Help diagnosing my crapola RM problems...

[Micahdawg]

Ok, hopefully you guys won't have to hear me moan about these issues again, but I've got several ideas of what might have happened to my bike and nothing really conclusive. Just guesses. So here is my situation and specific info.

Bike, 2001 RM250 - pc pipe/silencer, uni filter, carbon reeds, milled head, Carb jetting = 162 main, 52 pilot (standard = 162/48). The needle appears stock and in 3rd position. Air screw 1/2 turn out.

Elevation is 500ft, temperature was 50-60ish when riding, gas was 32:1 Klotz and 93 octane, and I rode for about 3 hours. Plug is a BR8EG (standard)

Before I took ownership of the bike, it had a couple hours on a brand new crank, wiseco piston and rechromed cylinder, however the previous owner ran race gas and did not tell me anything about this until after the fact.

How the bike acted- It had an up down rythmatic idling. Realy jerky and bucky down low. At the right throttle (1/8th or so) it would pop and knock a bit too. Over 1/2 throttle or so and it would run pretty good. Top end was fine.

Compression test on a cold cylinder showed 200 psi. Upon removing the head I noticed a small piece of the piston gone. Small chunk the size of a square keyway right in front of the arrow on the piston top. Cylinder showed no scoring so it looks to have passed out the exhaust.

Piston was very dark in color and had some slight pitting around the middle to exhaust side top of piston. No major pitting, just minor stuff. Cylinder head showed no pitting. Top side of the rings were very dark (kind of blued) and bottom side of rings were clean looking.

I'm wondering about three things at this point. (1) Me running too low a grade of gas causing intense heat and detonation (2) jetting being way off (3) powervalve protruding too far into cylinder causing chunk of piston to go flying.

If anyone has some hypothesis on what was the most likely scenario it would be great. Someone said when their cylinder was rechromed, the rings were clipping the valve and caused the same thing. Last night I manually moved the piston past the exhaust port and couldn't see any contact with rings to exhaust valve, but maybe if it were hot it could contact? Thinking I may want to take a sanding roll to the exhaust valve just in case.

At least this time I've got pics of the bike:

http://members.tccoa.com/micahdogg/100_4136.jpg
http://members.tccoa.com/micahdogg/100_4137.jpg

Micah

 

[Studboy]

Micah,

I think that your speculation is right on. You were either lean or needing race gas or a combination of both. You should be able to easily check if the powervalve is capable of hitting the piston by reinstalling it and pushing the main valve in (towards the intake port) until it stops. There should be a little bit of clearance before it enters the bore, but not too much.

 

[ETS]

If it were mine I would get rid of that milled head. Or send it to Eric and let him tell you if the squish band, cylinder base, and such has been modified or how much was removed from the head. Theres a lot of places around that are to quick to remove material and not really know what they are doing. Besides that, why run race gas if you don't have to. I mean certain levels of competition it might be mandatory but the average guy isn't gonna use the pump gas power. Your decription sounds lean on the bottom end but I don't believe that with a 52pilot and .5 out on the air screw. Mine with a 42 pilot and 1.5-2 turns out runs fine. Is it getting to much fuel down low causing poor response until it gets into the needle and cleans out that extra fuel? Do you think it might have gotten some sand in there to cause the pitting? Doesn't take long for a little bit of sand/dirt to make a mess. You should easily determine if the powervalve is hitting like studboy said and you should be able to tell if the rings can grab a port. Did you happen to mic the piston to cyl clearance? Just curious how good of a job the platers did. Just because it was new doesn't mean it's always right. I'm not a big fan or porting as I've had several failures with snowmobiles with ported/head cuts. But.. I just sent my cylinder this morning for my 01' to Eric for low/mid porting. I've heard so many raves that I thought I would give it a try. I don't want more power, I want to move the power where I need it. In the snowmobile world more is never enough so I think this is a different game and should be just as reliable. I hope.

 

[a454elk]

01 RM here as well, but the difference is my rings failed the first time the top end went, causing the ring to go forward and catching on the exhaust port, down stroke, tore the top of the piston off. Caused major gauges in the cylinder wall so it had to be replated. Now, last week, about 20 hours of riding on it and the upper ring broke again, (Using stock rings and psiton again), this time a small portion of the piston broke off and I believe it went out the exhaust, same with the missing piece of ring.

My powervalve has been a problem for about 2 years now. It tends to stick in the port and the last time it did that it broke the bottom portion of the powervalve actuator. I found that it is extremely important to keep the valve clean at all times. I don't think my valve is hitting the ring and piston, If so, it should have done it a long time ago. I think the ring fails and it moves out of it's seat, then it catches. Your jetting seems fine but the airscrew should be out 1 1/2 turns I believe. Check your reeds to make sure they aren't chipped or broken too. The up and down idle sounds like an air problem to me. Check your boot around the carb, engine side too. Spray some carb cleaner around while it's running to make sure it's not sucking in air.

Also, if race gas was run for while and it wasn't cleaned out, your jets and passages could be plugged, mainly the slow jet, pilot ones, that affects idle and just off. Clean out the carb completely, and I mean COMPLETELY. Race gas, when sitting, gums up like no tomorow. The idle problem could be associated to a clogged jet as well.

Sorry about the lengthy reply, hope it helps.
Elk

 

[Micahdawg]

First off....I sent the head to Eric Gorr last week. Just put a note with it to get it back in 92-93 pump gas range. Also, I have not mic'd anything yet, but I have a brand new wiseco piston ready to go in. The cylinder is bare and is going to the local shop today to get mic'd and see if it needs another hone. The cross hatch looks good still, but I just wonder if a fresh hone would help the rings seat.

With the cylinder bare and in my hands, I reinstalled the piston and the power valve to see the clearance. I moved the piston up and down carefully in the bore with the powervalve all the way in. It never snagged even once. BUT.....I wonder if a hot cylinder (expanded metal) would cause a clearance issue?

The reeds are good and unchipped/cracked. I also do not believe that there was any air leak. Seems like air leaks just cause throttle to increase period (500, 600, 700, 800, 900, 1000, 1100+ rpm, etc...) My idle was surging up and down in rythm.

I checked the stator and it looks to be in the original position (I.e. no advancing). At least no glaring evidece like old screw marks where it was relocated.

I also thoroughly cleaned the carb. It was missing two vent lines and one was plugged. Plus gas was dumping out the overflow frequently. A good clean and inspection of the float needle, float hieght, jets, etc... got it to quit dumping. It looked pretty clean in there though. So i don't think the pitting on the piston is sand or debris.

The pitting on the piston is not something I would really worry about if the bike had a fair amount of hours. But after about 5 hours on a new top end it looked like the wear was a little advanced.

Thanks for the tips and stuff. Just looking to cover all my bases up front so this doesn't take a 6 pack of pistons to figure out :)

Micah

 

[a454elk]

I hear you on that! Looks like you've covered it all. The head may have been worked in order to take the race gas so going to stock specs would do good. Taking off a portion of the power valve wouldn't hurt but see if Rich R can help answer that. I did mine and the only thing I see happening is when the ring broke, it moved forward, catching at that point. The valve itself doesn't have any marks on it, just the damage to the ring hitting the port shown on the piston and port. I still think it's not getting enough air from the air screw though, 1 1/2 - 2 is normal. With that little air, you'll get lots of oil in your vavle not burning through, too rich. Good luck!