RM125 : Catastrophic Engine Failure(s) ... Please Help???

CarnageZ28

Member
Mar 25, 2008
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So at this point I dont know where to begin.

So I happened to come across a 1991 RM 125. Really clean, but needed a top end. Kid said he rode it home after it let loose, still ran, but rough. He tore it down, saw it needed sleeved, and sold it to me.

I get an iron sleave pressed in, port matched, bored and honed. New Wiseco Pro-Lite piston, ring, wristpin, bearing. Since he rode it home, we assumed the crankcase was clean, but split it anyways to be sure. Tore it down, split the case, cleaned everything thoroughly. There was absolutly ZERO metal in the motor. Rod bearings WERE good. Slight side to side play, but ZERO up and down play. Reassembled, using plenty of oil on all parts. All fresh fluids. New premium gas, mixed at 20:1, just as the owners manual calls for. I know, its rich, but its what Suzuki calls for, even for my 89 Quadracer . Used SuperM oil.

Fire it up, fires on 2nd kick, idles great. Let it warm up for several minutes. Turned the choke off, good idle, took it out. Made several light-throttle (Less than 1/2) rides up and down the street, not going near redline on it yet, and it started to die on me. Had 50psi compression.

Tear it back down, looks like foriegn material got lodged between the piston and cylinder head. Took out the head, piston, and required cylinder boring. There were pieces of some type of magnetic metal jammed into both the head and aluminum piston. We assumed somehow a piece of metal was wedged somewhere that I missed, but I was 100% positive I was clean with it.

So I tear it all back down, split the cases again, this time there WAS metal in there from it blowing up. Last time there wasn't. I clean it all out, reassemble, get a .060 over Wiseco piston, bored, honed, cut powervalves, etc. Reassemble again, using plenty of oil on assembly. Ring was seated correctly, circlips were put in properly. Reassemble the top end. Get it together. Again, bike fires on the 3rd or 4th kick. Idles nicely with the choke on. I let it warm up for a few minutes, cut it to half choke, let it run a little longer, and then turned it off. Ride the bike a few light passes up and down the street, making passes in 3rd gear running it up to 60%redline, let it compression brake back down, run it up a little higher to say 65% redline, let it compression brake back down, to help seat the rings properly. Did this proceedure for several passes. Never took it up to what would be redline. Just enough to come into the powerband of the motor, then let it compression brake back down. All of the sudden, same old thing ... It starts sputtering coming up the hill, I downshift it, and it siezes up..

Pulled the engine out, took the head off, and the new cylinder head, and new forged piston are mangled, just like last time. Foreign material wedged between the piston and the head. It is magnetic, and is NOT Rings, Circlip, or Wristpin material. Light scores on the cylinder. Will need bored, piston, cut powervalves, honed, and head on the top end. However, this time, the big-end rod bearings are shot. I assume from the piston hitting the head? There is TONS of play vertically in the connecting rod, that wasn't there before. I am 100% absolutly positive there was no foriegn metal left in the crankcase from when I split it last time. I was VERY clean, and have done literally 30-40 of these types of motors before in tech/shop classes/for friends.

Any ideas WHAT is causing this? I know, FOR SURE, there was no metal down there on the last rebuild. And I have no way of contacting the previous owner. I am open to any/all suggestions. Its late, I'm frustrated. Im going to bed on this one.
 

kx125412

Member
Mar 30, 2006
341
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Well for starters you never ever use engine braking on a 2-stroke if you can help it. You're using high rpms when on the gas and getting proper lubrication to everything then you let go of the throttle getting enough lubrication for idle, not high rpms, from your pilor jet and nothing else. 20:1 is a little much, 32:1 is basically standard mix ratio and has worked for many, many people. The magnetic material sounds like the iron bore. Are you sure its not flaking, or coming apart anywhere? Possibly the piston snagging a port, although you should notice that damage. It could be your big end bearings came apart. It happened to me on my RM125, mind you it was a 1982.
I don't even know if its worth to fix man, cut your losses and sell it blown up. I got $350 for mine with it totally messed up and beaten. Mind you this is Canada and we pay too much for stuff.
 

IndyMX

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Jul 18, 2006
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I'd guess that the big end bearing was coming apart the whole time. It just didn't show itself. These things happen.

Unknown engine, with previous damage, should have inspected it better.

Just my guess.
 

kx125412

Member
Mar 30, 2006
341
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Well when my I had my RM and tore it apart when I got it to redo the topend there was lots of gouges in the head. I wasn't sure why I knew nothing about motors or bikes so I just threw a top end in there, no new wristpin bearing even. I could move my flywheel all over the place, I knew it wasn't normal but what did it matter? Lasted about 10 hours if that and gave its last hurrah as I raced this kid on a 79 YZ250. Smoked em though ;)
It never ran again to my knowledge. But I learned alot from that bike and this site.
Just sell it and save up money for a new bike. It's hard cause all you wanna do is ride but get something that needs as little work as possible.
 

CarnageZ28

Member
Mar 25, 2008
109
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First off, I am not getting rid of it blown up. I'd fix it and keep it at my cabin before I just cut my losses. The rest of the bike is immaculate. Seriously, its as clean as most late 2000' model bikes.

Second, selling it to get something new, is retarded. I have a super clean 01' YZ250. Just picked this up as a project. Figured worst case scenario, I have a new bike for the cabin.

I'm not trying to be a dick, but I dont want to be told sell the bike. I asked what caused it. I have rebuilt something around 35-40 2-Stroke bikes in the past 6-7 years with no problems. This one threw me off. It's not like I dont know what I am doing down there lol really. I just wanted a 125 as kind of a beater bike.

The piston/ring is not catching the cylinder. The ports were chamferred, and cylinder still looks fine with the exception of a score from the metal, which I think, is big end rod bearing material.

So it is possible to have a big end rod bearing go bad, without losing noticable smootheness in the bearing? Myself, and my cousin, a mechanic with 15+ years experience, double, and tripple checked the crank bearing for smoothness. It was smooth. There was no up & down play in the conn. rod. Only slight side to side play, which I have seen on most of the 2-strokes I have had torn down.

Anyone have any other ideas as to what could have caused it? This is the only thing I can think of. I really want to get this thing together though. I love how the bike handles/feels and I really would like to get this thing running.

Selling it blown up is definatly not an option though as it is, other than this, a very very very clean bike.
 

CarnageZ28

Member
Mar 25, 2008
109
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KX125412 - Let me better explain myself. I wasnt downshifting to slow it down. It has brakes. I was putting the engine under light loads, building some RPM's, then letting it slow itself back down, on the level. Then I would tach it up slightly higher the next time, and so on. Not like I was downshifting the thing coming down a huge hill. It was all done on the level. Just varying the RPM's and slowly working my way up higher and higher, applying light, part-throttle loads to help seat the rings. Hope that makes better sense.
 

CarnageZ28

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Mar 25, 2008
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And as for the 20:1 thing, my 1991 RM125, as well as my 1989 Quadracer both call for 20:1. It works fine in the quad. Doesnt foul plugs. And runs excellent. So I figure if the 91' calls for it, its what I am running it at. I realize it's rich, but, if its not fouling plugs, and its what they reccomend for it, why should I lean it out?

I do run my 01' YZ250 at 32:1 however.
 

Jeromeo

Member
Feb 26, 2007
187
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I thought you were suppose to have a richer mixture than 20:1 for break in.... You should sell it and buy another bike.... Just kidding. The magnetic is probably due to the iron in the bore like someone else has mentioned. Good luck.
 

jschiller

Member
Mar 5, 2007
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I'd check the jetting 1st off, carb boot and all gaskets on the inlet side next, sounds to me it's running very lean and the then siezing, a big end going will deffinately be noticed by someone with your experience!
 

IndyMX

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Jul 18, 2006
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CarnageZ28 said:
I realize it's rich, but, if its not fouling plugs, and its what they reccomend for it, why should I lean it out?

I do run my 01' YZ250 at 32:1 however.


you misunderstand the term RICH/LEAN. It's got nothing to do with OIL/FUEL mix.

It's the fuel/air ratio.

So, more oil in the mix, means less gas, which means the AIR/FUEL ratio is LEAN.

More oil in this case may not have been a good idea.

If your carb was jetted perfect to lean when you got it, and you ran it with more oil than the bike was setup with, you ran it leaner than it maybe should have been run.

Add to that lower ambient air temps, and you may have toasted it just due to running it too lean.

Again, I could be wrong. This is just my opinion.
 

todd36

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Sep 3, 2007
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I agree with IndyMX your big end bearing was probably shot. It is hard to visual inspect bearing clearance. I use an indicator and bench center. When you put it back together this time I would pull out all the stops on checking specs. Check piston clearance,chamfered ports,leak test,ignition timing,jetting and squish clearance just to list a few
 

2strokerfun

Member
May 19, 2006
1,500
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I have no problems with 20:1 and don't think that made any difference here. But I think indy is probably right and you had a bearing letting go without being noticed. How's the lower rod bearing?? Powervalve was all right??
 

2-Strokes 4-ever

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Feb 9, 2005
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Perhaps the former owner ran the mixture at 40:1 and THEN jetted accordingly. Now if you choose 20:1 you've created a lean mixture. Your bikes problem is a mystery to me, but I would think it safe to look for stock jetting specs and put it there if doing 20:1. A "start from scratch" approach.
 

whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
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Merrillville,Indiana
You seemingly did everything except actually measure the side deflection, thrust washer clearance. Takes a second or two and a feeler gauge. Complete engine kits go for around 300 dollars.
 

CarnageZ28

Member
Mar 25, 2008
109
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Oh, I understand where your coming from now. I thought you meant I was too rich with oil. I realize if it was jetted for 40:1 and then I ran 20:1 through it, there would be less gasoline for the same volume of fuel mixture being drawn through the carb, causing a lean condition. Previous owner told me he DID run it at 20:1 though. As for jetting, I am on the stock pilot jet, a clip richer on the needle jet, and a 340 main jet. Stock is a 330. I'm not saying your wrong, but none of the warning signs of detonation/lean mixture are there. Ceramic portion of the plug was correct color, plug looked good, no "burned" look to anything. Just kinda looks like you throw some chunks of iron/steel in the combustion chamber and let the piston go to town smashing them into the head/piston top.

But anyways, that said, the ceramic portion of the plug is a brownish color, not wet, and certainly not whitish like it was detonating from a lean condition.

The boot from the carb to the reed cage is new/not torn. Clamps were tight. And all new gaskets were used. Old gasket material was scraped clean with a razor.

I really think somehow it was a bearing that just wasnt showing signs of wear? :bang: Or it did, and somehow I overlooked it, twice...

I'll tell ya though, I feel pretty retarded at this point lol. And if anyone has a motor that will bolt in on a 91 RM125, I'm keeping my options open.

Actually, I wonder how bad it would be to do a 250 swap if I got the carb, ignition, and pipe. I love how these 125's feel so if I could get that handling, with 250 power :p Anyone ever do something like this before? I imagine its not very common at all since most people just do sell bikes, but it has the potential to be a cool project.

That or maybe a 144 big bore ...
 

BigRedAF

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Jan 9, 2005
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IndyMX said:
you misunderstand the term RICH/LEAN. It's got nothing to do with OIL/FUEL mix.

It's the fuel/air ratio.

So, more oil in the mix, means less gas, which means the AIR/FUEL ratio is LEAN.

More oil in this case may not have been a good idea.

If your carb was jetted perfect to lean when you got it, and you ran it with more oil than the bike was setup with, you ran it leaner than it maybe should have been run.

Add to that lower ambient air temps, and you may have toasted it just due to running it too lean.

Again, I could be wrong. This is just my opinion.

Indy, it may be your opinion but it's factually correct. :cool:
 

sparkysakitas

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Aug 31, 2005
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just fix that motor you are 100% about it being a fun bike
i have one! well a 90 which is the same. it is a little bulky
but when its running its hard to notice and i think all i have is blown up motors here, although i think i have a 89 cylinder and
oversize piston with a sleeve already in it just may need a honing to clean it up from sitting in my shed
 

Coop

Member
Jan 21, 2000
153
0
I used to work as a mechanic and my opinion is it was the lower rod bearing. I rebuilt a 1984 KTM125 about 8 years ago. I tore it down, split he cases, the lower rod bearings felt great so I didn't open the crank. Rebuilt it, first ride the exact same thing happened as you described. Even though there wasn't any play, the inside of my rod and the pin both were pitted from the bike sitting and that gave the bearings room to wiggle and explode. It was an expensive lesson. Now when I am rebuilding a bike of unknown history I spend the money to inspect the rod, bearings and pin.
 

CarnageZ28

Member
Mar 25, 2008
109
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Well at least I wasnt the only one, with my fair share of experience, that did something dumb ... lol ... or expensive...

Mark my words though, I will get this thing together. Probably will be a month or so though before I do anything with it ...
 

whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
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Merrillville,Indiana
And for the record, catastrophic is when the rod is sticking some place it shouldn't. Like, out of the cases. My oldest son is a mechanic, he has done hundreds of top ends, blah blah blah. He grabbed the rod on his 250f, wiggled and pulled, good to go. He caught it in time, cost only around 500 in parts. You have to check it thoroughly! Measure, get a magnifier and light. And if in doubt, cough up 200 more dollars for a fresh engine. There is no time table for changing. Once a benchmark is set you can keep track of time and wear.
 
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