2003 RM125 Top End Inspection Help

[GEOF]

It could well be debris also. Have you checked the bottom end? Does the bore show signs of the same debris effectig it? If it's got to the top of the piston then it's had to travel through the bore. Is the bore scored as if it's had sand through it? Another sign of Detonation will be slight or medium four corner seizure marks on the piston. Is there any marks on the piston? Is there aluminium on the bore from where the piston looks like it has picked up on the bore? 

Edited October 24, 2014 by GEOF

[GEOF]

 

Here is an excert from 

Engine Basics: Detonation and Pre-Ignition

Written by Allen W. Cline

 

Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of look if detonation occurs. It is a swiss-cheesy look on a microscopic basis. The detonation, the mechanical pounding, actually mechanically erodes or fatigues material out of the piston. You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because if you think about it, you would ignite the flame front at the plug, it would travel across the chamber before it got to the farthest reaches of the chamber where the end gas spontaneously combusted. That's where you will see the effects of the detonation; you might see it at the hottest part of the chamber in some engines, possibly by the exhaust valves. In that case the end gas was heated to detonation by the residual heat in the valve.

 

Detonation is influenced by chamber design (shape, size, geometry, plug location), compression ratio, engine timing, mixture temperature, cylinder pressure and fuel octane rating. Too much spark advance ignites the burn too soon so that it increases the pressure too greatly and the end gas spontaneously combusts. Backing off the spark timing will stop the detonation. The octane rating of the fuel is really nothing magic. Octane is the ability to resist detonation. It is determined empirically in a special running test engine where you run the fuel, determine the compression ratio that it detonates at and compare that to a standard fuel, That's the octane rating of the fuel. A fuel can have a variety of additives or have higher octane quality. For instance, alcohol as fuel has a much better octane rating just because it cools the mixture significantly due to the extra amount of liquid being used. If the fuel you got was of a lower octane rating than that demanded by the engine's compression ratio and spark advance detonation could result and cause the types of failures previously discussed.

 

Food for thought. 

Edited October 24, 2014 by GEOF

[mikel112]

There is scoring on the piston below the ring, it's pretty smooth - nothing deep. Still can't figure out what could've gotten inside the cylinder to trash it like this.. All parts are in place...

[717 MOTORSPORTS]

That piston is a mess! Something went through that motor. In the very first picture of the piston, it sure as shite looks to me like something is missing....I do not see a piston locate pin.

[717 MOTORSPORTS]

Here is an excert from 

Engine Basics: Detonation and Pre-Ignition

Written by Allen W. Cline

 

Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of look if detonation occurs. It is a swiss-cheesy look on a microscopic basis. The detonation, the mechanical pounding, actually mechanically erodes or fatigues material out of the piston. You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because if you think about it, you would ignite the flame front at the plug, it would travel across the chamber before it got to the farthest reaches of the chamber where the end gas spontaneously combusted. That's where you will see the effects of the detonation; you might see it at the hottest part of the chamber in some engines, possibly by the exhaust valves. In that case the end gas was heated to detonation by the residual heat in the valve.

 

Detonation is influenced by chamber design (shape, size, geometry, plug location), compression ratio, engine timing, mixture temperature, cylinder pressure and fuel octane rating. Too much spark advance ignites the burn too soon so that it increases the pressure too greatly and the end gas spontaneously combusts. Backing off the spark timing will stop the detonation. The octane rating of the fuel is really nothing magic. Octane is the ability to resist detonation. It is determined empirically in a special running test engine where you run the fuel, determine the compression ratio that it detonates at and compare that to a standard fuel, That's the octane rating of the fuel. A fuel can have a variety of additives or have higher octane quality. For instance, alcohol as fuel has a much better octane rating just because it cools the mixture significantly due to the extra amount of liquid being used. If the fuel you got was of a lower octane rating than that demanded by the engine's compression ratio and spark advance detonation could result and cause the types of failures previously discussed.

 

Food for thought. 

HEY!!! That's is pretty gaw-damned cool that you were able to copy, and paste that, just for me!!!! THANKS!!!! Now find something else to occupy my time that pertains to a 2 stroke, please

Edited October 24, 2014 by 717 MOTORSPORTS

[GEOF]

HEY!!! That's is pretty gaw-damned cool that you were able to copy, and paste that, just for me!!!! THANKS!!!! Now find something else to occupy my time that pertains to a 2 stroke, please

Actually apologies for that as I pushed the wrong button and have ended up with a copy and paste of your quote and posted by mistake. It was not intended for you specifically but has ended up on that post as I could not delete the post. The post was for general comment and for Mikel112. Good luck with the repair. I have pmed you. 

Edited October 24, 2014 by GEOF

[mikel112]

the locating pin is in place, I've taken out only one circlip another one is still in the piston (LAZY )... Someone asked me about pictures of my bottom end so here they are. Rod does have side to side play but no up and down.

[racin4ds]

That piston damage is most certainly NOT detonation! It was likely a spark plug electrode that did that damage from using the cheap plugs... this is why Suzuki used that crazy $29 plug in the 125's of that era! Look at the square shape of the indentions on that piston crown! The PO just reused the piston and threw it back together for a quick sale, Funny thing is the 97 RM250 I just bought had the EXACT same damage and guess what, it had a plain BR8ES plug in it when I bought it...

 

New head, quick hone, new piston and rings and you should be fine. Just watch piston to wall clearance and make sure your cylinder is not too far out of spec. If you have to, go with a piston other than wiseco so you can get the larger "C" and "D" sizes. Thats the one thing I hate about wiseco, one size only...

[Lindsey97]

listen to what these folks are telling you about using the proper spark plug.  NGK R6918b-8 I think. 

 

when you get your new head, measure the squish, and consider sending it to RB Designs to have the squish set and compression bumped up. 

 

I think what you got was a motor put back together after the damage had been done, and the bike was then sold to you. 

 

be sure to use premium fuel with no ethanol, or preferably race fuel.  if it was detonation, the race fuel will stop it. 

 

while you have the bike apart, verify your mechanical timing settings at the stator.  and tear the carb down, clean it, and notate what jet sizes are in it. 

 

My 2002 Suzuki rm 125 is currently running a 30 pilot, JD blue needle 3rd groove, and a 430 main.  my elevation is 900 feet.  also running FMF fatty/shorty silencer.            

[Excitable]

it really sucks when people slap the crap back in to get it running just to sell to the next sucker.

 

on a good note though, you will know for sure you got a good top end in her.

[pomoco]

I had the same thing happen to me. Now I always ask to pull the pipe when I'm looking at a two stroke

[mikel112]

Just got some goodies last night.. Piston doesn't have lubrication holes - no biggie, I'll just drill them out. Head will be here tomorrow.. Plan is to put everything together tomorrow and begin annoying my neighbors rippin up n down the street again

[717 MOTORSPORTS]

Check your crank for ANY up and down play. Even if you just feel a click, that's too much. Rotate the crank, and check for play in multiple crankshaft locations. Make sure nothing is in your pipe, or stuck in the power valve either, as obviously something bigger, and harder than dirt, or aluminum went through that motor

[moto356]

Currently running NGK br9eg - was there when I bought it... Manual says r6918b-8 but damn that thing is pricy...

Using pump gas (highest octane).

I figured out how to measure squish clearance, but angle, how do you measure angle?

dont mess with the plug. it is expensive but you need to run it. on a plus side ive noticed these last long than the chepaer ones other bikes use. ive had 4 rm125s 01 02 03 and 04. the plug used was a special design to avoid problems

it looks like a combination of detonation and a broken elctrode. ive had a bike that was leaking coolant and it ate the material off the head, look eroded not pitted. 

Edited October 28, 2014 by moto356

[Lindsey97]

I started out with a $900 rm 125 just like this.  it's not a $900 rm anymore, now it's a $2500 rm 125.  oh well it's only money. 

 

take the expansion chamber off and shake it vigorously, listening for small metal bits of anything.  blow it out before installing it back onto your rebuilt engine. 

 

also clean your fuel tank out, things could possibly enter your engine there  too.  you never know with a used bike. 

 

I have have bought used stihl chainsaws off craiglist in the past, and took tools with me to remove the muffler so I could inspect the exhaust side skirt of the piston. 

 

I think the "special" plug deals with harmonic vibrations inside the combustion chamber.  the regular br8es ground strap would break free and fall into the piston crown wreaking havoc on everything.   

 

another thing to check while rebuilding this bike is to see if the top cover of the airbox is intact.  on most bikes this part is missing, as was mine.  my opinion on this part being removed is that it causes a lean condition which leads to piston seizure.  just my opinion.  

Edited October 29, 2014 by Lindsey97

[Excitable]

I think the "special" plug deals with harmonic vibrations inside the combustion chamber.  the regular br8es ground strap would break free and fall into the piston crown wreaking havoc on everything.   

 

 

how exactly would a plug make a difference to the harmonic vibrations of the exhaust?

[Lindsey97]

If I remember correctly, an ordinary NGK Br8es has a very high chance of losing its ground strap if used in the RM125.  This is due to very high pitch vibrations that are unique to this family of engine.  So Suzuki uses the special plug that is recommended in the owner's manual.  The downside to all of this is the cost of the plugs:  $38.00 each. 

 

Do a search on this, it has been a while since I read about it. 

 

I have yet to foul one of the special plugs, they last quite a while.