One thing we continue to experience each year with many sleds, particularly the Ski-Doo REV models, is where the machine will begin to run erratically, to the point of maybe even missing and backfiring. You clearly know there is something not quite right with the running quality.
This is often caused by poor contact between the spark plug wires and the spark plug terminals. This area is an electrical weak-spot in the ignition system integrity, as there will be a collection of black residue that eventually acts as an insulator, as well as arcing that deteriorates the spark plug end terminal. This is one reason the harder metals used on the solid-plug end are preferable to the softer aluminum ends on plugs with screw-on ends (cheaper).
OK, so your sled isnâ€™t running right, and it seems like an ignition problem. You carefully remove the plug caps, noticing how far down on the plug they were, as caps are often not fully seated and this too can lead to the continuity issues. On the REVs, you need to lift the caps with extra caution, as weâ€™ve seen these easily damaged if you yank on the wire instead of the cap itself. Notice the plug ends as theyâ€™re exposed; ideally there should be some jelly-like material, dielectric jell, on them that could be dried up by now, as well as an abundance of the black electrical residue. Clean all of this crap off of the spark plug if youâ€™re going to re-use it. (Dielectric jell can be bought at most any snowmobile dealer or auto parts store. It is a conductive medium that isolates components from vibration and reduces the amount of arcing that occurs in press-fit connections like a spark plug cap to terminal.)
If youâ€™re doing this out on the trail, it is better to have them clean and dry than leave the old dried up dielectirc jell on there. Often, simply replacing the plugs will remedy the immediate situation, but ideally you want to clean out the inside of the caps, the plug ends, and re-apply dielectric jell. If the plug end terminal is deeply arced and contact with the plug cap connector is questionable, replace it.
Weâ€™ve been blowing the crap out of the plug caps with the air gun hooked up to the compressor, and have even sprayed WD-40 up in there to clean it all out, blown it dry and the applied the dielectric jell when you put the caps back onto the plug terminals.
What kind of mileage will one begin to see this? Seems like anything over 1500 miles it starts to be an issue. Many times it is the first thing one actually does to a REV other than add gas and oil (past keeping an eye on the track tension, hyfax, wear bars, drive belt and greasing the suspension).
You also want to pay special attention to the EXACT routing of each spark plug wire. They should not ride on or come in contact with ANY metal part of the engine, as this will provide a ground return path for the ignition to take. This can be a gradual thing, where the plug wire will start to arc to the metal part it is in contact with. Youâ€™ll even see witness marks on the wire and the metal where the arcing has occurred.
This pretty much applies to all of the two-strokes, and seems to be more frequent on the REV models, but weâ€™ve also dealt with it on Polaris 800s. If your REV starts to miss and run funny, this is first place you should check. The Firecats are particularly susceptible to the plug caps not being fully seated on the plug. If youâ€™re using plugs with the screw-on aluminum terminals, make sure they are screwed on tight as they can cause similar problems as mentioned above when they come loose and the electrical path sees a higher path of resistance, with intermittent contact occurring.