The
demand for instructions to jet a carburetor has been so overwhelming that
the following article was written. |
After modifications to an
existing carburetor or the installation of a new carburetor, final adjustments and jetting
changes are required to get the most power from the modifications. Without a dynamometer
and air fuel mixture test equipment, making jetting changes to your carburetor can be very
difficult. Here are a set of procedures that will work with most carburetors, without
regard to the type of carburetor nor the type of motorcycle.
(These procedures are most effective for single carburetor motorcycles)
Check your work:
Whether you are tuning a carburetor on a stock street bike or a
modified race bike, the procedures are essentially the same. Start by making sure the
carburetor's jets, adjustable settings and float levels are at a reasonable starting
point. These would be the recommended setting that came with the installation instructions
for the carburetor or re-jetting kit. While checking the carburetor look for fuel leaks,
signs of air leaks, make sure the throttle works smoothly and opens to the proper full
throttle position. Often overlooked is fuel tank ventilation, fuel line and shut-off valve
operations. All these items are required to ensure maximum fuel flow from the tank to the
carburetor.
Starting the Engine:
Assuming everything is OK, you are now ready to test your handy
work. Warm the bike up to full operating temperature. You may have a little trouble
keeping her idling, but one thing to remember is you now have a carburetor which is not
running as lean out as the factory setup, so DO NOT follow the owner's manual directions
for cold starting. Here is a new starting procedure to use: If the temperature is below
70°, pull the choke out all the way, above 70° pull the choke out half way. If the bike
has been run in the past two hours and the engine is still warm, the engine should start
without using the choke. After starting the bike, quickly adjust the choke to reduce fast
idle to a reasonable level. After 30 seconds, push the choke in all the way, use the
throttle to keep the engine idling.
Warm the engine up for 8-10 minutes by riding a few miles at a slow
pace. The Evolution engine is sensitive to the warm-up time. The engine must be up to
operating temperature before setting the idle and idle mixture. A word of advice, find a
small screwdriver that can be used to adjust the idle speed and mixture screws BEFORE the
engine is hot. Fumbling around trying to determine the correct screwdrivers will probably
result in burned knuckles if you are not careful.
Preliminary IDLE MIXTURE Adjustment:
With the engine at proper operating temperature and at idle speed,
turn the idle mixture screw inward (clockwise) SLOWLY until the motor starts to falter. If
the engine will not idle on its own when you begin this procedure, turn the idle speed
adjustment screw until it does. Throughout this procedure try to keep the RPMs at 900-1000
RPMs. Having turned the idle mixture screw inward until the bike falters, now back it out
slowly, keeping count of the number of turns outwards until the motor begins to run
smoothly. Re-adjust the idle speed set screw as necessary to maintain the RPMs about
900-1000 RPMs. The Harley-Davidson engine oiling systems needs 700 RPMs or greater
to deliver proper lubrication. Blip the throttle a time or two, and observe the results.
If the motor responds with a gratifying blast without backfiring through the carburetor,
the idle mixture is correct. If it backfires through the carburetor, back the idle mixture
screw out another 1/4 turn. Do not go too far, as too rich an idle mixture can cause
problems and poor gas mileage. This sets the idle mixture and idle speed for initial
testing.
FINE TUNING:
Adjusting Idle Mixture
Adjusting Low Speed Circuit(s)
The details of carburetor tweaking and plug reading is a very involved subject, so
you may want to refer to a higher authority after this. If you do not have access to a
Dyno facility, here is a very basic guide that will get the adjustments close.
You will need to perform these tests while the engine is at proper
operating temperature. Accelerate through the gears at full throttle. The bike should
accelerate smoothly, without spitting or hesitating. If at low RPMs, backfires through the
carburetor above 1500 RPMs, increase the size of your low speed jetting (slow speed
or intermediate jet) a couple of notches and try again. If the engine feels sluggish,
sounds flat, blubbers or emits black smoke from the exhaust, turn the idle mixture
adjustment screw in 1/4 turn or reduce the size of the low speed jetting. You will find
the part numbers for the Harley-Davidson CV carburetor on another page in this section of
the Performance Guide.
Adjusting Intermediate Circuit(s)
Adjusting High Speed Circuit(s)
Adjusting the main jetting requires a long stretch of open road with no traffic. You
must be in fourth or fifth gear and running at 3500-4000-4500 RPM. then open the throttle
all the way to the stop, noting the feel of the bike. Immediately let off about 1/8 turn
and note the feel of the bike. If the engine just slows a little, the jetting is very
close. If it seems to accelerate some when you let off the 1/8, your main jet is too lean.
If it hesitates or the top speed is poor (i.e. less than 80 MPH) you are too rich. Adjust
the main jetting accordingly. Use your common sense and seat of the pants feel and you
will get close enough to do plug reads.
Plug reading is as much an art as a science for most people. Lots of
experience is needed to REALLY do it up right. For most street riders, making sure your
plugs are a nice tan color is usually good enough. Don't bother reading the plugs until
you have done preliminary fine tuning. You will need several sets of new plugs, gapped and
ready to use. After your bike is properly warmed up, after installing a fresh set of
plugs, you will need to do a full throttle acceleration test, making sure the bike gets to
full RPMs in 4th gear. (The Motorcycle Performance Guide staff
recommends you take the bike your local drag strip for this testing. Your local sheriff
usually has no sense of humor about a motorcycle running down his roads in excess on 90
MPH). At the peak RPMs, you will need to shut down the engine before you let off
the throttle, and coast to a stop. Remove the plugs and look at the general color of the
porcelain. If the porcelain is black, the engine is running rich, requiring the
installation of a smaller jet. If the porcelain is bone white or a very light tan the
engine is running lean, requiring the installation of a larger jet. If the porcelain is
tan to light brown, the jetting is close. If additional tuning runs are required to adjust
the carburetor, make sure you install a fresh set of plugs before each run. You can not
read the condition of the plugs unless the plug is fresh.
If you are in doubt as to the jetting, make sure you jet on the rich
side (tan>brown>black). Because of potential engine damage, you are better off
slightly rich rather than lean. A lean engine can detonate, causing permanent damage.
FINAL NOTES:
Large changes to the low speed jetting may require additional
adjustment of the idle mixture again. You should expect to end up with a richer setup if
your bike is light and/or you are running exceptionally good flowing pipes (i.e.
Thunderheader). There are so many variations here it is hard to predict, but the ranges
listed here are for most normal applications. If you try these and your bike still runs
like hell you may have some other problem (clogged fuel screen/filter, bad petcock,
ignition trouble/electrical malfunction, timing wrong, fouled plugs, bad gas, etc. etc.)
You may want to get a seasoned wrench involved if you are in doubt.
|
| The 5 second
overview: When you are limited to street
riding, take note on how easily and quickly the engine reaches the proper RPM for shifting
gears. The engine should accelerate smoothly and quickly through all gears. When the main
jetting is to rich, the engine will feel sluggish and acceleration will 'feel' slow.
You can check the main jet by quickly closing the throttle from wide
open to 7/8 position when the engine's RPM is greater than 4500. If the engine accelerates
slightly, the main jet is to lean. A larger main jet is needed. If the engine hesitates or
misses slightly, the main jet is to rich. A smaller main jet is needed. If the engine just
slows a slight amount, the jetting is very close to correct.
Another test is to accelerate through the gears at full throttle. If
the engine backfires through the carburetor, misses, cuts out or quits running, the main
jet is lean. Increase the size of the main jet. If the engine acceleration seems sluggish,
does not react to the throttle or sounds flat, the main jet is rich. Decrease the size of
the main jet.
The main jetting can be set quickly at a 1/4 mile drag strip. For
this test, you can ignore the ET (elapsed time). Making sure the engine is warmed up, make
your run down the strip making note of your final MPH. Keep increasing the size of the
main jet until MPH begins to drop. Then drop back one jet size. Your jetting should now be
correct.
Information paraphrased from information found
in the Big Twin High-Performance Guide, D. William Denish |