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Old 08-10-2002, 08:03 AM
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sixtysix sixtysix is offline
No XS but XSive as can be!
Join Date: Jun 2002
Location: Central New York State
Posts: 1,945
Fuel Mileage Addendum
By Gary Granger

Now that Jerry has covered the easiest and most common ways to improve fuel mileage, I will cover some other things (less likely) that may be causing bad mileage.

Exhaust Baffles - If your baffles rattle around or have become loose or are rotting out, this may cause mileage to drop suddenly. I think it has something to do with the loose stuff blocking the exhaust. I replaced my old drilled out Jardine slip-ons with new ones, (the same exact Jardine slip-ons) and picked up about 3 MPG. If you hear the baffles rattling around it may be part of your problem.

Wheel bearings - Anything that causes rolling resistance can cause increased fuel usage and that includes 20+ year old bearings packed with 20+ year old grease. Check them when you have the wheel off and replace them if they seem stiff or notchy.

Petcocks - On my 80 Standard I found the vacuum line to one of the petcocks to be wet with gas when I pulled the tank off. The petcock had failed so that fuel was being pulled directly into the vacuum port in the carb boot and into the cylinder. A rebuild kit fixed this up. A special may not be able to fail this way (not sure), but if any vacuum lines are wet with fuel then some repairs are in order.

Carbs - Besides the already mentioned cleaning and tuning, there are a couple of things that can cause bad mileage with carbs. One is leaking diaphragms, I inspect these with a small intense light in a darkened room to insure there are no pinholes. Another is worn main needles or needle seats (emulsion tubes). Either can cause too much fuel to get by at any given RPM, please see the Factory Pro site for more info on how this works. On early model carbs the needles could also be set to high, which does the same thing.

Weight - Increasing weight and wind resistance by adding a fairing, windshield and/or bags can easily add 75lbs or more. More weight is less mileage.

Engine Condition: If you have poor compression, stretched out timing chain, worn cam lobes, etc. You will not be able to get optimum mileage out of your bike.

Bottom Line on Mileage: A myriad of factors including those mentioned above as well as, wind, towing, ambient temp, driving style, and many other things, can affect mileage. Even different fuel blends can cause mileage to suffer. An XS/XJ in good mechanical condition should get between 33-40 MPG, don’t expect miracles, start with Jerry’s recommendations if you feel the need to improve mileage.
Gary Granger
Remember, we are the caretakers of mechanical art.
2013 Suzuki DR650SE, 2009 Kawasaki Concours 1400, 2003 Aprilia RSV Mille Tuono