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MG MGB Technical - Missfire on warmer days

I have a 1972 mgbgt with twin SU's. I use slightly richer needles due to K & N filter being fitted but the car is pretty much standard.

The problem I have is that I get a missfire lower down the revs on warmer days. This happens consistently after the car has been sat in traffic for a bit and the temperature needle has got up to the N mark or above. The symptons are that the car coughs and splutters when I accelerate from lower down the rev range. However, on colder days (i.e. normal running temperature) the car runs very perfectly.

I have balanced the carbs, checked the mixture (via colour tune), changed points and condenser etc and chekced the timing.

I beleive that the B series run hotter at the rear of the engine and wonder if this is affecting the mixutre for the rear carb.

Any comments or advice would be greatly appreciated.
C Broadhurst

I have the same problem, I was told it was due to Vapour lock - on my '76 1800TC Marina engine there is a mechanical fuel pump just to the left of the manifold and the fuel pipes run near this, the fuel is heated when not under higher pressure and vapourises causing slight mis-fire. When under load the pump is working harder causing fresh petrol through... I notice this regulary but on a hot day a week or so ago it was so bad the car almost stalled - pumping the accelerator and it ran fine for the rest of the day (both times the engine had recently been stopped).

There are a couple of solutions to that problem that I know of, firstly heat wrap or a heat blanket on the manifold but this can't be done cheaply on the cast iron unit or replace the mechanical pump with an electric item sitting at the rear of the car. I tried putting tin-foil on the pipes and that has helped considerably (the fuel pipes)

Then again, I was told recently Vapour Lock is shown when you try to start the engine not when it's running and am still learning about engines and the like.
Stefan Carlton 1

Hi.

Vapour lock can cause a slow running engine to splutter or even stop, as well as being hard or impossible to start.

I believe that the underbonnet area of the B does get hot quite quickly, especially on the manifold side, and am considering drilling some holes in the inner wing to allow hot air to flow into the wheel arch, which I understand is a low pressure area when on the move.

I am also considering making some arrangement to switch on the electric radiator fan before the rad thermostat cuts in, maybe using a hinged flap to operate a switch when there is no forward vehicle motion.

Don

Don

This thread was discussed between 11/08/2004 and 13/08/2004

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