Triumph Repair: NG replica of MG, electric overdrive, autumn evening

Question
QUESTION: Hi. I have an NG based on a 1965 MGB with 1800cc 4 clynder engine with
electric overdrive.  I think I have a problem with VAPOR LOCK.  The little car
starts, runs till hot then gets rough and stalls.  After half an hour of cooling
off it goes again.  It is fitted with standard twin SU carbs and Lumenition
electronic ignition. On an Autumn evening it is possible to see the vapor
rising! Would exhaust wrap be  a good idea? I am a little afraid of producing
too much heat in the manifold.  Any advice on how to cure this annoying
problem would be appreciated. Thanks a lot.

ANSWER: Hi Ronnie,
Vapor lock as it is called is the gas in a line turning into a vapor in the line usually before a fuel pump. When a fuel pump draws fuel from a tank it lowers the pressure in the line between the pump and the tank. When you lower the pressure on a liquid, you lower the boiling point so it takes very little heat to vaporize the fuel in a line. When that vapor gets into a pump and tries to push it on the the carburetor/s, the valves in the pump have a difficult time sealing to the vapor and thus it stops drawing any new fuel. I am sure that is at least part of the reason MGs put their pump in the back close to the tank. The fuel from that pump on to carburetor/s is under a small amount of pressure which raises the boiling point and thus lessens the possibility of vapor in the line. Some brands of cars run a return line system to return much of the fuel that the pump brings forward thus keeping the fuel moving in the lines at all times. Jaguar goes to the next step and runs the excess fuel through their AC system to further cool the fuel. The 65 "B" had a heat shield behind the carbs to help keep the exhaust manifold heat off of the carbs and as long as you had some air flow in that area it was not a problem and most of my dealership experience was in FL.

Insulating the exhaust (if you are using headers)will help keep the under hood temp down and you might insulate the fuel lines themselves. (If vapor lock is your real problem)
Howard

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

QUESTION: Thanks a lot for the info.  Any idea where I might find a small fan that could be
installed to blow air on the carbs?  I heard that some Toyota cars had this and
thought it might be an answer to my problem.
Ronnie

Answer
Fiat did that also on one model of the 124 Spider. I am considering doing that to my "Project" MG V-8 conversion and am using four 2 brl down draft Webers from a 308 Ferrari. I plan to run a fresh air duct to the base of the carbs and put a small fan at the front that is switched on as soon as there is no forced air coming in from the front. I plan to use one of the small 12v fans available at any auto parts stores. They are used to blow air on the driver and have a universal mount.
You may or may not want to go to that extreme but those small fans are not expensive and could easily be mounted.
Some car manufactures just run their electric radiator fans on after the engine is shut down just to keep air circulating under the hood until it cools down a little. Trouble with this system was that when the thermo switch failed it ran the battery down and stranded the owner.
Howard