R100RS Top end rebuild.

[blkvelvt]

My 1981 R100RS is getting a little tired. Clutch seems to be good. Doesn't smoke. Starts and runs fine but has a fuel (unburnt) smell when riding. It will roll in gear (not running, clutch out) and compression is low. I think it's the top end (valves). Any thoughts. If it were rings, shouldn't there be smoke and other smelly things happening. Lastly, it has an EML sidecar and goes along fine. The low compression is the worry.

[too old to care]

From my experience the compression on airheads is usually 125 to 140 psi, at least that is what it has been on my bikes. Being even from side to side is also important. Currently I ride a 1978 R100RS; past bikes included an R90S, R80RT, R100RT, and a 1988 R100RS. The smell of un-burnt gas seems to be normal on older bikes; it may just be running rich. When I ride behind my R90S, now owned by a friend, I think I could get a few miles on just his unburned hydrocarbons.

 

An old trick that I used to use to check compression on car engines was take the reading, then squirt an ounce or two of oil in the cylinder and do it again. It the reading goes up, it is probably bad compression rings, if it does not; low compression is probably due to bad valves. I never have tried this with a BMW engine, and it may not work because the cylinders are horizontal and the oil will probably pool in the bottom.

 

IMO, I would pull the cylinders and do both a ring and valve job. Easy job.

 

[blkvelvt]

Thanks for the input. It is the logical choice. I just hate to mess with removing the sidecar (brake lines, wirings, fairings and attachment struts). Looks like that's what I have to do.

[louisvillebob]

Blk:

 

'Fore u do all that, why not check the valve clearance? Maybe they've tightened up on u--valve recession into the seat and all that which afflicts the '80s airheads. U may be able to restore proper clearance before the valves burn.

[blkvelvt]

Blk:

 

'Fore u do all that, why not check the valve clearance? Maybe they've tightened up on u--valve recession into the seat and all that which afflicts the '80s airheads. U may be able to restore proper clearance before the valves burn.

 

If I can get to the right hand side. The sidecar is real close and I may need to remove the sidecar just to check the valves. I'll let you know if there's room. This is the initial plan of attack

 

[Penforhire]

If you do end up needing head work, I used Motor Works near San Diego to rebuild by /5 heads (http://www.motorworkspromach.com). The work was good at a reasonable price. Hah, "reasonable" is a relative term, I know! They advertise in the Airhead Beemers Club newsletter.