Rivera Primo Common Plenum Dual 42mm Mikuni Carburetor Kit (DMK) - Manual (Page 8)

The jet that gives the shortest time is the correct
jet. This method is simple but effective.
FUNCTIONAL TUNING RANGE OF
COMPONENT PARTS
For best results, the accelerator pump nozzle
should be pointed directly at the jet needle. The
nozzle is held in place by the friction of an O-ring
and can be turned easily with a pair of long nosed
pliers. Nozzle adjustment should be made with a
minimum of pump strokes to avoid flooding the
engine.
SERVICING
There are very few moving parts in the HSR
series carburetor so it will not require much serv-
icing. There are a few items to be mentioned to
assure good performance season after season.
If it is ever necessary to disassembly your dual
carburetors, be certain to assembly all of the
parts back into the carburetor they were removed
from as some parts are not interchangeable
because of the custom nature of these carbure-
tors.
A. If the motorcycle is to be stored for any length
of time drain the floatbowl.
B. The float drain plug (main jet access) should
be removed periodically and cleaned of any sed-
iment that may have settled during long periods
of use.
C. DO NOT drill or modify any part of these car-
buretors for any reason as the result will surely
be more problems. Refer to this Tuning Section
to correct any problems that you may have.
D. If a jet or passage does become plugged, use
only carburetor cleaner and compressed air. DO
NOT push a drill or any other object through the
jet or passage to clean them.
E. Typical air filter service is done after about
2,000 miles of use. If the air filter is dirty (50% dirt
coverage) and requires servicing, follow these
steps.
1. PRE-CLEANING: Tap the element to dislodge
any large embedded dirt, then gently brush with a
soft bristle brush.
2. PAN CLEANING: Soak or roll element in a
large shallow pan of K&N air filter cleaner (1/4 the
depth of 1 pleat). Remove immediately and let sit
for approximately 10 minutes.
3. RINSE OFF: Rinse off the element with low
pressure water, tap water is OK. Always flush
from the inside of the filter out. This removes the
dirt and does not drive it into the filter. IMPOR-
TANT: Air dry only.
4. OILING: After the filter has dried always re-oil
before using. Apply K&N air filter oil with one
pass per pleat. Wait 10 minutes and re-oil any
white spots still showing. IMPORTANT: Do not
use gasoline or cleaning solvent to service the fil-
ter element as this will damage the filter.
portion has its range of operation. The tapered
portion has an increasing effect on fuel mixture
from about 1/4 throttle opening as the tapered
section leaves the opening of the needle jet.
Between 1/2 and 3/4 throttle the influence of the
jet needle is the greatest and it controls most of
the fuel flow.
ll jet needle and main jet testing should be done
with the engine near the middle of its rpm range.
Start your acceleration tests at about 50 mph.
The best jet needle position will give the
strongest acceleration.
With the engine at operating temperature, accel-
erate at 1/2 to 3/4 throttle, in top gear from 50
mph or so. If acceleration seems soft or flat and
the engine is slow to respond when the throttle is
quickly opened from 1/2 to 3/4 throttle position,
the mixture is too lean. Raise the jet needle one
notch and repeat the test.
On the other hand, if acceleration is crisp but the
engines hesitates or staggers as the throttle is
suddenly shut down from 3/4 to 1/2 throttle, the
mixture is too rich. Lower the jet needle one
notch and repeat the test.
The jet needle position will be near correct when
acceleration is crisp at mid rpm yet the engine
does not load up during throttle shut down.
TUNING THE MAIN JET
You could in fact remove the main jet from your
Mikuni HSR and the engine would run fine until
the throttle was near the 3/4 mark. The jet needle
and needle jet restrict the amount of fuel getting
into the engine until about that point. There is no
point in changing main jets if a mixture problem
exists below the 3/4 throttle position.
The main jet is the last jet you need to deal with
and is the easiest to get right. If you have the
room, the most effective method for getting the
main jet right is to measure the time required to
accelerate between two points. The start and end
markers should be spaced so that starting at
about 35 mph at the first marker will have you
going past the second at near 55 mph.
Set up markers that are far enough apart, on a
safe road, to meet the conditions mentioned
above. When you pass the first marker roll the
throttle fully open and have a friend (it's easier
with help) measure the time need to get to the
second marker.
DUAL DUAL CARB INST CARB INST ALLA ALLA TION TION DUAL DUAL CARB INST CARB INST ALLA ALLA TION TION
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14
CAUTION
NOTE
1/4 throttle full throttle 1/2-3/4throttle