One normally raises or lowers the needle to fine-tune mixtures within this range. Nearly all riding is done within this off-idle to ¼-throttle range.įrom about ¼- to ¾-throttle, the taper of the needle controls the main mixture. Either the diameter of the straight part of the needle, or the inside diameter of the needle jet, must be changed to affect mixtures in this most used throttle range. This is the range that is too lean for best engine performance on stock motorcycles. Off-idle to approximately ¼-throttle mixtures are controlled by the straight-diameter part of the needle together with the inside diameter of the needle jet, in which the needle rides. Both the jet size and screw setting are important. Idle and just off-idle air/fuel mixtures are controlled by the idle jet which is fine-tuned with a screw. And therefore, it shares similar parts which perform similar functions. Harley’s Keihin CV (constant velocity) carburetor is based on the basic Amal slide carb design from the early post-World War I era. However-and this is important-at idle and above ¼-throttle the jetting is pretty good. This is also true of all road-going bikes sold in America for the last quarter century. Stock Harley jetting is very lean from just off idle to about ¼-throttle. By contrast a stock or correctly jetted engine should get to at least 6,000 feet before getting grossly rich, 7,000 feet is better and achievable. A 36-mpg bike will probably start misfiring due to its over-rich condition by 4,000 feet, maybe even 3,000. Climbing ability is a more important concern for those of us who need to go up or over mountains. Range is an obvious possible problem, although some riders aren’t too concerned about range as they like to stop more often than the bike needs a fill-up anyway. That too-rich condition has consequences. The big touring machines usually get closer to 32 with the Dynojet kit.Īn FXD or FLST that delivers 36 mpg at 65 mph is running too rich.
MIKUNI BSR42 TUNING SERIES
From your bike’s mileage I would guess that you have either a Dyna or Softail series motorcycle 36 mpg is about right for a Dynojet-kitted FX Harley. I have talked with many (easily more than a hundred) owners who have installed the Dynojet kit and who have been disappointed with the results. However, that loss need only be a couple of mpg, not 10. When we modify carburetors to get rid of the “lean staggers” during warm-up and to smooth out throttle response during acceleration within the lower throttle settings, we can expect somewhat lower fuel mileage at cruising speeds. Keep in mind that stock engines are tuned very much on the lean side of correct jetting. I have found that properly jetted Evo, Sportster and Twin Cam Harleys deliver mileage between 42 and 51 mpg, using the test standard defined above. Using this standard, stock Harleys typically deliver 45 to 55 mpg-the lower for the big touring rigs and the higher for the Sportsters. These are conditions most of us can find and safely use. Here is my standard: a steady 65 mph on a flat, windless road. When I talk with someone about fuel mileage, I find it useful to set a test standard. Total gross weight has little influence at steady speeds however, carburetor jetting has dramatic effects on fuel mileage. Headwinds, climbing and elevation all affect fuel mileage. An FLHT touring rig needs about 12.8 horsepower to go 60 mph, while a Sportster gets along at 60 with about 10. Mileage plunges dramatically above 60 mph or so-a bike that gets, say, 45 mpg at 60 might only record 30 at 80 mph.Īnother important influence is the size of the hole you and your bike poke in the air. The mileage one records is dependent upon a number of factors. Before addressing this, I need to set the stage about fuel mileage. I get a steady flow of questions regarding carb jetting and the Dynojet kits, and I’d like to answer them once and for all. From the August 2003 issue of American Rider
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