Breathing, And Brake Lights (Again)

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Roger Parker’s explanation in his Q&A piece on engine breathing in the August issue was quite enlightening, but I’m not sure it answered the issue we face with our little MG1100. Ever since I fitted the 1275 engine a few years ago, I have had a lot of trouble with oil contaminating the air filter and escaping through the filler cap, especially after sustained high speed (well, 60mph) driving on the open road. The issue reached a peak one day recently when, after a fairly long burst on the open road, we zoomed up a reasonably steep hill only to lay a smoke screen behind that would rival anything the Royal Navy might have done during World War II. Clearly the air filter canister had filled with oil to the level of its lip into the intake manifold, and the incline had sent the oil straight into the carbies.

I have now installed an oil catch tank in the breather line from the rocker cover to the air filter, and also sealed up the filler cap. I don’t think crankcase pressure and poor breathing is the issue in our case, though no doubt it has contributed. I’m advised by a friend who is quite adept with the A-series engines that if one of the camshaft bearings is installed the incorrect way, oil to the rocker gear which comes via this bearing is pumped up continuously rather than in a pulsed feed. Then, especially at sustained higher revs, the rocker cover can fill with oil as drainage seems to be quite slow anyway, especially on the 1275 engines.