1988 RM 250 Power Valves Popping Motor

[Nova427]

I searched through the forums for a while and wasn't able to find anything about this, who knows I might be the only on it happens to... Anyway My first bike was a 1987 RM 125, I had the bike rebuilt 3 times and would get about a month out of it, and something would happen where the power valves let go and would hit the cylinder wall or the piston and blow the top end...All 3 times I had it rebuilt by a different person, 1 being myself and my older brother, 2 was a close family friend that worked for a Honda Yamaha Dealer as the top honda mechanic for 20 years, and 3rd by a Suzuki dealership, all 3 times resulting in the bike blowing up after a few weeks. Then after the bike got stolen after rebuilding it a 4th time, I went without a bike for years, then I decided to get one again and my older brother said someone gave him a 88 RM 250 I could have. I got the bike, he told me it needed a water pump seal before I ran it.. Did that and a few other things, Rode it around my buddies yard for about 10 min and the motor locked up, when I took it apart the same thing that happened to my 87 125, the power valve hit the sleeve in the middle of the 2 valve ports and broke it out, and the piston came up and whacked it... So my question is, has anyone ever heard of this, and or do you know what all these people including myself could be doing wrong that the power valves keep letting go and blowing up the bike?? I have had the 250 sitting in my basement for the past 10 years or so, and have finally decided to fix it, but I don't want to go through all the trouble to just have it pop again...Thanks for you time...

 

[rmc_olderthandirt]

the power valve hit the sleeve in the middle of the 2 valve ports and broke it out, and the piston came up and whacked it...

I am not sure I am understanding this.

It sounds like you are saying that the power valve hit the "bridge" that goes down the center of the exhaust port, which broke it and then the piston hit the broken part of the sleeve.

I seriously doubt that could happen.

The power valves on my bikes are spring driven such that if the power valve jams (which happens from time to time) it won't destroy the linkage or governor. There just wouldn't be enough force available to the power valve to break the sleeve.

If the problem is that the bridge (which some bikes don't have) is breaking out then I would suspect that it is due to lack of lubrication of the bridge.

My bikes don't have an exhaust bridge so I haven't had to do this, but the instructions that are packed with the piston explained what was needed if you had a bridge. Basically you are required to drill a hole(s) in the piston (below the rings) over the bridge so that oil will make it onto the bridge and lubricate it.

I can believe that lack of lubrication on the bridge could result to it getting really hot, seizing, breaking and then all hell breaking loose and all sorts of things breaking.

Rod

 

[Nova427]

Sounds possible, when I took the top end apart, the actual valve was diagnal inside the port up against the bridge, and it was broken out and the piston had hit it and squashed it down on the rings which made it loose compression...

Of course maybe from the bridge breaking due to lubrication like you said that could have maybe broke the power valve? I'll have to take a look at the piston but I don't think that it had a hole drilled in it, and I don't remember if I ever heard of anyone doing that to my other bike ( the 87 125) either...

Thanks for taking the time to reply, like I said I have had this bike for over 8 years and I rode it for about 10 minutes, I just couldn't bring myself to scrap it, so I've decided to fix it, I just wanted to get some opinions on what happened, I hate to have it happen again...

Toby

 

[julien_d]

what are you running for mix?

 

[Nova427]

I honestly have no idea, the bikes been blown up for about 8 years...I couldn't even tell you what kind of oil I used...

I was talking to my brother, cause I couldn't remember exactly what was happeneding with the other bike I mentioned, apparently the 3 times the 125 blew up the power valves broke and fell into the motor...Each time they were replaced, they would do it again, and with the 250, the bridge part of the sleeve was broke out, I thought it was from the valve letting go, because it was up against the side of the port hitting the bridge where it was broken...

I honestly don't know what to think, because I have searched tons of forums and can't find anything about this type of problem...I think I am the only one it seems to happen to...

 

[tlb]

Had an 88 rm 250 do the same thing.The power valve bores were worn verticaly allowing the valve to hang down and the piston would barely hit it.The ring would be pinched and the stem would break off of the valve.

 

[Nova427]

What did you do to fix the problem? Did you have to buy a new jug? And how exactly did you find out that they were worn, is there a measurement for what they should be? Is there any kind of safeguard to prevent that from happening... Thanks for the reply I was starting to think I was the only one this ever happened to, but it happened on 2 different bikes, so i couldn't figure out how my luck was just that bad...

 

[tlb]

put a valve in the cylinder in the closed position,if its worn there will be considerable up and down play. Yes new cylinder was needed.

 

[Nova427]

Awsome man, thanks alot for the help, I unfortunatly don't have the top end for that motor anymore, so I can't check to see if that was the case, but sounds like exactly what happened, I found 2 jugs on ebay, so I'll at least check them before I put the bike together again...Thanks again..

 

[rmc_olderthandirt]

another thought:

If your cylinder has been bored out oversize as part of the top end rebuild it is possible that the power valve will need to be ground down to match.

On my old YZ-125 I could bore to a very limited oversize amount before there would be interference with the power valve.

Rod