Main Jet Size

Tune ups, maintenance, and general help on your sleds

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Main Jet Size

#1 Post by john » Wed Dec 31, 2008 8:00 pm

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 here.

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 so.

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.

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