Jump to content

125 SX Toasting Conn Rod Bearing

Recommended Posts

I bought a 2003 125 sx and after about 5 minutes of riding I knew I needed to clean up the power valve. Once I reassembled the bike it ran great... for about 10 miles until the connecting rod welded to the crank. I'm sure I contaminated the crankcase when I had the bike apart so live and learn... Fast forward a couple months and I clean everything and reassemble the motor with new seals and a Wiseco crank and piston. It started and ran no problem for about 20 miles and the bearing on the big end of the connecting rod disintegrated.

I'm getting good at tearing the bike apart and I have some cool "trophies" but a new crank and piston every ~15 miles is not very sustainable due to budgetary constraints and I don't think I should have to inventory replacements. I am running Amsoil interceptor at about 40:1 and both sets of piston and main crank bearings looked good to me after disassembly. I cleaned the carb and made sure the settings were at factory spec for conditions (main jet size was 2 pts above recommended). No damage to the jug or cylinder coating that I noticed (I will check again). The spark plug was pretty dark and there was a lot of spooge on the rear of the bike.  

So I'm here looking for a cause (or 2) of my woes...

I'm not sure if it was too much time at WOT? I'm too used to sleds.

My other thought was I didn't clean everything properly when I reassembled.

I can't think of any other problems except maybe a parts defect the second time around but that seems unlikely to me.

Share this post


Link to post
Share on other sites

I checked it compared the the manual when i put the last crank in. Air screw was way off but only thing different was the main jet being a little rich:

Air Screw Open = 1 1/4

Idle Jet = 48

Needle = R1469D

Needle Clip Position = 3

Main Jet = 190

Capture.JPG

Share this post


Link to post
Share on other sites

Stay away from Weisco cranks...seriously.  Find an oem and have it rebuilt, or go with hot rods.  Weisco are known to be problematic. 

  • Like 1

Share this post


Link to post
Share on other sites

If you put in the new lower end and spent much time at WOT right away, that could explain it. A thousand heat cycles and mondo babying isn't necessary, but a bit of break-in before flogging it is probably wise.

  • Like 3

Share this post


Link to post
Share on other sites

I've had good luck with Wiseco in the past but I will definitely look into trying out something different. 

Where's the fun in breaking it in properly? I did some easy riding around the yard and in the street while I was trying to get the carb adjusted but it probably wasn't enough...

Share this post


Link to post
Share on other sites

Old weisco stuff wasn't as bad, in more recent years most of their stuff is Chinese, QC has gone down.  Look in the Yamaha 2 stroke forum, ripe with blown cranks.  Some of their stuff is fine, cranks seem to be their biggest issue.  Hard to beat an OEM rebuilt crank. 

Share this post


Link to post
Share on other sites

I recently blew the lower conrod bearing in my 2001 125 engine. I installed it used in my 2000 bike about 7 years ago. The original 2000 engine got water logged 3 years in a row and its lower conrod bearing failed several months after the last dunking. It gave some notice and didn't do any cylinder damage. This time the rod bearing gave no notice, failing noisily at 70mph. It didn't damage the cylinder or head either. Good engineering these KTMs. I have a spare 2003 engine, but when I checked it out, its lower conrod bearing is bad too. 

My 2 engines that were in the bike ran many thousand street miles, so my feelings are not hurt. I do wonder if the lower conrod is a weak point or if I can protect it any better than Motul 40:1? I think it is probably the first fatal flaw. I have replaced pistons and wrist pins and bearings along the way. First engine went 7 years untouched and both have original clutches. Mind you, I am not a hard rider, other than high speeds, long miles and water...

Edited by sbest

Share this post


Link to post
Share on other sites

Both time have been without notice. The first time the rod stuck to the crank so it was a pretty abrupt stop from halfway through 5th gear. The second time was about the same speed. There was some notice but it was still too late and the damage was done.

I don't think there is much more protection to be offered to the lower connecting rod. It's directly in the path of incoming fuel and oil and mixture oils are mostly a matter of preference in my opinion...

Share this post


Link to post
Share on other sites

You have to take it easy with the engine for a wee while when you've rebuilt your crank. As you will know the crank 'runs' in a bath of 2 stroke oil which is caused by the heat burning off the petrol from some of your mix while in the crankcases, allowing fairly neat 2 stroke oil to continually lubricate the bottom end bearings and bore. If you just rebuild it and go blasting, then the con rod bearing in particular will overheat and lock up. By the way agree with poldies4 - try and use oem bottom end. Bob.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×