Springtime arrives slowly over here but today I went to the shed and gave my '69 DT1 a little check up wether it would be alright for the new season. Checking the gearbox oil I noticed that the gearbox is completely full of oil. I wondered where it came from and the only explanation I found was that some two stroke oil must have found its way into the gearbox over winter as the oil level in the two stroke oil tank has lowered considerably too. I remember filling that up before I stored the bike three months ago.
Looking at the parts list there is one little seal on the oil pump drive shaft (93101-10002 OIL SEAL S-10-22-7) that could be faulty. Has anyone had the same issue? Is it this bad seal or a problem within the oil pump? Any experiences/ideas?

Cheers,

Nils

I have not had this issue with the 1968 to 1971 250s or 360s but I have had many issues of this on the 74-76 250,360 or 400,that is two stroke oil leaking into the transmission side of the engine.

on your 1969 DT1 I would suspect that seal is bad. the oil pump drive is on a jack shaft and there is a bushing that supports that shaft and the seal that is in the primary cover. make sure the bushing and oil pump drive shaft has no excessive play or else the replacement seal may not last long. if that bushing is ok,then I would replace the seal and it should solve the issue.

the jack shaft should rotate smoothly in that bushing that goes into the primary cover but should have no axial play in it,no front to back or up and down play. I have heard of those wearing out and cause this but I never experienced it personally. when you get it apart,you should see what I am referring to.

on my 1975 DT400 for example,the oil pump is driven right off the crankshaft and it needs a new seal.if you park it for a few days,it will drain all oil from the oil tank and the 2 stroke oil ends up in the transmission thus over filling the transmission.

I have same problem with a CT3, when left for long periods, so decided to change the oil seal. I was a little unsure if the original seal had been put in the correct way so trawled for diagrams. (Not sure if there is more pressure at the pump side of the seal or the gearbox side). Think the third diagram just about explains the seal direction, and is the same direction as my original seal, so am opting for that.  Thought diagrams may help someone with any other pump problems.  

A standard lipped oil seal always faces the fluid it is intended to seal in (the side with the garter spring is the front face). If keeping two fluids apart as with the pump and transmission oils, then it must face the fluid with the higher pressure.

Reason for this is that an oil seal is designed to exert a greater sealing force on the shaft as the pressure on it rises.

Great explanation thanks Tinkicker, guessed that was the case, but was a little unsure if the pump or the gearbox was potentially at a greater pressure. Seems the gearbox wins the battle of pressure, as the two stroke oil entering the pump is only gravity fed from the oil tank, whereas the gears and clutch are thrashing the oil around the box.

No.  In fact to introduce an Americanism. Hell no.  The gearbox has a breather. So it is always at atmospheric pressure.