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RM85 engine issue.

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Well, to make a short story long, neighbor brings me his 05 RM85. It literary sounds like maybe a deeper tone of valves slapping when I start it. It only runs for about seconds then shuts off. Obviously, there's no valves to be slapping but what can it be? The first thing I did was drain the fuel so I can start troubleshooting. I'm not a two stroke guy but it doesn't seem that hard. First thing I notice, no color to the fuel. I don't know if they make 2-stroke oil that is the same color as fuel but I am curious. I run a compression check and it's at 85 psi. I don't even know how it starts at all with that. So I figure it will need a top end but I still don't know where that clanging sound is coming from. Being 2-stroke stupid I said I would change the top end but that may only be some of the problem. He agrees, I send the cylinder off to Millennium Technologies and they quick turn it back to me. They notice the seriously lean condition and seen some detonation spots they cleaned up. I threw together the top end and get an increase to almost 120 psi. That doesn't seem like enough but it did nothing to solve the problem of the motor not running. It still has the same clanging sound so I imagine it's in the lower end. I just don't know where to start. Any ideas????? Never being into a 2-stroke, I imagine I'll need special tools but I am clueless.

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hard to tell if i can't hear it usually on a 2 stroke there are no valves and usually the clanking either comes from the crank bearing or when you over bore the cylinder and the metal head gasket is stock bore the piston will hit the slack and cause an annoying sound, but in this case some of the rm85 models the model K5 actually has an exhaust valve which adjusts the port size and adjusts/tunes the intake pressure giving it broader power band, from what i'm reading with the bike running for a few seconds and shutting off shortly after, the problem seems to be with flow and i believe something is up with your exhaust valve which is blocking flow possibly something broke and it's running free in there clanking

 

just my 2 cents

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Thanks for the reply. I thought about the exhaust valves as well. When I sent the cylinder in, I had Millennium check those and assemble them while they were putting the nikasil back on. When I had the top end out, I did rotate the rod to see if I could hear anything odd. I know yanking on the rod is really no indication of anything because I can't spin it fast enough to get anything out of it. I'll open up the clutch side and see about the governor. Maybe something will stand out when I take that off.  

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Well crap, no dice. I pulled the clutch cover and I didn't notice any problems however, when I drained the tranny oil, it was almost a black sludge. Just shows how they took care of it. Other than that though, there was no obvious problem. The only thing I noticed was about 1/8 of an inch of play from the connecting rod. That seems like a lot but I really don't know. Since I have nothing but basic tools and no need for two stroke tooling since I don't own one, it might be going back to the owner. Kind of sucks since he shelled out 400 bucks to get nikasil and a new piston for it.

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If you have a new cylinder and piston at correct piton to wall clearance that thing should have 150psi or better. If that "play in the rod" is verticle that is where the problem is. If the play is horizonal (side to side) that's about standard. Go on youtube and see how to check squish, that also should give you an idea if that piston is coming all the way to TDC. If the bearing is bad you should see heat blueing around the lower big end of rod.

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I was certainly curious as that compression seemed low for a newly reworked cylinder and ordered piston from a reputable business. The play in the rod is definitely vertical and it is blued on the lower end of the rod. I questioned the crank bearing but it still turns freely. Just seems odd. The symptoms you are describing is exactly what I see. Sounds like a case splitting and new crank is in order. Thanks for the reply.

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