
Howard