Jetting a sled and reading plugs, piston wash by MrViper700

How to Jet a snowmobile and read the plugs and piston wash:

This is another example of something that people ask for all the time and is quite easy for them to do. When jetting a sled you’re going to be working in steps, first will be the low speed circuit, this involves the pilot jet and the fuel screw settings.

1.) Low speed circuits: With the correct pilot jet and fuel screw settings the sled will smoothly gain rpm and not cough back thru the carbs or backfire thru the exhaust, it will cleanly gain rpm from a dead hit off idle.

A.)Too much pilot jet will result in overly rich idle, fouling plugs and "wet" looking plugs. The piston top will be washed clean of carbon and be a new like silver color. Anytime you clutch a sled very aggressively and or make motor mods you most likely will need to richen up the low speed circuits, this can be from simply turning out the fuel screws more or bumping up the pilot jet size. Again you’re looking for good clean stable idle and dry plugs.

(Rule of thumb)- if you need more than 2 turns out on the fuel screws to have a stable idle speed and smooth rpm gain, you need to bump up to the next biggest pilot jet.

2.) Needles: This next step will be where you need to ride the sled for a distance, because the carb circuits all overlap and it takes a bit of distance to get the circuit you want to tune to be free of the overlapping circuits. The needles are where most all people tend to overlook, it’s also one of the biggest reasons why people burn down their newly piped viper, big bore srx/viper engines, modded engines etc. The needle circuit overlaps with your low speed circuit we discussed above, it controls from 3/8-5/8 throttle settings, your mid range cruising speeds.
To check your needle circuit you need to take the sled to an open field and run it at half throttle for a good 500-600ft, steady throttle and then with your left hand, reach over your hand holding the throttle open and hit the kill switch. Keep the throttle at the same exact opening till sled coast to a stop. Remove the plugs and observe the burn on the electrode, ground strap and porcelain insulator, then observe the piston dome, it should be somewhat dry, slightly black/brown appearing and have no signs of detonation on the edge (little pick marks)

Needle tuning tip: There are 2 little white plastic washers in your carbs, these will stick to the slide and to the mounting block, so be careful to not lose them or neglect installing them correctly. They will allow you to place the needle in a half clip spot, by either shimming under the clip or on top of the clip, this allows you to super fine tune the needle settings for crisp performance and perfect mixtures.


3.) Main jet circuit: Lastly you’ll need to adjust your main jet size, this jet is what everyone thinks can keep an engine from blowing up but actually the needle is far more important in a trail sled where steady speeds(midrange) are held for long periods of time, so don’t skip the needle!!!

This jet controls the 3/4-wide open throttle settings, it is responsible for the coloring of the piston/plug on long runs over 600-700ft, you won’t be completely on the main jet till after 500ft, the carb circuits all overlap and it takes a bit to get completely on the main. The pilot jet you tuned earlier also does contribute to a small 2-3% total fuel flow even at wide-open. I have posted pics below to show you different piston tops to help guide you on the coloring of them and what a sparkplug should look like if you get all 3 circuits tuned in perfect. I also will add some pics of different plugs as I do tuning here in the field to show you so check back to see more plug pics in various stages.

In closing there is actually allot more things to look for in a finely tuned race sled or if you want to be at your best for the weekend grudge match on the lake. There is a hidden indicator that you can look at if you happen to be freshening up your engine, if you look to the underside of the piston dome you’ll see an indicator of how well you have jetted your engine overtime. The perfect looking underside dome will be a dark Hershey bar brown and about the size of a dime or nickel, this means you have the optimum mixture and temp in the piston to make good power. If it’s large and filling entire dome top and flakey black it’s too rich and also too rich in the way of oil/fuel ratio. If you see the underside is a grey/whiteish color and dry, it’s called "death ash", you can guess as to how it got its name, this will be shown in a blown up seized engine on a lake run.

If you place your cursor over the pic, it will identify the number:

Pic# 33= Good wash, very good overall jetted trail riding piston

Pic #34= slightly lean, you can see the dryness and the piston top wont color its whole surface, this is where the sled will feel "flat" just like its making noise and not gaining speed.

Pic #35-Detonation starting, the pic is bad or you’d also be able to see the spotting on the ring lands, its best described as looking like you washed your car and the sun made water spots on it, the ring tops will look just like spotted car, this is the first sign of too lean. The edge of the piston also gets a small picked looking appearance to it; best described as little ice pick indentations on the squish band edge of the piston, too low of octane and or lean jetting contribute to this.

Pic #36- Too much oil and rich, too much carbon build up top and little wash marks(clean spots from swirl of transfer ports on dome edges), You can’t tell from the pic but the carbon has begun to stack up in the dome, layered effect. This is usually how a stock jetted sled will appear. A piston can be even more rich than this but will be a wet, silver color (none). This piston is as close to a stocker as I had on hand to show an example. It’s not the most prime example but a pretty close one.

Spark plugs

sparkplug observation: there’s 2 pics here, (LEFT) is a right on the money spot on jetting, but this would leave you not very much room if there were a temp drop in the night, but you can clearly see the center electrode of the plug silver across the tip, the ground strap is burned halfway back into the curve, the porcelain has a nice brown finish to it, if the plug would have been newer instead of a used plug you’d see the porcelain white with a "smoke ring" about half the way down it, this means you’re in the sweet spot! Also, if you’re running octane booster it will show up on the porcelain with a rusty red finish! Race gas will make the plugs a dull grey color and very hard to read, that’s why you’d need to cut the race gas with some pump gas to get some "coloring" on the plug to be able to accurately read it.

The other pic (right) is of a trail friendly plug with a small bit of safety in there for a temp drop at night. If the same characteristics from the spot on plug are seen but the center electrode core is just silver on the outside edge, with a darker to black center on it, you have a little bit of safety margin in there, this is a excellent trail sparkplug and will yield excellent power from the sled. The first plug is what I would want when I am racing. (Remember the center will be more white if a NEW sparkplug is used then an old trail ridden one)

Inside the piston dome

This is the hidden indicator of how well your tuned in! Small sized brownish spot, means very good tune-(right pic), a large amount of coloring, black, and flakey means rich and too much oil as well(left pic). (Move your cursor over pic to identify the number)

Different oils can affect this though, yamalube will burn black and bone dry, synthetic oils will be more wet and brown in color.

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