Wilsons Sandbox

> This should start a good thread. I have this bike (say a KLR) that > burns 1 gal of per hour (gph) at 50mph. Lets also say for arguments > sake that it takes 10hp to maintain a steady speed of 50mph. Way is > it when I change to a larger jet in the carb the milage goes down? > Now know the typical Harley tuner says "because your burning more > fuel", duuhhh... But if I still require 10hp to do 50mph then the > burn rate should stay the same. If I'm burning more fuel then I'm > making more hp and I need to back off (reduce throttle opening) to > maintain 50mph. Of course I know that the bike is now more fun to > ride hard and I have the 'normal' reaction to twist even harder but > if I'm just cruzin down the 'slab then burn rate should be the same. > right??

Hoo, boy, there's gonna be a gun fight. At this point, I agree with your statement. You're running closer to stoichiometric (HA! Now THERE's a big word! Means the ideal fuel/air ratio for complete combustion.), so you should be putting out more power. I agree, for a given speed, and all other things the same, you should be back on the throttle, so your overall fuel consumption should be the same. Not sure how much the gearing would affect that, at least for small carb changes. Even though you're running at the same rpm, the throttle is closed further, so the vacuum should be higher, meaning lower density and less mass flow. HOWEVER, I don't know all that much about M/C carb operation, so maybe you don't get that compensation, and you will be sucking a comperable mass of air through there. In that case, the gearing change would help. Waaaaaay back when, we had a homework problem in which we had to find the max mileage point for the professor's diesel Rabbit. I think it was the lowest speed in the top gear. I'm curious now. I'll have to dig into this a bit. The only other thing I can think of is incomplete combustion, which would result in wasted gasoline going out the exhaust. But, I don't see why that would happen. I always figured that the main point of gearing changes is to put the peak power at the speed where you want to use it, and maybe that's what happens in this case. Re-jetting the carb may shift the power peak, rather than shifting the whole curve straight up, in which case the gearing change could re-align your cruising speed with peak power/efficiency. Now, if you want to talk hydrogen combustion in severe nuclear power plant accidents, or aircraft fuel tank explosions, I'm your man! Chris -- Dr. J. Christopher Krok Project Engineer, Adaptive Wall Wind Tunnel Graduate Aeronautical Laboratories, California Institute of Technology MS 205-45 Phone: 626.395.4794 Pasadena, CA 91125 Fax: 626.449.2677