Need help w/full top end rebuild

Before you start saying wrong forum, understand that I dont like the YZ/WR250F forum because most of the people on there are younger kids who dont know what they are doing. That being said, we need help rebuilding the top end on my dads '02 WR250F. We havent cracked the case yet but we are just going to go ahead and replace everything. valves, valve springs, cams, cam chain, piston, piston wrings, gaskets, and everything else that I am leaving out. The bike is 5 years old so I figure that I might as well go ahead and replace everything. (Do I really need to replace cams? If I need to I will prolly do the auto decompression cam and may even get aftermarket)

Part of the reason why we are doing it now in the middle of the summer riding season is because it wont start. It hardly has any compression and we are thinking that this is why it wont start. When we first got the bike we COULD NOT even begin to move the kick start when it was at TDC w/out the compression release. Now, it feels like my KX125 did right before we rebuilt the top end. My dad and I both think that the lack of compression is the reason why it wont start seeing as it was getting gradually harder to start as it lost compression.

My dad is offering to take it to the shop and have them do it but I dont want him to have to pay $1000 and besides, I want the experiance of messing with a four stroke engine. I was going to get a motopower video and use that but there wasnt one for my dads bike so I figure that I would just use the manual we have. The only problem is that I dont know what tools I will be needing to do the stuff that I am talking about. I do have a basic tool set. I am kinda nervous about doing this major rebuild. Should I be worried?

I am considering refreshing the motor on my '03 450 while I am rebuilding my dads top end. What should I do to it? I havent done anything AT ALL except for basic maintenance. I have been very good about the maintenance but I have had the bike since novemeber and I figure that it is probably time to do something.

I appreciate all of your tips!

Before you start saying wrong forum, understand that I dont like the YZ/WR250F forum because most of the people on there are younger kids who dont know what they are doing.

some may argue, but i know what i'm doing.

(Do I really need to replace cams? If I need to I will prolly do the auto decompression cam and may even get aftermarket)

check the cams for wear and for warpage. if there ok then no, don't replace, if you want more power, try an after market cam.

Should I be worried?

absolutly not. if you have time, patients, skills, and tools you'll do it no problem. any of the mentioned are lacking and it may be a head ache.

I am considering refreshing the motor on my '03 450 while I am rebuilding my dads top end. What should I do to it? I havent done anything AT ALL except for basic maintenance. I have been very good about the maintenance but I have had the bike since novemeber and I figure that it is probably time to do something.

you could re-ring, check valve seats, valves, make sure there in spec. thats about all i can think of

good luck and best wishes in getting your bikes running good :ride:

When was the last time you checked your valves? A tight exhaust valve could make the bike loose compression.

Before you start saying wrong forum, understand that I dont like the YZ/WR250F forum because most of the people on there are younger kids who dont know what they are doing.

Roger that...with a few exceptions like GreyRacer and Bill P (and others...if I didn't mention you, please don't take offense, those two are just the first that come to mind).

We havent cracked the case yet but we are just going to go ahead and replace everything. valves, valve springs, cams, cam chain, piston, piston wrings, gaskets, and everything else that I am leaving out. The bike is 5 years old so I figure that I might as well go ahead and replace everything. (Do I really need to replace cams? If I need to I will prolly do the auto decompression cam and may even get aftermarket)

You probably don't need to replace the cams...unless the journals are worn. I've never heard of the lobes wearing beyond service. I will tell ya, though, that putting the auto-decompresor exhaust cam in my son's 250 and my 426 were just about the best mods we've ever done. No better time than when it's apart.

Part of the reason why we are doing it now in the middle of the summer riding season is because it wont start. It hardly has any compression and we are thinking that this is why it wont start. When we first got the bike we COULD NOT even begin to move the kick start when it was at TDC w/out the compression release. Now, it feels like my KX125 did right before we rebuilt the top end. My dad and I both think that the lack of compression is the reason why it wont start seeing as it was getting gradually harder to start as it lost compression.

That's characteristic of a bad valve. Have you checked the valve clearances? One thing I think is worth doing...once you have the motor back together, break it in using your preferred method (and break in methods seem to be like butt holes or Trans Ams in the eighties...everyone has one!), then run a compression check so you have a baseline of what a "fresh" motor puts out. Then, you can periodically take readings to gage the health of your scooter.

My dad is offering to take it to the shop and have them do it but I dont want him to have to pay $1000 and besides, I want the experiance of messing with a four stroke engine. I was going to get a motopower video and use that but there wasnt one for my dads bike so I figure that I would just use the manual we have. The only problem is that I dont know what tools I will be needing to do the stuff that I am talking about. I do have a basic tool set. I am kinda nervous about doing this major rebuild. Should I be worried?

No worries. It's a pretty easy job. In addition to basic hand tools, you will need a 1/4 drive, in-lb torque wrench, a ring compressor (cheap at Auto-Zone), a set of micrometers or a machine shop that will measure your cylinder bore, cam journals, valve guides, etc., and a valve spring compressor (check this thread... http://www.thumpertalk.com/forum/showthread.php?t=222358&highlight=valve+spring+compressor I used 1/2 inch EMT conduit instead of a socket...worked great!). If you are replacing the valves and springs, I would replace the keepers, too. They are cheap, and just a little bit more insurance. Make sure you get the valve seats machined as well. Eric Gorr and Engine Dynamics both have great reps. I couldn't find a local shop in my little slice of paradise that I felt comfortable with, so I sent mine off to Gorr.

For the 426/450 crowd The jury is split about 50-50 between Kibblewhite Stainless valves and springs and stock Ti parts. For the 250's, I think it's about 75-25 in favor of stock. You may want to do a search and read up on the opinions and experiences of some of the smart guys with respect to valves. I went stock on my 426, as I believe the higher spring forces required for SS valves put higher loads, and thus cause higher wear, on the valves and seats.

Another thought...if your bore is out of limits for either size, taper, or ovularity (out of round), by the time yuo add up all the costs of stripping and replating, gaskets, piston, rings, etc, it's just about a push to do a big bore kit. I've had great experineces from both Eric Gorr and Luke's racing. I think Lukes is a bit cheaper, and usually has the barrels and pistons in stock, where Eric usually has to turn yours around.

I am considering refreshing the motor on my '03 450 while I am rebuilding my dads top end. What should I do to it? I havent done anything AT ALL except for basic maintenance. I have been very good about the maintenance but I have had the bike since novemeber and I figure that it is probably time to do something.

If you open up both bikes at the same time, be sure you keep ALL the parts separated! I take digital pictures of EVERYTHING before I take it apart, and I use a crap-load of zip-lok baggies to hold parts. I mark on the bags with sharpie pens so I know what went where. That would make it a whole lot easier to keep from getting parts mixed up.

As far as what to replace...as Way Fast Whitey said, rings, and inspect everything really well. Look for cracks along the wrist pin boss (not a problem on the Yamahas, but good general practice anyway). Check the bore, and check the piston dimensionally. If you pull the head apart, be sure you put the valves, keepers, collets, etc back in the holes they came out of! Check the big end bearing for side play and radial play while you have it apart, as well. Also, if you do take the valves out and they all look good, you may want to think about new springs (a set is only 25 bucks thru the TT store) and they are what fatigue first. Oh, and replace the valve seals if you take it apart as well.

Good luck. It's an easy job if you take your time, follow the book, and work carefully. Might be a good project for you and your pop to do together...my 15 year old son and I work on bikes all the time together (on my bike he's the "tool bitch", and on his bike, I'm the "tool bitch". I also still help my 78 year old dad work on his toys, and really enjoy that time together.

Best of luck. Birdy

that happened to me and the problem were only the valve seats and valves, 2 were leaking and the valves needed adjusting, after that she was ok again :ride:

There is a great online article around for the 250, that walks you through step by step. I think it was was from dirt rider magazine but I could be wrong. I know that I have seen links posted to it here before. I used it the first time I tore apart my 426. It really isn't a hard job to do, even with no experience. Maybe someone could post the link again? If you can't find it and want it PM me, I've got it on the hard drive some where and can e-mail it....

Like the others said, have you tried adjusting the valves? If they're out of spec, it'll be hard to start.

Wow, thanks for all the help guys, I really apreciate it!

Create an account or sign in to comment

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

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now