2001 rmk 700 - spark plug gap closed - need advice

[kdecelles]

We bought a 2001 rmk 700 which is not much to look at, but seems to run fine. The sled is stock through and through but had some electrical issues (voltage regulator) and the speedo doesn't work (3300 showing). Other than idling a little high (2K), and drinking oil (1 quart @ half tank) it runs fine.

We were out the other night in a field running passes at 3/4 throttle and it all of a sudden just dies. We got it started and limped it home and found that the mag cylinder was not firing. Upon closer inspection the right plug gap was closed and the the prong/electrode were touching. After much googling, I'm down to 3 scenarios that could cause this:

1. piston hit the plug - likely cause is bearing failure in the crank
2. debris entered the cylinder, came between piston and plug - likely cause is piston ring/skirt implode, or bearing bits from crank?
3. detonation.

The engine is not seized, and compression test is 110 / side. After regapping the plug the engine starts and will run with one plug from left/right.

My questions;

1. Does anyone have any experience with this exact scenario?
2. Is there any scenario whereby this is just a fluke and I continue riding as-is?
3. What is the recommended course of action? A top-end rebuild is obvious, but is a complete tear-down in order to inspect the crank?

Any help would be appreciated

 

[ABMax24]

I do not believe this is a fluke to be ignored.

Is there aluminum flakes on the bad spark plug? If there is the piston is done.

Is the engine noisier than normal, in particular a rattle indicating piston slap? This could indicate piston skirt failure.

Have you removed the pipe or can to look for debris?

The reason I ask is I had a 98 rmk 700 that broke off an entire piston skirt and chewed it up and put it through the motor, the only way I knew was due to a slight rattle in the engine, aluminum on the spark plug, and it ran on 1 cylinder for 20 seconds. When I tore it apart all I found was a piece of aluminum about the diameter of a toothpick stuck in the connecting rod, the rest of the skirt went through the crankcase into the cylinder and out the exhaust. The motor had 120 psi compression and ran for 200 miles after it lost the skirt, I didn't connect the dots until I got home from that trip to the mountains.

Personally I would pull off the head and look for signs of foreign debris on top of the piston. As well I would check the bearings on the driveshaft as they can cause the speedo to quit functioning as they go bad.

 

[Trashy]

Here's a similar thread I started a few years back, with the same issue's you seem to be having. Pics are about 3 pages in... https://www.snowandmud.com/ski-doo-rev-sleds-461/04-rev-800-ho-shortblocks-48130.html

 

[kdecelles]

Thanks for the confirmation guys, we'll start pull it apart this week.

How about the oil consumption? 1/2 tank is about 20 litters, so running at a rate of 20:1. My dirt-bikes run 32:1, but I only know bikes, not sleds.

 

[ABMax24]

Thanks for the confirmation guys, we'll start pull it apart this week.

How about the oil consumption? 1/2 tank is about 20 litters, so running at a rate of 20:1. My dirt-bikes run 32:1, but I only know bikes, not sleds.

I used to run about 1.25 liters of oil to 50 liters of gas, for about 40:1. I would shoot for 40:1 but with oil, more is better than not enough. But I wouldn't trust the gas gauge for an accurate reading, fill up the tank at a gas pump to accurately determine the quantity of fuel used for a given amount of oil.

 

[kdecelles]

We split the cases tonight and found the problem. One of the thrust washers on the crank (MAG side) was missing and we round remnants in the crank, but mostly a lot of gold flakes. Based on the how the piston looked, most of it went out the top.