Help please, burning piston after piston?

[RobbyB]

I picked up a 02 YZ125 for my son, I know the folks personally, they had only put a couple of hours on the bike since the rebuild. The bike came with receipts for a complete rebuild of top and bottom end with a renickeled jug, from a reputable shop. They set the ring gap at 18 thous.

The jetting is all stock with clip on middle notch on needle, low speed set at 1/5 turns. We take the bike out after purchase, warm it up and ride for 5-6 minutes fairly hard. Stopped the bike for a couple of minutes, start it up and idle away a few feet and it stops. Not compression. I pulled it apart and found a broken ring on the exhaust side. Piston was still mint so I installed a new ring and reassembled. The bike ran well for maybe five tanks of fuel, then, after a hard ride, the bike is stopped and left for about five minutes then no compression. Pull it apart and find the exhaust side of the piston is burnt slightly. I then install a new piston and ring and reassemble with ring gap at 20thous. I figure to lean so I lower the clip on the needle and set the low speed so I get a good idle. Take the bike out and break it in. Take it to the sand pit and run it around a bit (10m tops) and come to a stop for 10m or so and again, no compression. I haven't had it apart yet but I'm thinking burst piston again. I haven't checked the plug yet either. The last failure the plug looked a nice brown.

I don't know what to do? The bike has an FMF pipe and silencer on it. The rads have a bit of a bend, but one can still see straight through them. The piston appears to be melting after it stops. The motor doesn't seem to be running any hotter than other bikes.

Anyone have this problem?

 

[m4i2k2e2]

when you stop do you trun the bike off or leave it running?
you could get a tempature sticker and put on to seems what the bikes running at. what mix ratio are you using?

-mike.

 

[_JOE_]

I would start by getting it torn down. It sounds like the ports may chamferred a bit more seeing how the ring got snagged. Measure piston to cylinder clearance before reassembly. I think the ring gap should be closer to 12 thousandths on a 125? Once it's back together swap the main jet out for a few sizes bigger. It would be a good idea to have somone do a leak down test to verify the crank seals. Make sure the intake boot and reed cage gaskets are in good shape. If all that looks good go ahead and do a plug chop to see how the main jet is working.

 

[RobbyB]

Thanks for your reply. My son shuts the bike off immediately after stopping. The mix is 32:1 using the Amsoil non-sythetic. After a hard ride I can hold my hand on the rad for 1/2 second or so, I did this to some others bikes and ours seems a touch hotter. I'm starting to think that after stopping with the piston in a certain position, that there is tremendous heat in that one spot that is cooking the piston. Does it help to let the bike run for a few seconds before stopping it? Thanks

 

[RobbyB]

Thanks Joe. The manual states a 20-24 thou ring gap for this bike. I didn't want to risk going smaller. The interesting thing is that you can ride the bike with no problem, its when you stop, let it sit for a few minutes, then get back on the theres no compression. I maybe should have started a bit richer on the high speed. It was set at the middle grove, I moved it down one, making it a bit richer.

 

[_JOE_]

Edit first post....ports may *need* chamferred.



I'm interested to hear what you find in there this time. Please take a few pictures, I would like to see where the piston is burnt.

And the high speed carb circuit doesn't have grooves, it is a fixed orifice jet. The needle is only used to tune the mid range.

I would look very close at those at those ports when it's apart seeing as how there's new plating on there. A broken ring a sure sign of a mechanical issue.

 

[whenfoxforks-ruled]

Is there a score mark running down the exhaust side of the piston? Has the top of the piston ever been missing pieces or have melted looking spots, on the exhaust side? If the manual said 20 to 24 thousandths ring end gap, and you had it at 18? Its too tight, snap/seize. Check the ports for chamfered edges, .002 skirt clearance, .0035 at least on the exhaust bridge clearance, and set the end gap to what the book says. Burnt spot on the bottom of the piston? If the spark plug has shiny drop lets, knocking or the plug is coming loose, you need higher octane fuel. When you go riding in the sand, richen the main.

 

[helio lucas]

one more thing to look at, the silencer. if it has too much back pressure will melt the piston right on the top on exhaust side. good idea to fresh the packing.

is the plug on correct heat range? you can verify this on service manual.

take of the ignition cover. have oil behind the stator? if yes you have a leaky seal that may cause the bike to run hotter.

 

[RobbyB]

These are lots of good points made, thanks. The piston is burnt a tiny bit on the exhaust side. You pointed out an important point about something I wasn't aware of. The needle only regulating fuel upto 3/4 throttle, then the main jet works above that. I need to check the main jet, and as mentioned, go a little richer when riding in the sand. I think the ports were chamferred properly, I think the ring broke due to insufficient end gap combined with heat?? This never damaged the jug. The bike also has the ProCircuit pipe and silencer on it. I read somewhere that this can lean the system as well. I haven't had the bike apart yet to see where were at after the last blow-up, but I'm thinking that the piston will look the same. The puzzling thing is that the damage occurs after the bike is stopped. This is the way it happened both times now, the bike was stopped left for a few minutes, then no compression. I think I going to go with the Yamaha piston and get the right one for my jug, as it has been nickeled.

I'll post some pics.

 

[Uchytil]

This may sound goofy but - pull off the water pump cover and see if the impeller turns when you crank the engine. My son's 04 YZ250 (after he did a top and bottom rebuild) was running hot and he burned two pistons (exhaust side). I told him it was running to hot and he checked the aforementioned item. He had sheared the gear teeth which prevented the water pump from turning -easy fix. Not saying that's your problem just one of those things.

 

[RobbyB]

Thanks, thats something I will certainly take a look at. I found when riding the bike that it would get pretty warm after a hard ride then after some light riding it would cool. I don't know what the operating temp should be but you can't keep your hand on the rad. There was a post that suggested checking the stator side of the crank to ensure that there was no oil. If there is, he said the bike will run hotter, I guess due to drawing air. Do you know if the performance pipe would require bigger main jetting?

 

[BSWIFT]

Uchytil makes a very could point. While riding, your bike will get "some" air cooling. When you stop, the heat will build up and if your ring gap was to small, the expansion from heat could cause it to snap while it is shut off.
When I first read this thread, I suspected a lean condition. After several posts and more info, sand riding, I think you have more than one issue. Proper jetting and checking/replacing you impeller will likely fix your overall problem. However, do all of the checks on clearences as previously suggested to cover all of your bases.

 

[RobbyB]

Sorry, no pics yet. I pulled the top end off and found that the piston was damaged slightly due to a small foreign object that made its way up one of the intake ports (a peice of piston I'm thinking). It nicked the intake side of the piston top pressing the ring groove and locking up the ring. I know for a fact that it wasn't something I left in there as I am extremely careful when working around the open crank area. And the one piston I did replace had no missing pcs. I'm sure it was a soft pc of material as there is no damage to the jug where it would have caught. The 'Reputable' shop that did the motor would have left something in the bottom end I'm sure. Where do I go from here. I was thinking about pulling the motor, adding some fuel to the bottom end and seeing if I can flush out anything else that might be in there, if anything. I checked the reeds/cage/carb and there is nothing that would have broken off from those peices. Other than the nick, the piston was not burnt, which is a good sign. Any ideas? Should this case come apart? Thanks again in advance.

 

[whenfoxforks-ruled]

Big end bearing coming apart?

 

[_JOE_]

Both circlips are in place?

Was the connecting rod bearing replaced or just the crank bearings?

 

[RobbyB]

Circlips are both there. The crank and rod bearings were all done, with a new rod. The material must have been soft as there is no damage to the cylinder where it caught. There are about 7-8 very small nicks in the head surface. I'm wondering if I should just pull the motor and wash out anything that could be in the bottom end, if anything now. I've never had a case apart to do a crank, but I can't imagine how material could be left in there after a rebuild. Are there places where a small pc of pist could hide? If it wasn't for the nicks in the head I might think that the piston caught the port. I saw a pic of a piston on another thread that looked exactly like what I have here and they claimed that the piston caught the port. Thanks/.

 

[Uchytil]

Are any "pieces" available? Are they steel or aluminum (magnet)? Lot's of people swear the piston is falling apart but it could be part of the bearing cage, ring, bearing - which are all steel. Did you check the water pump to see if it rotates? Check all of the above before you go splitting the cases.

 

[RobbyB]

No, there are no pcs available yet, but I haven't had a chance to dump the motor upside down and rinse anything out that might be in there. I haven't had a chance to check the water pump yet but certainly will do that. I'll post back more results when I get a chance to pull the motor out. Thanks.

 

[2bitsamile]

did previous owners shatter the piston? debris could cause the "There are about 7-8 very small nicks in the head surface". i'm guessing in the squish band.

 

[RobbyB]

I do beleive the piston was broken a bit and the crank/rod bearings were toast. Is it possible that during a rebuild that one could miss a small piece of debris in the crank area? I did check and the water pump impeller is turning as it should.

 

[whenfoxforks-ruled]

There are a LOT of people who swear by douching it upside down and calling it clean. Leave that non sense for your better half. A 2 stroke dirt bike needs to be torn down completely, you can not flush between the crank seal and bearing. Anything out of the ordinary will fry the big end bearing, little bit of sand, dirt or piston pieces. You have a bike that has about a 15 hour mark on replacing the top end. The bottom will last a long time, if you take care of the top end and basic filter maintenance, or once a year.

 

[Uchytil]

Bob, douche - really - yuk! At this point you should just do as he says and split the cases, it's not too hard but you could always have someone else do it. There's a guy named Eric Gorr that does good work, you can search for him by name and go to his website. Good luck.

 

[RobbyB]

There are a LOT of people who swear by douching it upside down and calling it clean. Leave that non sense for your better half. A 2 stroke dirt bike needs to be torn down completely, you can not flush between the crank seal and bearing...

I'm wondering if the damage I'm seeing was due to the piston catching the port? I went with a Namura pist and I found that the motor was making a bit more noise that when the previous Weisco was in there. Someone posted a piston pic on another thread that they claimed had caught a port and the damage looked identical to what I have here (I wish I could find the pic again). Is it possible that only a very small area of the piston top could catch a port (3/16ths wide)? All ports appear to be chamfered. The direction I'll likely take is to pull it right apart to be certain that its clean. This is frustrating due to the motor being completely rebuilt with new bottom/top ends with a new rod. I have the receipts and know the guy that did it. He does the bikes of many of the pro racers in the area. These shops won't warranty motors after they leave running as to many run lean, don't clean filters, etc and abuse. Thanks

 

[RobbyB]

Question, is it possible for a piece of foreign material (i.e. small pc of previous piston) to lay around in the crank cavity for 7-8 tanks of hard riding then finally make its way to the top?

 

[whenfoxforks-ruled]

A small piece missing from the top of the piston, or skirt? When the fuel comes in, it goes to the cases and flushes up top through the transfers. Did it leave a mark in the head?