Tech Notes from Bill

Are you just guessing?
by Desert Custom Cycles
April 2007

Most of us customize our bikes to personalize them after we buy them. Some of the changes we make do not change the performance of our bikes. Chrome here, handlebars there, seats and about anything we think of to make our bikes look the way we want. But and there seems to always be that but, when we start changing the exhaust, the air intake, the carburetors and all the various stage kits from Harley what happens to our bikes. Motorcycle engines whether carburetor or injected all need air, fuel and spark to run. Air fuel mixture is what I would like to address here.

The first thing most of us do with our bikes is put on a nice loud set of pipes. Louder pipes are usually pipes that have fewer or no baffles in them. In doing so we have allowed air to leave the engine at a much faster rate. To compensate for this we also put on a hiflo air box to allow more air into the engine. We can also put different jet kits into our carburetors and after market fuel injection control modules to increase or decrease fuel flow. Changing any of these things can greatly effect the performance of your bike. Are you guessing at what these changes have done to your bike.

To burn all of the fuel and have no excess oxygen left requires a mixture of about 14.7:1, air to fuel, by weight. Your engine will give its maximum power at a slightly rich mixture of about 12 to 13:1. Your best mileage would be at a leaner mixture, sometimes as extreme as 18:1. Leaner engines run hotter than rich engines. Your engine can run at different air fuel mixtures throughout its power curve depending on changes that have been made. We like to try to get a bikes air fuel mixture to between 13.8 to 14.1: 1 through the power curve, as this will give good performance and reasonable fuel economy.

We recommend having your bike dyno'd after exhaust, air intake or fuel deliver changes are made to your bike. With the dyno you can be shown your bikes horsepower, torque and air fuel mixture very easily. We use the Dynojet 250 here at Desert Custom Cycles and would be happy to run your bike and let you take the guessing out of your bikes performance.

Call us at 480-835-6666
if you have any questions.