Triumph Repair: Fuel Vaporizing, fuel pressure gauge, spitfire 1500

Question
Howard,

I recently had my carb rebuilt on my 1978 Spitfire 1500.  Now it seems like there was more issue than a bad carb in that after the car has been running for even a small amount of time, when I start it the car starts but it wont get fuel into the spark or something.  I talked with someone who said it may be vapor lock and that I may need to get a heat shield put on above the exhaust manifold. Could it be the octane level of fuel? yesterday, I put in some octane booster hoping to do the trick and it seemed to run a little better, but I didn't have a chance to run it that long. Any thoughts?  the manual suggests a minimum 97 octane.

Answer
Hi Ethan,
When an engine falters (acts like it is not getting fuel) you need to run some tests at the time it is running bad to divide the problem. Meaning it is either fuel or fire. compression will not usually come and go.
Vapor lock as it is called can cause problems on many British cars in the summer time when it is hot and most British cars don't have good hot engine compartment air exhaust. Triumphs use a mechanical fuel pump on the engine so they have to suck the fuel all the way from the tank. any time you lower the pressure of a fuel you lower the boiling point thus the fuel boils at a lower temp than normal. The fuel in the line from the pump to the carb is under higher pressure so the boiling point is raised. However, this is also where the fuel is heated the most. and the fuel in the float chamber is at normal pressure so it is most likely to boil when the engine compartment is hot. A exhaust manifold heat shield is a good thing to have if one is not already installed.
Octane rating is ONLY a measurement of a fuel's resistance to detonation, nothing more, nothing less.
You need to get a fuel pressure gauge and a "T" for the fuel line. connect the "T" just at the fuel line going into the carb and connect the fuel gauge to the "T" and run it out form under the hood and put it under a wiper arm so you can read it while driving. Be careful not to pinch the line at the hood. Get the engine to when it falters and read the gauge when it runs bad. If you have 1 1/2, or 2 PSI or above, You do not have a fuel supply problem so you need to look at carb adjustment or ignition problems.
Let me know the results,
Howard