After upgrading the carburettor, exhaust and air filter the car initially went well. I had plans to have it properly set up on a rolling road to really get the best out of it but unfortunately it developed a peculiar stalling issue. The car would perform well under heavy throttle application and light throttle application but would really struggle in between the two. So much so that going along at a constant pace was impossible as the car would stall and then pick up again quite violently as the power dipped and then returned.
Further investigation under various load levels revealed that the problem was intermittent - intermittent but happening most of the time as opposed to intermittent and happening only occasionally. This ruled out anything as rigid as ignition timing. The most obvious solution was fuel supply. Given that the newly installed carburettor was believed to be in good working order the fuel pump seemed like the next logical component to look at.
The fuel pump on the Mini is driven by the camshaft and works by moving a small diaphragm up and down to pull fuel from the fuel tank and provide enough fuel for the carburettor to feed the engine. In short, it turned out that the fuel pump on my car had been replaced at some stage in the past - not uncommon, it is 20 years old after all. However, it had been replaced with the wrong pump. An aftermarket alternative designed for the 998cc model. The 1275cc engine originally came with the larger fuel pump. The aftermarket, smaller, cheaper pump is capable of providing just enough fuel for the hif38 to power the 1275cc engine but when I upgraded the carburettor size it demanded too much.
So it was time for a shiny new part!
At around £50 the proper SU fuel pump wasn't the cheapest way to rectify the problem. I could have installed an electric pump for about £35. For simplicity of installation it's hard to argue with the mechanical option though.
The new part on the left is quite a bit bigger than the old one, which was quite reassuring when I compared the two side by side.
Before I had realised the fuel pump was the issue I'd tried adjusting the mixture screw to no avail. It didn't solve the problem because technically there was nothing wrong with the mixture. The SU carb allows you to set the mixture at idle and then the profile of the needle within the carburettor takes care of the air/fuel ratio further up the rev range. The small capacity fuel pump seemed to perform fine when the car was idling. It gave a false impression of the fuelling overall though so a correct adjustment would have been hard to achieve. With the new fuel pump installed and a tweak to the mixture the car ran right again!
Now it's MOT time again. That came around quickly..
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