600HO Carbed Motor Rebuild?

[GoBigParts]

This is a new one for me.. I have a question about my 2008 RMK. Its a carbed version of the 600HO. My wife rides this sled now days and I hopped on it and it didn't seem like it was making top RPM. However, I never really checked because I had a ski alignment issue I was more concerned about. So I decided for good measure I better pull the exhaust valves and make sure the bellows aren't blown and the valves are moving freely.

First valve I pulled was the PTO side. No sticky valves and it slid right out. I disassembled and started to clean everything up. The bellow had a grey greasy substance, so I sprayed a little WD 40 on it and wiped it out. As I continued to wipe and clean it I found a lot of the gray must have been metal shavings because I started seeing them.

At this point I finished cleaning, replaced that bellow and did a compression test. PTO is at 120. Well that works. Then I checked the Mag side and I was at 130. So there is for sure a difference. Pulled both exhaust port and depth miced the valve stops. Both appear to be around .480" deep from gasket. Then I just layed the valve blade in against the stop and turned the motor over by hand with the clutch. I for the life of me cant feel any contact or see any movement of the valve as the piston goes by. So I believe the stops are still good.

I will attach some pictures for you to help me figure out the metal fileing issue. From what I see the lower ring looks pretty worn. Could simply putting new pistons and rings in fix the issues, or do I just run it (that one makes me nervous for future trips) or do tear it down to see more?

 

[Jay]

Has the lower ring collapsed?

 

[BeartoothBaron]

When my sled ate a piston, I had a bunch of gray gunk on the associated EV bellow. Those flecks definitely look like aluminum. On the other hand, the pistons in those pictures look ok to me. I'd lean toward at least digging in a little further over trying to run it as-is. Also, it might be the difference between just doing pistons and having to re-plate a cylinder (or worse). Hopefully nothing worse than a top end job at this point...

 

[volcano buster]

Any metal flakes on the business end of the plug(s)?
Might not hurt to pull the Y-pipe off and get a little better look at the front side of the pistons.
Do you have access to a bore scope or leak down tester?

 

[GoBigParts]

I have torn the entire motor down and really don't see anything wrong.

 

[GoBigParts]

See video:

Sent from my Pixel 6 Pro using Tapatalk

 

[BeartoothBaron]

Yeah, I'd agree nothing looks that bad. Would have been interesting to see what a leakdown check had revealed. Could be one or both piston skirts have just started to collapse. If it were me, I'd put in new pistons, maybe give the cylinders a light hone, and it should be good to go.

 

[GoBigParts]

Yeah, I'd agree nothing looks that bad. Would have been interesting to see what a leakdown check had revealed. Could be one or both piston skirts have just started to collapse. If it were me, I'd put in new pistons, maybe give the cylinders a light hone, and it should be good to go.

Yes, no leak down but did do compression test. The side with the most aluminum was at 120psi and the other side was at 130. New pistons, rings, and wrist pin needle bearings going in as well as new seals & gaskets..

 

[05rmksteve]

I see 2 things slight wear marks on the piston and rings from the corner of the valves and a little bluing on the connecting rod and bearing (might just be the lighting in the picture). In my experience when the valves start to make contact with the piston or ringing it usually shows wear marks on each side of the ring from the points of the valve. But yours look good. I've rebuild a few of those motors cfi and carbed 3 where because of crankshaft failures 1 was seized and 1 was the valve caught a ring.

 

[GoBigParts]

I see 2 things slight wear marks on the piston and rings from the corner of the valves and a little bluing on the connecting rod and bearing (might just be the lighting in the picture). In my experience when the valves start to make contact with the piston or ringing it usually shows wear marks on each side of the ring from the points of the valve. But yours look good. I've rebuild a few of those motors cfi and carbed 3 where because of crankshaft failures 1 was seized and 1 was the valve caught a ring.

On this same sled with a different motor my valves did make contact with the pistons and rings. That is why I replaced. I could have fixed but found a really low mile used motor for about the same or less than the fix. This motor looks pretty good for being in the chassis 10 years. First time its been opened up.

With the pistons out and the cylinders setting on the bench I installed the valves into the cylinders so they are sitting on the seats. I am a long way from inside cylinder wall. They are sitting back maybe .060"? I kind of see it now if I really zoon it. Not sure if that is where from the port opening or if the valve made contact. But with inspecting it, I am not sure how it could have.

 

[05rmksteve]

Motors can do some unexplainable things.
I've had a few 800 cfi pistons whith wear marks in the same area. On 1 motor the rings where almost Wore flat. I checked the clearance and there was plenty. Only thing I could think of is that the piston and valve expanded quicker than the cylinder. I ground a little bit of the end of valves to give it a little more clearance. I don't recall exactly how much clearance there was.

 

[GoBigParts]

As an Engineer I like to know the cause. If the valves did make contact it was supper light because other than a little color difference there are know burrs or dents in the pistons or rings.

 

[GoBigParts]

I found a spot in the upper case half on the mag side where the crank rubbed against it. left a bur. The weirdest part is the crank rod bearing play is in spec. Also, found one rod tight and on the bearing and wrist pin. So I ended up ordering a new remanufactured crank and installed it with new pistons and rings. Cut he valves back maybe another .032" just to make sure. Got the engine back in the chassis and have a few more things to install tonight and should be ready to fire it up.