I currently have a '78 standard 308 degree cam in my '77 R100RS squish band engine (40mm exhaust etc.) I'm thinking about putting a '77 cam in (or making an adjustable cam sprocket.) How much would it affect the performance of this otherwise stock engine? I really would rather avoid all of the work and ride instead.

I did a little research to be sure my memory was accurate, and I re-discovered that the only difference from 77 to 78 in the cams isn't in the cams. It's the cam SPROCKET, which has the 6-degree change in the keyway. Since the 77 and 78 sprockets will interchange, you can just fit a 77 sprocket (11 31 1 250 253) and retard the cam timing. But you should check your cam timing first to be sure that you actually have the advanced timing in your '78. As most '78 models that came into the US were made and imported before Jan. 1, 1978, which was the emissions cutoff date for non-emissions-regulated motorcycles imported into the US, they didn't HAVE to have the advanced timing. those produced AFTER Jan. 1, 1978 did HAVE to have the advanced timing and other modifications that BMW used to meet '78 emissions control regulations.

There's a pretty easy way to check whether your bike has "symmetrical" vs. advanced cam timing. It'll take about a half hour. Here it is:

1) remove the spark plugs and valve covers.

2) shift the transmission into fifth gear.

3) turn the rear wheel in the driving direction, until the exhaust valve rocker arm pushes the valve in and returns almost all the way out.

4) as a exhaust valve starts to come out the intake valve will start to go in. This is the overlap phase.

5) lay a straightedge across the adjuster locknuts (not the end of the threaded adjusters), from the exhaust rocker to the intake rocker. Looking straight down on the rockers and straightedge from above, the straight edge will change angle relative to the valve cover gasket, as you turn the rear
wheel When it is parallel to the gasket, STOP.

6) now remove the timing plug and observe the position of the OT mark. If the OT mark is perfectly lined up with the mark on the case (center of the window), then the timing is symmetrical. If the OT mark is at the bottom of the window (about 3/8" BELOW the mark on the case), then you have the advanced timing cam sprocket.

The 77 CAM setup was symmetrical, the 78 and later was the advanced set up.

-Tom Cutter

(Originally posted to the Airheads list)