It's probably easier and safer to take somebody along who does know them. If you find one just put a message up on the board, I'm sure the lads will be more than willing to lend a hand
Bevel boxes - check for leaks and noise when rotating the back wheel (on the stand).
Electrics - the charge light will be on at tickover but should go out just above that. Ideally take the tank off and check for corrosion and the overall condition. Expect heated grips not to work.
Clutch - at tickover the clutch may be noisy but any rattles should stop when the clutch is pulled in. If it still makes a noise then the gearbox may be suspect.
Gearboxes - I've had four airheads and the gearboxes have all been slick and worked without clunking (unlike the 1100/1150) check for gear selection problems while testing the bike.
Drive shaft - The paralever driveshaft was a known weak spot (more so on the 100) spin the wheel backwards and forwards and check for ominous clunks. I think a new drive shaft is around £200. Knocking sounds could simply be the swinging arm not set up correctly but be cautious if it does make a noise.
Shock - check for leaks and good damping. An upmarket replacement will cost at least £300 or a replacement original around £200.
Main stands - the rubber bungs come adrift on the stands and they scoop up water. A replacement just cost me £85 so worth checking out.
Camchain rattle. if there's a rattle from the engine it's probably the camchain - just over £30 and a couple of hours work to sort out.
Oil light - should not be on when the engine runs - if it's on at tickover it could be simply a gasket fitted to the oil filter that shouldn't be there or could be the oil pump worn (or something more serious).
Rubber gaitors on the drive shaft - check for splitting and signs of oil leakage.
Paralever bearings - check for sideways play on the back wheel.
Exhaust - Most will have stainless systems fitted. The paralevers run much better with the standard collector box fitted, instead of the usually fitted Y piece, but these corrode badly. A decent original collector box is a plus point. The later Paralevers had stainless silencers fitted as standard; but came with chrome downpipes and collector box.
R80's put far less stress on the drive train than the 100's but the extra grunt of the 100 is nice. Don't rule out the 80 though - the engines sweet at lower revs, the 100 is much lumpier at low revs but happier at higher speeds (unfortunatly the brakes aren't
)
This list could go on forever.
Don't try and buy the cheapest. It's worth paying a few hundred pounds over the cheaper ones to get a bike in better overall condition. The newer the better. mainly because of their age Monolevers are a minefield unless you don't mind taking the chance that you could end up doing some serious spanner work.
Try an airhead before you think of buying one, they aren't to everybodies taste, if you're ever in the East Mids come and try my R80 out.
