Older Ski-Doo Engine Issues - I'm Stumped

[skiroule]

OK, I’m looking for some input from you good engine mechanics. My cousin wanted me to take a look at his ice fishing machine (96 Skandic 500 fan). He said it was running great when he started it this fall and gradually ran worse and worse.

It was immediately obvious that the PTO cylinder was dead (although it seemed to have spark and had a new plug). It looked like a mouse had his way with the airbox foam so I thought maybe a dirty carb. Dismantled the carb, cleaned it, and tried it again – no change. Switched the carbs and the carb worked fine on the good cylinder. Ruled out carb and fuel pump issues.

Along the way I did discover that it seems to have intermittent spark on both cylinders but that’s a whole different can of worms.

Here’s the part I don’t get: it looks like the engine is not pulling fuel on the PTO side cylinder. Plug is bone dry even though the cylinder is not firing. It should be soaking wet. Checked compression - 128 on the dead side, 120 on the firing side so don’t think it’s burn-down.

Am I missing something here? Thought it might be a crankcase seal but it seems like both cylinders would have the same issue. Any ideas on anything that would interfere with the intake vacuum on one carb.

Also looking for suggestions on intermittent spark: coil? CDI? Stator? Not sure which is most likely but thinking I should maybe deal with that first.

Sorry for all the questions but I’m stumped at this point and trying to decide if it's worth putting much effort into this sled.

Thanks Much!

 

[doodad]

I would try a leak down test / Pressure test

Unlike all 4 cycle engines, the two cycle engines in all two-stroke engines must have a completely air tight lower end.

The normal vacuum in the 2 cycle engine's lower end will draw a mixture of fuel and air from the carburetor though the intake manifold. Once this fuel air mixture has been sealed off by the reed valve or a piston skirt, the resulting crankcase pressure moves the mixture through the transfer ports upward into the cylinder. A small "air leak" at a crank seal or an intake gasket will cause the usually perfect fuel/air mixture in the crankcase to become slightly lean...sometimes very lean. This lean mixture can quickly result in a seized piston. Air leaks are, by far, the number one cause of vintage engine piston seizures.

http://www.klemmvintage.com/airleaks.htm