G'day blokes, I'm after some advice or opinions...

I've noticed a 'stain' of oil (I guess it must be oil) on the engine crankcase at the base of the cylinder. It's not much, can't see it on my finger when I wipe my finger over it, and I don't know for sure how long it's been there, but I do wash the bike reasonably often. So I'm assuming it's oil getting past the cylinder base gasket. When I looked close, with the engine running, I could see a tiny bubble form and burst where the stain was (on two sides of the rear cylinder) and again assume it's pressure from the crankcase pushing oil past the gasket.

So, any opinions on what might be the cause and my best course of action. I don't want to pull it down if I don't have to, but that seems it might be the go. I guess taking off the rocker boxes and re-tensioning the cylinder head bolts is probably the minimum? Or is it likely to just be a failing gasket and only a nuisance, and not the first sign of catastrophic engine failure!! 

Stock 80 cube evo, done less than 40,000 km's and never been pulled down. Bike was cold when I noticed it, haven't checked it when hot (which I probably should do, thinking about it).

Thanks....

nuisance only, stock evo common fix, may as well freshen the top end up as well while you have it off.

Very common on Evos....(and a nuisance only)

Tightening the head bolts will not help as its the cylinder base gasket that's gone - most likely the rear cylinder too. Just pull the heads & jugs and replace the gasket with a good quality MLS one and you should be good to go. While you have it apart you should look at running a hone through the jugs and fitting new rings (pretty much a standard thing to do when the gaskets go). Some folks also drop in a new Cam wile they are at it as well - gven you have that much of the engine apart.

and also a set of adjustable pushrods, just in case...

Yep as said above. Seems to be a common issue on the Evo engines. Keep an eye on it and hope it doesn't get any worse.

I wouldn't get upset to quick I put up with a weeping base gasket for years ,but if it is leaking you
Don't have to use adjustable push rods anless your doing a cam upgrade , top set gasket set ,evo base gasket leak fix kit ,hone ,new rings, and maybe svc the heads whilst there off but if you want more power at same time whilst your fixing the leak than that's a bigger parts list

Thanks for the replies everyone, some good advice there. Yup, rear cylinder Tussock!

From all that's said, it seems non-urgent, so I can keep it on the road for a while without fretting. None the less, once I get some spare cash I'll at least do the minimum (gaskets, rings, hone), and I'll be looking at my options to tweak it a little. I've always said that once it started to feel tired I'd freshen it up and add horsepower, although it's a bit earlier that anticipated... hadn't expected it to all start with a failed gasket!

Oh well, every cloud has a silver lining!!