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Centrifugal Advance on 78, 79 & 80 XS11 Motorcycles
by Blaine HoopesA few days ago I asked if anyone had a newer service manual to confirm what I suspect is a mistake in my Yamaha manual that is repeated in Clymer's. Both claim you should test your centrifugal advance by disconnecting and plugging the vacuum hose and verifying that the timing advances to 36 degrees for every model except the 79SF which they claim should be only 31 degrees.
I just tested my '79SF and verified that the centrifugal advance does indeed behave as I expected which is the same as the all the other models except the '78E. The 78E has a centrifugal advance that only adds 26 degrees to the static so with proper 10 degree static it will advance to 36 degrees at high rpm. All later models (I've only tested a '79SF and '80SG) have a centrifugal advance that adds 31 degrees so you achieve the same 36 degree advance at high rpm with only 5 degrees static. So unless someone swapped out my centrifugal advancer from some other model bike the service manuals are in error.
On models other than the 78E I use 2 small nylon wire ties on the centrifugal mechanism to limit the amount of centrifugal advance to the 26 degrees the '78E had. Otherwise when you change your static to 10 degrees your overall advance will be 5 degrees too high and performance/fuel economy could suffer. Those of you who don't use the vacuum advance/retard unit might as well keep the extra advance for better mileage at cruise speeds.
Cheers,