Mark came to our dyno at Biketoberfest in 1996 because his motorcycle - rather suddenly - began to run really bad. The flat graph on the first run told John that the slide wasn't raising. Upon removing the cap from the carb John found a slit in the rubber diaphragm. A hole of any kind in the rubber prevents it from working properly. Since we just happen to keep a spare slide on hand at all times, John replaced the slide and Mark was on his way pronto!

The question remains - what tore a hole in the diaphragm? When John took the carb apart everything appeared fine (except for the slit in the rubber). Maybe a backfire when Mark started up or shut off his bike? No one can really know for sure, but the fix is fairly simple.