Carb Jetting Simplified by Joe Minton
From the August 2003 issue of American Rider (mpg översatt till
liter milen på vissa ställen)
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 100
km/h or so – a bike that gets, say, "0.52 liter milen" at 100 km/h
might record "0.78 liter milen" at 130 km/h.
When I talk with someone about fuel mileage, I find it useful to set
a test standard. Here is my standard: a steady 105 km/h on a flat,
windless road. These are conditions most of us can find and safely
use. Using this standard, stock Harleys "drar normalt 0.5 liter
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 (0.65 liter milen) is
about right for a Dynojet-kitted FX Harley. The big touring machines
usually get closer to 32 (0.75 liter milen) with the Dynojet kit.
An FXD or FLST that delivers 36 mpg at 65 mph (0.65 liter milen vid
105 kmh) 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.
Rejetting stock CV carbs
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
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
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.
Get your stock carb right
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.
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
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