OK. I did some reading, and the overwhelming opinion is that you will get the best mileage using fuel for which your engine has been tuned. The LS650-S40 has been tuned to use 87. One test report showed that a vehicle tuned for 87 lost 4 percent of its mpg using 89 and 9 percent using 93.
As I said, my ear has been telling me that my Suzy likes 89 better. That is purely anecdotal and has more to do with feeling than fact. I get 50 mpg on the little iron horse now. I will try two or three tanks of 87 and report my results ... assuming however, that the thumper doesn't start shake rattle and roll with the introduction of the 87 fuel. I will (obviously) use the same brand from the same service station. I currently use BP, and I am such a creature of habit I almost always buy at the same station near my home.

Lupo, what you report on backfiring seems to make sense. Higher octane grades burn more slowly and are more difficult to ignite. Unburnt gas is accumulating in the exhaust no matter what octane grade you are using. The lower octane grade is more likely to ignite in the exhaust system and backfire than the higher octane grade. Lower octane gas is designed to ignite more easily, so more backfiring.

Have you turned your idle circuit adjustment screw (right side of carb, high up near engine intake sleeve, under a brass plug) counterclockwise (CCW) 1/4 turn? This got rid of most of my backfiring on 87 octane. (I still get over 50 mpg if I take it easy.)

I'm not sure why this works the way it does. Doing so should make for more unburnt fuel accumulating in the exhaust. Maybe the richer mixture lowers the temperature of whatever is igniting the mix. Lowers it just enough that it's less likely to ignite?

BuckRYCA wrote on 09/23/10 at 05:56:39:


As I said it is very slight and only if I chop the throttle as done on some windy roads. I'm getting 48 mpg playing with it and I'm sure a more mature 59 year old could do better. Now you do give me an excuse to mess with the tune a bit and go for a ride later. When I adjusted the mixture screw I had premium in the tank.  
This octane topic could be as   as an oil thread.

And gap  

floor the car or put under heavy load

87 octane in in the Savage, the Helix, two Kawasaki Voyagers, and a Goldwing...never had a problem with this octane.

Now if you are going to let your bike sit you might want to try something different.

I've always used 87 in all my bikes except the 1980 V50 Moto Guzzi which called for 98 octane Standard Oil gas work the best it would ping 1 time and no more all other gas brands would keep pinging .I once found 95 octane leaded gas in southern Ill. but it would ping on that the same as other gas.If I put in octane booster it did have more power but I didn't use it except once.Standard is now BP.Or was bought out by BP.