2005 YZ250F Top-end rebuild

I purchased a used 2005 YZ250F two years ago. It looked decent but defiantly would need a few things soon. One was the top-end needed piston and rings within a year. The bike had a carburetor issue that took me some time to figure out which made for little riding the first year I had it. Note: make sure you check all jets and needle settings.

Now was the time for rebuild. This was only the second time rebuilding a 4-stroke. I have done a few 2-strokes and had no issues. There is a lot of “things” in play with a 4-stroke. Valves, cams, timing chain, pulling flywheel to access timing chain…the list goes on.

I started out reading the service manual and watching youtube videos. Many videos. The best was the RM-ATV videos on rebuilding the YZ250F.

I researched parts and part numbers for my dealer to order. I got those parts on order and started disassembly. One thing you need to do is clean the bike. It needs to be spotless. You do not want dirt falling in and going for a ride on your main bearing. Bad stuff.

I followed the service manual for disassembly. Make sure you go slow. Make sure you take out your cam chain tensioner before removing the bolts off the cam.

Check all the parts. You can see that my cylinder doesn’t look bad but it really is. The coating on the cylinder is gone. Junk as my dealer said.

Cam chain guide was actually broken at the bottom. Yikes!

Piston looked okay. I’m replacing so no measurements taken.

I was not doing the valve seats this time. I tried to see if they leaked. They seem okay. Search YouTube for how people do this. I’m not so hung up on it since it is a Yamaha motor. 🙂 Note: the valves were all in spec before I removed them. You will want to make sure you keep valve caps and shim in order.

Keep yourself organized! Key to getting it back together without missing parts.

I went OEM with most of my parts. Price for everything vary depending on where you purchase. The dealer hooked me up with a Millennium Tech refurbished cylinder for me. It was nice and pretty.

Don’t mind the Kold Kutter screws…I’ll show you those after I get this running.

I again watched more videos and looked over the service manual. Am I really going to do this? Ugh. Here I go…

Not many pictures once I started. The worst part was compressing the rings and installing the cylinder. I really needed an extra set of hands careful get the rings into the cylinder! I had to go really slow for this. Torqued everything down and now I have this!

Done for the most part. Note: the exhaust header bolt is a pain. I stripped out three threads on the motor! The new crush gasket was really thick. I could barely get the threads into the motor. I ended up reusing the existing gasket. Prefect…no. I stopped after the stripped bolts. Tired and working on a motor is not a good plan. I believe it took me about 8 hrs over two days to come to this point.

I now need to bolt the rest of the exhaust on, radiators, plastic, seat and add oil and coolant.


Completely back together and it started!

The bike would not stay running because the idle was so low. It would run 30 seconds and die. After playing with the idle a few more time it stayed running on the 4th re-start. The bike sounded great! I ran it about 6 mins and rev’ed it a few times. Nothing crazy. I shut it off and let it sit for 5 mins so I could check the oil height. I was a little low and added 100cc more of oil. The total I added is now at 1100cc. Most oil changes are in the 1100-1200 range.  It may need another 100cc but I’ll have to see.  I’m saving the next re-start for the weekend when I can put a load on the motor.

Also, changed out the rear tire and put in Kold Kutter screws.


About the author: Molti
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