Fuel Vaporization? (MG TD) food for thought

These cars are reported to suffer from fuel vaporization when the weather is particularly hot.

I found that during recent hot summers, when parking the car for, say 5/10 minutes, difficulty would be experienced within five seconds or so after starting normally, the running being very erratic, without power, necessitating kurb-side throttle blipping for up to half a minute before normality resumed.

Starting straight away after parking or leaving the car for longer periods was without problems.

Reading the Octagon magazine it seemed that a heat shield between carbs and manifold would solve this problem. I made one from two pieces of aluminium sandwiching a 1/8 Hallite gasket.

The results were slightly better; but I did not consider the problem cured. This was 1994.

During the summer of 95 I travelled from Sheffield to the Lincoln Steam rally, a distance of about 55 miles. On arrival I found the classic cars were heading for the arena (display was brought forward) so without stopping I pointed the car in that direction and within 100yds the engine stuttered and stopped in just the same way as it did when suffering from so called fuel vaporization!

Up with the bonnet I felt the carburettors, expecting these to be hot (after the run from Sheffield); but no, they were cold. However the petrol pump was very hot! I gave the pump a good clout and tick-a-tick away we went. [I'm glad I didn't make the arena, it was bound to have happened then].

Thinking cap on. Modify pump to do away with the points. (Yes, I know it is done). I wanted to do it my way to try to prove my belief that the main problem with the fuel is the pump starving the carbs of fuel at any time. (I recalled that my TD always seemed to go better when nicely warmed up than when it had run for, say half and hour, ie when perhaps that pump was hot?)

When the car has stood in hot conditions for a few minutes, heat will pass to the carbs and some fuel is bound to evaporate; but as long as this is replaced immediately on switch-on, the float chambers will be full and no problems should arise. (This did not happen at Lincoln).

To the work bench. As a model engineer [hobby 2] and an electronic engineer [many years of working at it, now retired] I soon had the points replaced by a drop-on printed circuit using infra red detection and without making any modifications to the pump. The spring contact is removed; but can, with only a screwdriver be retrofitted in an emergency.

The system worked straight away, without any mods. There has been no further trace of fuel vaporization, the car now runs as well throughout a journey as when just warmed up and the pump only runs aired. This test period is about 3,500 miles in 18months. QED.

Another part of the equation is fuel supply and consumption. The max output of an SU pump is eight gallons per hour. Any car fitted with such a pump would return a fuel consumption of 10 mpg at 80 mph, so to my mind the allowance for missed strokes on the pump should be enough. But it obviously isn't or we would not find this to be such a problem. Don't forget that some tuning manuals recommended fitting two pumps also the 1500 TF had a high pressure one.

In my view these were all vain attempts to correct what is effect a faulty design; but nevertheless only really able to be corrected by the use of technology that was just not available at that time.

Anyway, I am completely happy with my modified pump, I would be happy to go into constructional details depending on the response to this article.

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