As an ex-mechanic the way I would approach if it was a customers vehicle, would be to carry out an oil pressure test to confirm one way or the other what the oil pressure was.Simply remove the switch, insert an adaptor and attach a gauge. Removes any doubt whatsoever before you embark on an expensive job.
Something like this maybe-

http://www.ebay.co.uk/itm/Oil-pressure- ... 2ec336eb00

In relation to the cost of an engine job it's peanuts.

I think that's been done 2up. But the tech has noticed it's a circulation problem which does seem to point to a deeper internal problem in this instance. I've had it on cars but not bikes, with almost no oil pressure showing on the guage. Not always easy to find the problem without mucho taking apart as you no doubt know. It can be a right bugger to sort.

Data wrote:I think that's been done 2up. But the tech has noticed it's a circulation problem which does seem to point to a deeper internal problem in this instance. I've had it on cars but not bikes, with almost no oil pressure showing on the guage. Not always easy to find the problem without mucho taking apart as you no doubt know. It can be a right bugger to sort.

Ah yes missed that. Took a V8 Range Rover engine out once and sent it to the reconditioner, a very experienced chap in his 60's. When it came back it knocked like hell so the car ended up back with him. Took him some time to find the problem, sludged up oilways.

Cheers, Data... this may well become my summer project.

Barry
Had a quick look at the B650 manual, the engine has two pumps and an oil pressure regulator which is a piston in a sleeve before it fits to the sump filter. Something I've never seen before. If the piston controls the pressure, could this be your problem, it's stuck.
I was looking for seals that control the feed to the crank, then saw this strange design.
The crankcases need seperating to get to it.