Rivera Primo Mikuni 42mm Carb - SH & XL - Manual (Page 7)

7
While a black and sooty spark plug is a sure sign of richness,
there are others that are a bit more subtle. If your engine
responds crisply at low throttle when it is cold, chances are the
needle jet is one size larger than it needs to be. Assuming, of
course, the idle circuit is correctly tuned & adjusted.
Poor fuel mileage is another sign of richness and because of
the way most of us ride our Harleys, that richness is usually the
result of a needle jet that is too large. The color of the end of
the exhausts is a sign of mixture strength. Dark grey with some
black is normal for today's lead free gasoline. If the exhaust
color is black you can reduce the size of the needle jet.
It may be that you will prefer a needle jet that is slightly on the
rich side of the correct range. A slightly over-rich condition lets
a Harley accelerate a little better at very low (1,000 - 1,500)
rpm and with very low throttle settings. Be aware that you will
lose some fuel economy if you choose to do this.
GENERAL MAINTENANCE
TUNING THE JET NEEDLE
Like the idle circuit and needle jet, the needle, within its range
of operation, has a gradually increasing effect on fuel mixture
as the throttle is opened. From the time (about 1/4 throttle) that
the tapered section of the needle leaves the mouth of the nee-
dle jet, it has a major effect on the amount of fuel entering the
engine. Between 1/2 and 3/4 throttle the needle's influence is
greatest and it controls most of the fuel flow.
All needle & 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 needle position will give the
strongest acceleration.
With the engine at operating temperature, accelerate 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 the 1/2 to the 3/4 throttle posi-
tion, the mixture is too lean. Raise the needle one notch &
repeat the test. If acceleration is crisp but the engine hesitates
or staggers when the throttle is suddenly shut down from 3/4 to
1/2 throttle, the mixture is too rich. Lower the needle one notch
and repeat the test. 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
HSR42 and the engine would run fine until the throttle was
near the 3/4 mark. The needle & needle jet restrict the amount
of fuel 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 & is the easi-
est 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 & 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, to meet the condi-
tions mentioned above. When you pass the first marker roll the
throttle fully open and have a friend (its easier with help) meas-
ure the time needed to get to the second marker. The jet that
gives the shortest time is the correct jet. This method is simple
and effective.
ACCELERATOR PUMP TUNING
The beginning point of the pump stroke is adjusted with adjust-
ing screw #1 on the white plastic pump lever. To start the pump
sooner, back the screw out. Turn the screw in to make the
pump start its stroke at a larger throttle opening. Most engines
will perform best if the pump begins its stroke as soon as the
throttle is moved from the idle position.
The end point of the accelerator pump stroke is adjusted by the
adjusting screw #2 located on the top of the carburetor just
behind the pump lever. Best performance is generally achieved
when the pump stroke ends at 3/4 throttle. It is important that
the nozzle be aimed correctly. The nozzle is held in position by
the friction of its 0-ring seal and can be turned quite easily with
a pair of long nosed pliers. Rotate the nozzle until the fuel
stream strikes the needle. This ensures an even distribution of
fuel between front & rear cylinders. Nozzle adjustment should
be made with the engine stopped & a minimum of pump
strokes to avoid flooding.
FINE TUNING THE PILOT SYSTEM
NOTE: Before you apply any of the information in this section
be sure that the basic pilot system is correct. Be sure that your
engine idles smoothly with the mixture screw between one and
three turns out from the seated position.
The pilot and pilot air jets have slightly different effects on mix-
ture strength within the effective tuning range of the pilot sys-
tem 0-1/4 throttle. When you change the pilot jet, it will have a
slightly greater effect on mixture strength at zero throttle than it
will at 1/4 throttle. On the other hand, when you change the
pilot air jet, it will have a slightly greater effect above 1/8 throt-
tle than it will below that setting.
If your engine has slightly soft acceleration just as the throttle
is raised from the off idle position, the size balance between
SHOVELHEAD/IRON HEAD XL
Installation Instructions