OK ... I promised a report on switching from 89 to 87 octane. Been through several tanks full now ... no difference in MPG ... no discernible difference in the bike's performance ... other than more backfires. The 87 gas does have a lower flash point, so it is going to fire more easily in the exhaust pipe. My 2006 S40 does not backfire all the time ... just perhaps two or three times per half hour. I see that the PO did drill out the "air screw" plug, so that is accessible. If I feel it necessary, I will screw that all the way in and then out close to 3 turns (believe that's what has been recommended). At present, I don't believe the backfires represent a serious threat to the engine. Besides, winter is closing in, and the pipe is cooler. Believe it or not, backfiring has been slightly reduced. Gittin my long johns out.