Motorcycle Safety & Driveability: Service and Fuel Economy, internal combustion engine, yamaha vmax

Question
Hi, and thanks for reading this question. This question pertains more to standard advice than safety, but maybe you'll be able to help. I brought my 1998 Yamaha VMax into a shop for some service and, after more than two weeks and $500, got the bike back last Friday, with a second exhaust I had lying around installed and the bike jetted - which the place did without asking me first and without using a specific jet kit (I also had the carbs cleaned out and an aftermarket fan switch installed). I've had the bike back for just a few days now and I'm noticing that the fuel economy has markedly dropped. Before, I'd usually get about 90 city miles to 3 gallons before the light went off and maybe 110 on the highway, which is the accepted standard for my model. In the last two tanks I've ridden about 25 miles city and 50 miles highway, and have had to refuel after only a little more than 75 miles. I'm not happy and I'd like to get your advice. Thanks again.

Answer
An internal combustion engine is essentially an air pump. By installing a less restrictive exhaust and changing the jets, you are pumping more air (and fuel) through the engine.

It is possible that it was not tuned correctly, and it's pumping more fuel than it should. The best way to tune a bike for optimum fuel/air flow is on a dynomometer. Check the plugs to see if it is running too rich (wet or carbon buildup on the inner plug; smells like gasoline). If you don't find these things, that's just how your mpg is going to be until you restrict the flow again.

Hope that helps,

Pat