Remove the carburetor per your service manual. You MIGHT get
away with leaving the throttle cables OR the choke connected to the bike, but don't count
on it. It would be much easier if you remove the carburetor completely and go to a work
bench. I usually leave the choke cable hooked to the carburetor and disconnect the
pull-handle end, taking the entire choke cable/carburetor assembly with me. Your call.
IDLE MIXTURE ADJUSTMENT:
Before we really get started, you might as well remove the
aluminum plug covering the idle mixture screw. Using about a 1/16" drill bit, turn
the carburetor over and CAREFULLY drill a hole in the small plug just on the motor side of
the float bowl. If the plug did not fall out when drilling, remove your drill bit and use
the back side to pry it loose. Some carburetors may require the CAREFUL insertion of a
self-tapping sheet metal screw the get a good enough grip to pull the plug.
Underneath you will find a slotted screw. Turn this screw
clockwise until GENTLY seated...jamming the screw down too tight will ruin both the
carburetor and needle. Now back off about 2 1/2 full turns. This gives you an approximate
starting point for tuning.
Remove the top of the carburetor (slide vacuum chamber cover)
being careful to loosen the throttle linkage stop plate. There is a spring under the top
cover so hold it with a finger until all screws are loose. Holding the carburetor upright
remove the top and spring, then the slide/diaphragm assembly (all one piece). If you look
into the top of the slide you can see some plastic stuff holding down the needle. Pour
this stuff in your hand as you will be replacing the main needle. Before reassembling the
slide you will need to modify it.
Looking at the bottom of the slide where the needle was
hanging out, you will see a second hole off-center from the needle hole. This is the
vacuum hole, and needs to be CAREFULLY drilled to 1/8". Be careful to make a clean
straight hole and keep shavings away from the rest of the carburetor and blow them out of
the slide. Remove any burring that may have occurred.
Now it is time for the critical part for a true craftsman.
The front bottom edge of the slide needs to be chamfered or "radiused". This is
the edge OPPOSITE the side the vacuum hole was on (the front). Use a fine flat metal file
to CAREFULLY smooth this edge from the 90 degree to a "rounded" 45 degree. This
smoothes the airflow. DO NOT file too deep or you will cut through the wall of the slide
creating a hole, thus ruining the slide. DO NOT file the flat parts which contact the
carburetor body as this could cause improper operation. You may need to take some fine
(400 or 600) emery paper and polish the chamfer a bit if the file has left grooves or
With this done you can set the slide aside for now. Open up
the float bowl CAREFULLY and replace the main and low speed jets. These are brass in
aluminum and both are soft- use a light hand and the proper tool. Do not over-tighten when
reinstalling: good and tight is better than stripped and loose. Note: be sure to use jets
numbered for the CV carburetor, and not for the older butterfly carburetors, as the
numbering system is different. Use the HD part numbers listed above to be sure.
Replace the float bowl and the slide. Place the 1/8"
washer over the needle. This will raise the needle, allowing a slightly richer mixture.
Install the new needle and related hold-down hardware, then the spring and top. CAUTION:
some diaphragms are a bit flimsy and are easy to get mis-aligned and pinched in the top
edge. I used a VERY small dab Hylomar applied around the lip of the carburetor body to
hold mine in place while I installed the top. The diaphragm has the appearance of being
"swollen" and seems to be too large to fit into the groove. The repeated up and
down movement of the diaphragm causes it to stretch and can make re-assembly tricky. In
addition to Hylomar, try using the cap to position the diaphragm. It will not only allow
you to evenly push the diaphragm into the groove but you can "wiggle" the top
and feel when the diaphragm is properly located. Later model CV's have a dowel in them at
the cable guides...this can make matter's difficult at times. A damaged diaphragm will
have symptoms of the motor being able to idle well but not being able to take ANY
throttle. If you think you may have a damaged diaphragm, check for any pinholes with a
bright light behind the rubber, pulling on the edges to stretch the diaphragm a bit. If
you find any, you must replace the entire slide/diaphragm assembly and start over on the
drilling and chamfering.
Assuming everything went OK, you are now ready to reassemble
everything and test your handiwork. Be careful not to over-tighten the choke cable
mounting nut as the plastic will break fairly easily. 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 as leaned out as the factory setup, so DO NOT
follow the owner's manual directions for cold starting. Here is the new procedure: Pull
choke out either half or all the way depending on ambient temp (below 70 degrees? Pull all
the way out) and if the bike has been run in the past 2 hours or so (if the motor is
totally cool, pull out all the way, otherwise half or none if the motor is already warm).
Start the bike, quickly adjusting choke to reduce fast idle to a reasonable level. No more
than 30 seconds later, push the choke all the way in and use the throttle to keep the bike
idling while warming up. Some use the throttle lock for this purpose. If you follow the
owner's manual you will certainly foul your plugs, so use your head.
Letting your bike idle for 15 minutes to warm up is not
always desirable, and your bike should be running well enough for a CALM test run around
the block. This will speed up the warm-up time and also give you a little bit of a feel
for the improvement of the modifications. DO NOT blast off and ride wheelies down the
street! Your Evo is as sensitive to the warm-up time as you have always heard, so be
gentle the first few miles. I would go around once to get the temp up then stop to tweak
the idle and idle mixture. Word of advice: find a little screwdriver you can use to adjust
this screw BEFORE the motor is hot, as fumbling about trying to use the wrong screwdriver
will probably result in burned knuckles if you are not careful.
ADJUSTING IDLE MIXTURE:
With motor hot and at idle, turn the screw inward (clockwise)
SLOWLY until the motor starts to falter. If the motor will not idle on its own when you
begin this procedure, bump up the idle set screw until it will. Throughout this procedure
try to keep the RPMs at or below 1000. Now having turned the screw inward until the bike
falters, back it out slowly, making a mental note of the position on the clock. Turn
outwards until the motor begins to run smoothly, then adjust idle stop screw as necessary
to bring RPMs down, but not TOO low. Evo oiling systems need better than 700 RPM to work
properly. Blip the throttle a time or two and observe the results. If the motor responds
with a gratifying blast without backfiring through the carburetor, you have your idle
mixture right. If it backfires through the carburetor ("carburetor farts") you
will want to back the idle mixture screw out another 1/4 turn. Do not go too far, as too
rich an idle mixture will certainly cause you many headaches and poor gas mileage.
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
Go run your bike while it's at operating temperature. Run up
the gears, giving hard throttle and messing with roll-on performance. If it is lacking in
this area, as in spitting or hesitating, increase the size of your low speed jet a couple
of notches and try again. If the bike blubbers and/or coughs black smoke from the exhaust,
go down a size or two. Below you will find the HD part numbers.
The main jet is another story. You must be in fourth or fifth
gear and running fairly high RPM (3000+) 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 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
your 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 us backyard
wrenches, since high-dollar equipment is needed to REALLY do it up right, but for most
street riders making sure your plugs are a nice tan color is good enough. Don't bother
reading the plugs until you have done the fine tuning above, and then use new plugs
(hopefully you started this whole thing with a new set also- and don't forget to check
that gap!). Do some riding which exercises either low speed or main jetting and stop
immediately and check the plugs for the most accurate reading. If your plugs are black you
are too rich, which sucks your gas and performance but will not harm your motor. Too light
or worse yet bone white you are too lean, and motor damage will soon follow if proper
steps are not taken.
Large changes to the low speed jet will probably require you
to adjust the idle mixture again. Keep this in mind when choosing and applying the RTV.
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.
Hope this helps you. And remember, after you flow your heads
and add a good cam, you may need another carburetor like an Mikuni HSR-42 to realize the
full performance potential, but the CV with these modifications works very well on a stock
or near stock motor.
Thanks to all the digesters who contributed to this. There is
really nothing new written here except it is in electronic form to be sent around the net.
Don't ask me why yet another CV article was needed, but I thought it might as well be
These modifications are for off-road use only and are not
approved by the EPA or DOT or Bill Clinton. Some people ride on the road with these
modifications, but I would not know who that would be ;-) Also, the authors nor their
descendants nor their ancestors are responsible for any damages due to following these
instructions. If you follow them correctly and do not screw up you will not have any
trouble anyhow, but there it is.