R80 RT 1982 - final drive problem?
 

Just trying to decide what things to try and fix this summer. One problem I've noticed is that at around 2-2.5 revs very small movements of the throttle often result in a nasty shlunk and thump from the rear section of the bike and an associated lurch forward as a consequence. I assume this is not meant to happen on the R80!

Is this likely to be a problem with the splines of the final drive? What other things should I look into to begin diagnosing this problem.

it might be the gearbox
 

I get a similar thing sometimes with my R80G/S, usually when I am pulling away, I think my problem is a worn shock absorber in the gearbox. I was told when it was rebuilt a couple of years ago that this was worn but the new parts were no longer available unless I had an expensive upgrade to the later type. With mine it is not a harmfull problem, just a bit of a nusance.
Your problem could also be as you suspect with the final drive, I would look at this first as it is easier to check and more harmfull if it fails.

...or you may encounter what I did...
"Dentist needed for final drive..."
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Pull the bike on the central stand.
Turn the rearwheel gently forward, then backwards - feel for any uneveness in motion. If ok, turn on the engine. See to that the rearwheel spinns freely off the ground. 3000rpm - go up and down through the gears; listen to the gearbox and transmission with a "stetoscope" (i.e. long screwdriver, tire iron, or what ever). If there are any bad bearing - you will hear them (sounds like rumbling at a quarry). Worn sprocket gives of a churny sound like chewing on cartilage sort of.
It simply should not be the shock-absorber...