Carb Jetting Simplified
Tuesday July 22, 2003
From the August 2003 issue of American Rider
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. Before addressing this, I need to set the stage about fuel mileage. The mileage one records is dependent upon a number of factors. The speed at which you travel is one. Mileage plunges dramatically above 60 mph or so-a bike that gets, say, 45 mpg at 60 might only record 30 at 80 mph.
Another important influence is the size of the hole you and your bike poke in the air. An FLHT touring rig needs about 12.8 horsepower to go 60 mph, while a Sportster gets along at 60 with about 10. Headwinds, climbing and elevation all affect fuel mileage. Total gross weight has little influence at steady speeds; however, carburetor jetting has dramatic effects on fuel mileage.
When I talk with someone about fuel mileage, I find it useful to set a test standard. Here is my standard: a steady 65 mph on a flat, windless road. These are conditions most of us can find and safely use. Using this standard, stock Harleys typically deliver 45 to 55 mpg-the lower for the big touring rigs and the higher for the Sportsters. 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.
Keep in mind that stock engines are tuned very much on the lean side of correct jetting. 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. However, that loss need only be a couple of mpg, not 10.
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. 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. The big touring machines usually get closer to 32 with the Dynojet kit.
An FXD or FLST that delivers 36 mpg at 65 mph is running too rich. That too-rich condition has consequences. 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. Climbing ability is a more important concern for those of us who need to go up or over mountains. A 36-mpg bike will probably start misfiring due to its over-rich condition by 4,000 feet, maybe even 3,000. 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.
Stock Harley jetting is very lean from just off idle to about ¼-throttle. This is also true of all road-going bikes sold in America for the last quarter century. However-and this is important-at idle and above ¼-throttle the jetting is pretty good.
Harley’s Keihin CV (constant velocity) carburetor is based on the basic Amal slide carb design from the early post-World War I era. And therefore, it shares similar parts which perform similar functions. Idle and just off-idle air/fuel mixtures are controlled by the idle jet which is fine-tuned with a screw. Both the jet size and screw setting are important.
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. This is the range that is too lean for best engine performance on stock motorcycles. 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. Nearly all riding is done within this off-idle to ¼-throttle range.
From about ¼- to ¾-throttle, the taper of the needle controls the main mixture. One normally raises or lowers the needle to fine-tune mixtures within this range.
The main jet takes over at about ¾ throttle and is virtually unimportant below that opening.
If you would like to learn more about how to diagnose and tune these carb sub-systems, I invite you to download the Mikuni HSR Tuning Manual (www.mikuni.com; click on the picture of the carb and click on the hot link “Manuals”). I wrote this manual for Mikuni, and although it directly addresses the Mikuni carb, the diagnostic principles apply to the Keihin CV and many other carburetors as well.
To get your Harley’s stock carb right, follow these instructions:
l. Buy and install a stock jet needle for a 1988 or ’89 1200 Sportster (H-D Part No. 27094-88). This needle was developed for the early Sportster Keihin CV carb that was not equipped with an accelerator pump. As such, it is richer in the off-idle to ¼-throttle range and works just right.
2. Remove the soft aluminum plug covering the idle mixture screw. Back the screw out to slightly richen the idle mixture (½ to 1-½ turns will do it).
DO NOT do any of the following:
Do Not change the main jet; the stock one is just right with a free-flowing air cleaner and mufflers. Yep, the stock main jet is rich. If you find this hard to believe, use the main jet test in the Mikuni manual to see for yourself. You see, the main jet size is not controlled by emission testing and the government is not very interested in mixtures at full throttle. The factories are free to use any main jet they want and, for some reason, all the stock bikes I have tuned over the past 25-plus years have had somewhat rich main jets, including Evo and Twin Cam Harleys.
Do Not change the slow jet; the stock one is just right with an open air cleaner and free-flowing mufflers.
And Do Not install straight, open pipes, especially long ones. If you do, forget everything I’ve said. Straight open exhaust-equipped engines run poorly in the 2,000- to 3,500-rpm range and no amount of carb tuning can fix that.
— Joe Minton