Bought my bike 1 year ago in bad condition, but It run, before i started to restore it. Ok, but heavily smoked out of the exhaust. Standing in my garage, the oil feeding pipe from oil pump to carb was removed, I noticed out of the pipe, oil was dripping. Just a few drops each day... Now , removing the cylinder barrel I saw, crank case was full of oil. Obviously 2 stroke oil passes by the pump for some month, but what`s wrong with the pump? Manual does not give any advice, opening the pump housing, maybe something is worn there after 32000Km?. Further investigation today, I saw no stroke on the pump piston in idle position (should be 0.20-0.25mm ?) The order of nut and shims I found is shown in the picture, but that can´t be right. Re-ordering the 2 shims below instead above the big grommet, clearance at max. stroke in idle is 0.25mm, what seems to be right.

Could this be the reason, oil was passing through? The piston of the pump before was always lifted a bit, maybe that let the oil pass by while the bike was standing?

Any idea from your side? I bought an oil pump repair kit from Yambits, but I do not see any parts in it, which could fit inside the pump housing.

On the outlet side of the pump (line going to cylinder) there is supposed to be a check ball and spring. You have to pull the fitting off the pump and there they are. Maybe missing or gummed up. Either way ......no check valve.....the oil from the tank will run past the pump directly into your cylinder. On the no movement of the pump at idle....you have to look very closely and you might swear it is not moving but at idle it only moves a small fraction...I swore mine wasn't moving either but upon very close inspection....and a feeler gauge it was moving. I don't have the min and max pump stroke in front of me but others will probably chime in. Good luck!

Somewhere on the forum one of the members put a set up clip.It's really good and shows you the displacement that should be coming from your pump.By displacement i mean he made a tool to turn the pump at certain RPM with a drill and has a displacement jug with a measure on it for idle and whatever thousand RPM. He also explains what each shim size will do i.e advance or retard delivery.In other words it's a dynamic not static check.
If i find it i will give you an idea where to find .
Dave

Yes, check valve ball under outlet fitting is the likely culprit... often debris under ball keeps it from sealing..

They have a very fine spring & small ballbearing in the fitting the pipe to the cylinder comes off. Have to remove that & clean out the bumfluff that gets caught in there over 40 years & she'll be good again. just gravity feeds past it if not seating.
Sorry, i see it's already been said pretty much.

I sell auxiliary check valves if yours won't stop leaking.

Thank you very much for support.
I saw the drawing in the manual, but didn't catch the valve seals in both ways. The description says, it avoids oil floating back.

But surely gravity will not push the spring.
I will check the ball seals properly after cleaning. Could hang the pump at 1 meter oil filled hose and watch some days.
Again, thank you all

Hello again chief.
It gets propped open and takes nowt to do it either . Biro springs are twice as strong as that wee spring.
You can get a build up of crap on the oil way orifice and it keeps the ball off it's seat.
I have wondered if stored pressure within the pump can lodge it open too for a time or in fact the weight of the oil it's self via gravity does
it.
Heres the link

Dave

Problem is solved, now. You were right, the ball in the valve was not sealing properly. I tried to clean it as good as possible, but still leaking. So I put a Q-tip in my Dremel and some diamante grinding paste on the batten. Rotate it in the ball-seat for some seconds. Worked well, no drop of oil after 4 weeks.

I have the same problem, fitted a new spring and ball and it still leaks, had it apart 5 times now, tried more new check balls and tried the old spring stretched a bit, even a slightly bigger ball 3.88mm (sold as a 4mm ball) the original is 5/32" and measures 3.84mm all parts are genuine yamaha
I was not sure if the seat was rubber or metal as when I cleaned it the first time it felt like rubber
I suppose the only thing left to do is either get a 4.5mm ball and try it or try and lap it in, but the thought of lapping paste getting into the pump puts me off doing this
is there any other solutions please