Jet Size: How to Get it Right
Mikuni HSR-series carburetors are
remarkably versatile instruments. The standard tuning seldom
needs more than small adjustments to accommodate a wide range
of engine set-ups. One of the more common required changes is
the main jet size.
Aftermarket exhausts have a wide range of flow volumes and the
best main jet size is closely associated with exhaust flow. Thus,
it is often necessary to replace the standard main jet with a
different size to accommodate the wide range of exhaust designs
on the market. However, it is easy to get the main jet right
for a particular exhaust system using one of the techniques described
on this page.
The standard main jet fitted to the HSR42 is a number 160. This
size is correct for stock mufflers. Typically, an HSR42 combined
with aftermarket exhaust system needs a 165 main jet. The general
rule is that HSR42s fitted to engines with loud exhausts usually
run best with a 165 main jet.
The HSR45 has a number 175 and the HSR48 a 190. These jets are
more suited to modified engines with free flowing exhaust systems.
Keep in mind that the main jet does not affect mixtures until
approximately 3/4 throttle. Below that throttle setting, specifically
between 1/4 and 3/4 throttle, air/fuel mixtures are controlled
by the jet needle and needle jet.
It is relatively easy to get the main jet correct. Follow either
of the techniques described below. Both are satisfactory but
the Roll-On procedure is more accurate.
The following tuning techniques might result in excessive (illegal)
speed and increased risk from the speed and the necessary distraction
of doing the test. We recommend that the testing be done on a
closed course (track) or on a dynamometer, if one is available.
The Roll-Off technique is the quickest and is almost as accurate
as the Roll-On method. First, one gets the engine warm on the
way to a safe roadway. If there is room, use fourth gear as this
allows more time to assess the result.
Now, get the engine rpm high enough that it is on the cam and
in its power band. This may need to be as high as 4000 rpm with
some cam choices. Apply full throttle. Let the engine accelerate
for a couple of seconds until it has settled in and is pulling
hard. Quickly roll the throttle off to about the 7/8ths position.
When you do this, the mixture richens slightly for a second or
If the engine gains power as you roll the throttle off, then
the main jet is too small and you need to fit a larger one.
If the engine staggers slightly or has a hard hesitation, then
the main jet is too large and you need to fit a smaller one.