So my dad probably can't help it, but he's got a soft spot for his very first XT 500 4 Valve, which is an odd-ball model and is a smaller bore XT 600. Now ever since this one turned into a smoke bomb, we said we're gonna investigate and fix the f*cker.
So here we are. The most harmless reason would have been that either the scavenge oil-pump (XT600s are a dry-sump setup) is leaking or for some other reason not sucking enough of the good juice back to the oil tank resulting in too much oil in the crankcase or that the check valve in the clutch cover is stuck resulting in the oil tank emptying itself into the engine, when park for a (longer) while.
That's said check-valve. Poke it with a stick to see if it's stuck. (It wasn't.)
Next step was to work towards the oil-pump. For this the clutch has to come off as it's driven from a gear on the backside of the clutch basket.
With the clutch basket off, a yellow-ish gear becomes visible, which is the oil-pump. Unlike on the older 2-valve XTs and SRs. It's fixed with nice hex-bolts instead of the old philipps, which always put up a terrible fight.
Undo those three bolts and pull out the oil-pump. There's a pretty self-explanatory check procedure to check the lobe-clearances between rotor and housing and even though the pump was working fine, it's over the service limit.
This means the Dre-XT-Stück will be degraded to a parts donor for the pretty one in the rather near future. Either it will donate the complete engine, if I can't find anything obvious, when I take the engine apart further OR I'll just swap over parts.
Now why did my dad park up this fine XT for several years AND didn't show quite the enthusiasm to fix it? Well the reason comes from Italy and you've seen her before. It's his Moto 6.5, a bike so nice, he only rides it whilst wearing a shirt. And it got even nicer now that we've added some progressive fork springs, which together with the modified SV650 rear-shock makes the fine Italian lady quite a respectable weapon in the twisties. (On the first rideout I managed to get it down to the pegs in a roundabout with great ease, just saying...)
The mark is to check the sag with my dad on it and it's perfect: slightly less than a third of the overall travel. (30mm vs. 130mm overall)
And as I wrote before, a bike so nice, you have to ride it wearing a shirt. Luckily it's pretty much sorted by now, so all that's left to do for my dad is to enjoy it this Summer and probably do an oil-change at some point in the not too distant future.