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In general
most people are confused or afraid to handle jetting their bike for new pipes or
High Flow air filter. Here are some simple guidelines that apply to Carbureted,
and also EFI bikes to determine if your jetting is correct or not for your
modifications.
In order for
Harley to comply with EPA standards for emissions, they have jetted or mapped
your bike from the factory in a lean configuration. In some instances, like my
personal 2003 FXDWG, the bike was so lean from the factory, that it coughed or
backfired, on acceleration. Unfortunately, Harley needs to keep a good standing
with the EPA so that you, their customer, can still be able to have a new
Harley.
The examples
used here will reflect that of a 40 mm CV carburetor found on most 96-2003
carbureted models and are written assuming you have purchased a Jetting Kit like
Dynojet. EFI models generally will
use a Power Commander or alternate stand-alone fuel management system to adjust
the mixture.
To properly
jet your bike you will need to obtain at least four new spark plugs for your
application.
For Twin Cam applications, the plug you can purchase at almost any local auto parts store is a Champion Spark plug #RA8HC. You can also use any brand plug that directly crosses to the Champion number.
For
EVO applications, you will use Champion RN12YC or equivalent, for your plug
reads.
Once
your jet kit is properly installed, you will need to adjust your idle mixture
screw to between 1 ½ to 2 ½ turns and set your idle (engine speed idle) back
to factory specs.
You
will need to warm the engine with your old spark plugs. Find a suitable location
that you can make partial and full throttle passes without endangering yourself
or others.
Make
sure you bring whatever tools you require to change your spark plugs, and the
new plugs. Once your jetting is correct you may re-install your original plugs
or keep the new ones you just installed in there. We suggest the use of an anti-seize lubricant on the threads.
Use a small amount only on the thread.
Be careful not to get it on the ground strap or the porcelain before
leaving them in permanently.
For this
tuning session we are only concerned with the front cylinder and its plug
reading. Later you will read you rear cylinder and makes sure it’s in
accordance with its relationship the front cylinder’s jetting.
Now that you
are in a suitable location, have your new plugs and tools, make sure your bike
is already warmed up to operating temperature. Remove your stock plug (front)
and install the new one you purchased. You’re ready to begin.
Start your
bike, DO NOT ALLOW IT TO IDLE, and immediately begin moving and keeping your
throttle at or about ½. Do this while you run through three gears and then dis-engage
your clutch and kill the engine immediately.
Again DO NOT ALLOW THE ENGINE TO IDLE.
Coast to the
side of the road, now you’re ready to read your spark plug (front) for the
mid-range enrichment portion of your jetting. Remove the front cylinder spark
plug, and it’s time to see how well your mid-range enrichment circuit is
jetted.
This illustrates a lean reading. This Illustrates a Rich reading. This Illustrates the Correct reading.
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Midrange
adjustments can be made by leaning out the needle (adjusting the E-clip up a
notch, and possibly changing pilot jetting). If you are lean in the midrange,
you will want to try to move your e-clip on the needle down one notch.
As long as
your midrange enrichment settings are NOT to the extreme RICH or LEAN end of the
spectrum you’re ready now to move on to your main jet readings.
Again to get
a proper reading for your main jets you will need to have an operating
temperature engine and replace the plug you just used with a new one.
Make sure
you are comfortable with your bike enough to make full throttle passes and that
you do not endanger yourself or others in doing so.
This time,
you will start your engine and proceed with a full throttle pass, through three
gears, and follow the same procedure for not allowing the engine to idle and
then shutting it off as mentioned above.
Pull your
plug (front) from the engine, and compare it to the above illustrations. If your
plug reads RICH, you will need to go to the next lower number jet size supplied
in your kit. If you plug reads LEAN, you will need to go up to the next higher
number jet size in your kit.
You may need
to repeat this process more than a couple of times to get your jetting correct,
and may also need to purchase additional spark plugs in order to get your
jetting correct.
Once you have
your jetting correct on your front cylinder, before you start the engine again,
you should read your rear cylinder spark plug. This involves removing the rear
spark plug, and installing a new one. Then again follow the above steps for your
main jet test.
Because of
the nature of how the V-Twin fires, the rear cylinder will always be a little
richer (darker) than your front. If
this is not so, repeat the process once more and check again, if the rear
cylinder is leaner than the front, you most likely have a vacuum leak or leaky
intake seal.
Once your
jetting is correct, screw your idle air mixture screw until the engine begins to
miss or run rough. Then back it out until your engine smoothes out. You may need
to re-adjust your idle screw (on the cable mount) back to factory setting.
Factors That Can Adversely Affect Spark
Plug Reading:
Harleys have
a common problem, in some instances, with the factory intake seals (manifold to
cylinder head) leaking vacuum.
Please make
sure if you end up using a larger than normal or recommended main jet, that you
investigate the possibility of these seals leaking. On older Evo style engines,
you also have the ability to adjust ignition timing, and this needs to be at
factory spec to get a proper read on your plugs. A poor or malfunctioning
ignition system (coil, plugs wires, etc) can also affect the read you get on
your plugs.
Proper
Jetting offers many benefits including; not bluing or overheating your pipes,
not overheating your engine, improved fuel mileage and improved general
performance.
Now go out
and ride with the confidence your jetting is right and reap the benefits of it!
Dyna Jim
©
2003-2005 Copyright