[meddlesomebilly]

Hello John, thanks for your reply.
As you say the source of the oil has to be through the pump shaft bearing although it does not seem unduly worn. The drain hole in the timing chain compartment can apparently not keep up with it, when I checked the drain hole it had sludge in it which wouldn't have helped, the hole was only 1.5 mm diam so to be on the safe side I opened it up to 2.5 mm. I also fitted a sealed bearing to the magneto drive shaft. I've since done a couple of seventy mile trips on it without a recurrence so I'm keeping my fingers crossed that I've cured the problem.
Tony.

[singleminded]

Mine did this for a while till the breather valve destroyed itself,I never did find the remains of the valve plate or spring..
The other place oil can get into the mag casing is from the oil pump drive being worn.
Someone else has told me that they installed a 'thrower' to the mag drive gear to stop too much splashing out the casing..John

[meddlesomebilly]

My 1929 M90 has a habit of evacuating its lubricant via the magneto drive hole in the back of the timing chest when about 40 miles into a journey, at 30/35 miles there doesn't appear to be a problem. The rate of evacaution is around 30 miles per litre of oil.
I have tried both the original spring assisted breather and the later type without a spring [and checked that they are not fitted upside down!] to no benefit.
The scavenge pump seems to be operating OK and there is no residual oil in the crankcase whilst the oil is being discharged.
After the bike has been left for an hour or so the cycle will repeat itself.
The engine has lots of compression so piston blowback is unlikely.
Has anyone else experienced this phenomena and if so was a cure found?
Tony Pashley.