MZ/MuZ Riders

if it hasn't been started for a long time, and the underseat tank's empty because it's all disappeared downstairs into the engine?

It's all very well saying start it up to see if it starts scavenging, but if the oil's dangerously low, you could cause engine damage.

Just curious ....... none of the British bikes I owned ever dumped all their oil into the crankcase <snif> despite being dry sump - so why does the Yam?

There is a non return valve behind the oil pump that sometimes leaks back a little and over along period will drain the oil tank.Have only ever needed to replace one on all the yammy singles I have ever worked on when employed as a mechanic or on my own bikes.
If your concerned about having a crankcase full of oil just remove the main plug in the cases and drain any excess and then top up the oil tank.Starting with a full crankcase most likely to be no problem but will blow lots oil out exhaust for a while until the pump scavenges it all out.
Friend with old british bikes fits a stop cock to his oil lines to stop the back flow as the bikes sit for a while between rides and back flow is a problem, start the bike regularly during your winter and keep that oil flowing and should not be a problem again.

all dry sump systems have a springloaded valve to prevent oil draining into the crankcase. The one in the Yamaha engine is contained the the right side cover and is a simple spring-loaded ball. The rubber seat is very cheap and easily replaced. Usually, it is the raised sealing lip that is damaged and not the ball seat at all. Either way, the oil can drain back to the crankcase over time. Unless there is a large puddle under the bike, assuming it to be your bike and there was oil in it last time it was used, the oil is of course in the crankcase. Where else could it be? If you can get it to start, just let is run at a quick idle for 5 minutes or so and check the level. No damage will be done.
If you don't know, drain the system entirely (as said above) and refill it as in an oilchange, pouring about 1/2ltr directly into the cylinderhead thru the back access cover for the intake valves. Put about 1 /2ltr into the oiltank and ride the bike sedately around to block for 10-15 minutes, then check the level in the tank.

If you do have a faulty checkvalve, replace it. You need the valve seat
93101-14092-00
and a gasket for the side cover if you were not careful removing the cover. You can just as well put the cover on with a silicon gasket maker without a real gasket. I only do it this way.
http://65.213.72.112/tds5/docs/5926-EN.PDF
In a pinch, you can supplant the damaged raised lip with the silicon.

That makes sense - thanks Bill.

It's not that my bike's done anything so dreadful ............ I was just curious as to how an unsuspecting owner could be sure of lubrication in the scenario I presented. I wonder how many folks have come ot a long-stood bike, checked the oil, carefully topped it right up ....... and then watched, horrified, as the bike then gleefully disgorged its oil all over the floor when started up!