Iâ€™ve got a 1999 Yamaha SX 600. Itâ€™s a pretty good sled, but I suspect (based on what the plugs look like) that it is jetted richer than it needs to be. It is bone-stock, except for the track, skis, and transfer rods. With gas prices looking like theyâ€™re going to be higher, improved fuel economy would be great.
What can you offer in terms of general jetting advice on stock sleds?
We ideally want to look at each model individually, as some engines are known to be more over fueled than others. From a general standpoint, most sleds come from the factory with carb jetting that is going to provide reliable operation. This means the calibration is generally going to be capable of providing enough fuel on a very cold day at very low elevation for a long pull. Generally. On fuel that isnâ€™t maybe the highest of octane, either. Thus, anytime you operate in less than ideal conditions, chances are thereâ€™s some extra fuel being used.
A big deal? Sometimes it is, often not. Some engines are designed to operate well across a wide range of conditions and fuel amounts, so theyâ€™re not as â€œfinickyâ€ when it comes to needed spot-on jetting to run well. Most of the Yamaha 2-strokes of late and many of the Rotax engines typically fit this description.
Other engines that are â€œsqueezedâ€ for even more performance can become more demanding in this respect, 440 race engines for example.
What one needs to do is first determine what the stock jetting spec is calibrated for. Most of the stock sleds are going to be low elevation and down to â€“20. With your triple 600 Yamaha, I know from experience that at our 1500 foot elevation and cold temps we ran smaller main jets and dropped the needle, but we would often run richer jetting in December and January (colder temps) then switch to leaner jetting for the rest of the season (warmer). Just watch your plugs.
Usually, most any sled can tolerate a one size smaller main and a half-position drop of the needle (using shims). Some bigger triples can take several sizes in mains, each model is different. This is why reading the plugs, analyzing the amount of wash on the pistons, monitoring exhaust gas temperatures give you the information so you can make informed decisions on what action, if any, can be considered.