Help: YZ125 Top End Won't Last

jackiemoon917
Profile picture for user jackiemoon917
Posts
25
Joined
2/27/2015
Location
Cedar Rapids, IA, USA
jackiemoon917
Edited Date/Time 3/27/2016 10:09am
So I have been experiencing a string of bad luck with my 2003 yz 125. The last 2 top ends have blown with under 5 hours on them. I've been running a Vertex 'B' piston and the cylinder is stamped 'B' as well. My jetting is slightly leaner than stock (400 main and 37.5 pilot) which I've seen recommended by many and seems to work very well.

The first piston blew with about 3 hours on it shortly after I had just switched from Amsoil Dominator to Klotz BeNol. The day it blew it was a pretty cold day and Klotz is notorious for separating in cold weather. I chalked that up as the cause and switched back to Amsoil. Also, I threw in a new OEM crank for good measure since it was still running the original.

I thought I would be in the clear until my trip to Texas last week. It blew up again; this time with 4.4 hours on the top end. I have included pictures of this piston as it looks very similar to the first one. The skirt on the the intake side has dropped on both of them and there is some slight wear from the exhaust bridge.

I have already ordered a replated cylinder to start with a completely fresh motor. I'm not really sure what else I can do but hopefully some of you can help me out. Just trying to figure out why this is happening.

Thanks







|
fulltime wrench 3/24/2016 6:37pm
3/24/2016 6:37pm
Are you keeping your air filter clean and properly oiled?
FGR01
Profile picture for user FGR01
Posts
6108
Joined
10/1/2006
Location
USA
Fantasy
926th
FGR01 3/24/2016 7:06pm
3/24/2016 7:06pm
Did you notice any pinging before it let loose?
jackiemoon917
Profile picture for user jackiemoon917
Posts
25
Joined
2/27/2015
Location
Cedar Rapids, IA, USA
jackiemoon917 3/24/2016 8:58pm
3/24/2016 8:58pm
FGR01 wrote:
Did you notice any pinging before it let loose?
Yeah I would call it a ping. I had thought it was sound of a chipped reed.
jackiemoon917
Profile picture for user jackiemoon917
Posts
25
Joined
2/27/2015
Location
Cedar Rapids, IA, USA
jackiemoon917 3/24/2016 8:59pm
3/24/2016 8:59pm
fulltime wrench wrote:
Are you keeping your air filter clean and properly oiled?
Yes every ride

The Shop

See All »
colintrax
Profile picture for user colintrax
Posts
4695
Joined
8/25/2015
Location
Taylorsville, GA, USA
colintrax 3/24/2016 10:26pm
3/24/2016 10:26pm
How's the cylinder look?
jackiemoon917
Profile picture for user jackiemoon917
Posts
25
Joined
2/27/2015
Location
Cedar Rapids, IA, USA
jackiemoon917 3/24/2016 11:42pm
3/24/2016 11:42pm
colintrax wrote:
How's the cylinder look?
The cylinder looks fine, visible wear on the exhaust bridge, but nothing substantial enough to feel it.
Paw Paw 271
Profile picture for user Paw Paw 271
Posts
3632
Joined
4/3/2013
Location
Benton, LA, USA
Paw Paw 271 3/25/2016 3:53am
3/25/2016 3:53am
What does the head look like?
Have you checked your squish?
Have you checked cylinder to piston clearances?
Was the ring end gap set?
What color is the spark plug tip?
Have you done a leak test?

In general it seems to be running lean and an excess amount of heat from the pinging is causing your issues.

Paw Paw
Markopolo400
Profile picture for user Markopolo400
Posts
1426
Joined
7/24/2012
Location
St. Paul-ish, MN, USA
Markopolo400 3/25/2016 6:49am
3/25/2016 6:49am
In my experience, breaking the intake side skirt off meant that the piston was rattling in the cylinder, which meant that the cylinder was out of tolerance big.
jackiemoon917
Profile picture for user jackiemoon917
Posts
25
Joined
2/27/2015
Location
Cedar Rapids, IA, USA
jackiemoon917 3/25/2016 8:04am
3/25/2016 8:04am Edited Date/Time 3/25/2016 8:13am
Markopolo400 wrote:
In my experience, breaking the intake side skirt off meant that the piston was rattling in the cylinder, which meant that the cylinder was out of...
In my experience, breaking the intake side skirt off meant that the piston was rattling in the cylinder, which meant that the cylinder was out of tolerance big.
This is what I was thinking. Hopefully that's all it is.
jackiemoon917
Profile picture for user jackiemoon917
Posts
25
Joined
2/27/2015
Location
Cedar Rapids, IA, USA
jackiemoon917 3/25/2016 8:12am
3/25/2016 8:12am
Paw Paw 271 wrote:
What does the head look like? Have you checked your squish? Have you checked cylinder to piston clearances? Was the ring end gap set? What color...
What does the head look like?
Have you checked your squish?
Have you checked cylinder to piston clearances?
Was the ring end gap set?
What color is the spark plug tip?
Have you done a leak test?

In general it seems to be running lean and an excess amount of heat from the pinging is causing your issues.

Paw Paw
Can't tell anything from the head it's basically caked in pieces of piston.

Have not checked squish or clearances. And yes I set the ring gap. I can assure you the bike was running slightly fat if anything. I was checking the plug basically every 15 minutes at the time chasing another issue.
project racer 3/25/2016 3:11pm
3/25/2016 3:11pm
Always use at least the next letter size larger piston when replacing them on older cylinders with a lot of hours on them.

The skirt broke because of piston rocking due to excessive clearance.

A "C" or "D" piston may have lived a bit longer ,but the cylinder was probably already out of spec on the first failure.

Measure things, instead of just throwing parts at it, and this can be avoided.
jackiemoon917
Profile picture for user jackiemoon917
Posts
25
Joined
2/27/2015
Location
Cedar Rapids, IA, USA
jackiemoon917 3/25/2016 6:42pm
3/25/2016 6:42pm
project racer wrote:
Always use at least the next letter size larger piston when replacing them on older cylinders with a lot of hours on them. The skirt broke...
Always use at least the next letter size larger piston when replacing them on older cylinders with a lot of hours on them.

The skirt broke because of piston rocking due to excessive clearance.

A "C" or "D" piston may have lived a bit longer ,but the cylinder was probably already out of spec on the first failure.

Measure things, instead of just throwing parts at it, and this can be avoided.
Will do thanks for the advice
mx5471
Profile picture for user mx5471
Posts
985
Joined
7/10/2008
Location
USA
mx5471 3/25/2016 7:33pm
3/25/2016 7:33pm
Been there, done that. Your answer is the gas you are using. you can't run it on pump gas if you are. You need above 100 octane. Had it with a stock 06. I bet your spark plug tip came off too. The start of the damage. Fix it and run higher octane gas, not from the pump.
chris225
Profile picture for user chris225
Posts
45
Joined
2/11/2016
Location
Henderson, NY, USA
chris225 3/25/2016 7:40pm
3/25/2016 7:40pm
I agree with the above. I had this happen with my 01. I started mixing half 110 with half pump gas and it fixed the problem
usafwx
Profile picture for user usafwx
Posts
1471
Joined
7/19/2012
Location
Trussville, AL, USA
usafwx 3/27/2016 10:09am
3/27/2016 10:09am Edited Date/Time 3/27/2016 10:09am
mx5471 wrote:
Been there, done that. Your answer is the gas you are using. you can't run it on pump gas if you are. You need above 100...
Been there, done that. Your answer is the gas you are using. you can't run it on pump gas if you are. You need above 100 octane. Had it with a stock 06. I bet your spark plug tip came off too. The start of the damage. Fix it and run higher octane gas, not from the pump.
I find that hard to believe as plenty of people run their 2 strokes on 93 pump gas without any problems, self included. My money is on the cylinder being out of spec in comparison to the replacement piston.

Post a reply to: Help: YZ125 Top End Won't Last