YZ250 rebuild for Trail Riding

[dk_250]

Last year I bought a used 1996 YZ250 and have been working on it to bring it to good working order. Its a super fast bike and has lots of aftermarket parts on it already, but this summer im going to break it open for my first rebuild...Im sure it needs at least a top end. There are problems of course....

At idle (and near idle) there is a knocking sound inside (piston slap?). I have an uneven idle most of the time, itll sound okay and then idle kind jumpy, cant really xplain it. It seems like the bike needs to idle fairly high to keep it to not die, so on hill decents there is no real engine braking. Is this because of low compression? or just the nature of the beast (I used to ride an IT250, more of a trail natured 2-stroke). Another odd thing is I need to have the kickstarter almost all the way down and kick only about 4in for it to do anything, if I kick from the top it wont do anything. 2 times ive fallen and came back up to see that my kickstarter is moved forward (like it retured too far). So I just took it off and moved it back, is something stripped inside?

I would love to do some mods this summer to get this beast back to shape and good for trails (desert riding, rocks, sand), and I guess then I can see if my crank or rod bearings are good. Im thinking of replacing my dented FMF fatty pipe (cant tell a diff in performance) for a gnarly pipe, a scotts stabilizer (which ive heard is a must especially since I get crazy speed wobbles in sandy woops). What else can I do to make this 2-stroke more of a trail boss than an mx'er?

Sorry so many questions in my post, I just have so many issues that id liked fixed. Thanks.

 

[dk_250]

anyone?

 

[Reesknight]

Yes that sounds like piston slap and if I were you, I would not ride it again until the top end was replaced or you run the risk of serious engine damage. As for the idle, i would think you either have a small air leak or the needle seat is worn and that bike will have very little to no engine breaking. It also sounds like the ratchet and/or spring is worn inside your kickstarter. As for making it more of a trail bike, I would say the garly pipe with a pipe guard would be good as well as a flywheel weight and you may want to look into getting it ported for more low end. Definately get the stabilizer but until then, you may want to try raising the forks in the clamps(lowering the front end) to help with the head shake. I'm sure you also want to do normal maintenance like wheel bearings and I'd bet the swing arm bearings are shot if the previous owner did not replace them. Good luck! :thumb:

 

[rethnal]

Sell it and buy a trail bike! That bike will be a money pit and never do what you want on the trails. You will foul plugs and be pushing it out of the woods cursing it. Trust me... I know!

 

[Lorin]

As to whether or not that particular bike is a money pit remains to be seen. But, the yz can make an excellent woods bike with a flywheel weight, and a little suspension tuning. Fouling plugs comes from jetting, not from the brand \ type of bike. The 96 actually has a pretty good motor stock.

 

[rethnal]

The carb is just plain to big for trail riding... I'm sorry but it will still foul plugs! The gearing is wrong and you will not stay in the powerband, the fork angle is wrong, a YZ is made for wide steering and will push corners.. I could go on and on but really it is a moot point. And yes... It still will be a money pit!

 

[Mully]

Ok, I have to ask. What does the size of the carb have to do with fouling plugs ??

Mully

 

[Lorin]

The size of the carb has nothing to do with the fouling of the plugs. FWIW, I am running a 39.5 pwk carb from a kx 500 on my 98 yz and havent fouled a plug in two years, and that is riding enduros, trails, and cross country riding almost exclusively. Steering the yz hasnt been a problem either. You need look no further than the National GNCC circuit to see a yz constantly on the podium (both Barry hawk and Raines). All said, the 96 yz is a solid platform that can do well in the woods. I dont usually get in these debates, but the guy wasnt looking for an excuse to sell his bike, just some ideas to make it easier for him in the woods.

 

[fuzzy]

Yes, all the mods lorin mentioned are a must. I would also recommend keeping your eye out for someone parting a WR, and snag the tranny. A 94-97 WR trans will swap right into your motor allowing you to get pretty close to a modern WR250Z, which is what the factory guys are tearing it up on. Too bad there is no production version of a WR250 2-stroke as their seems to be more of a market for enduro 250's now than ever--damn 4-stroke assimilation--NEVER! Modding a MX-er like this is the only choice off-roaders have to buying a euro enduro 250 (KTM, Husky, etc).

You will spend some bucks on it, but hopefully you got yourself a good deal on the bike in the first place. In the end you will end up with a modern YZ chassis adorned with all the off-road goodies for less than you would've paid for a '04 YZ250, and actually have the bike you want--a potent woods weapon at that.

 

[dk_250]

I got a fairly good deal on the bike, and its paid for so im not planning on selling it. I have NEVER fouled a plug in this bike, so im not worried about that. Where can I get flywheel weights? How does this help, and what weight should I get? How much would make a difference on raising the fork tubes, theyre set at stock 0.22 in, it will take a little time to save up for a stabilizer.

Right now im kinda worried about my engine. Yesterday i went out and when i finished with my ride my bike was smoking more than usual, even at idle. Im pretty sure my rings are gone. Im not going to ride anymore until i do my top end, what should i be looking for when i open it up???

 

[Rcannon]

I would look at your powervalve and see if it is brushing the front of the piston. This has been common on the YZ's of this era. You can take off the pipe and look in the exhaust port. If there is a shiny or damaged spot on the front of the piston, you have your answer. The only good solution is to replace the p/v and cover.

The YZ's are great trial bikes with a flywheel weight. I can use my 97 to follow my nephew around on his XR 50 in the morning and ride the track in the afternoon.

I have had very little plug trouble. Jetting for your conditions is very worthwhile.

 

[dk_250]

okay i took apart my top end. my head is good, my cylinder looks good with no significant wearing on one spot, but i dont have an inside micrometer to check wear. Ive never done this before so i dont really know what to look for. Piston had slight wear on the back and has alot of carbon on top.

Here is the problem, my rod moves side to side ALOT. both on the piston side and crank side. Its hard to feel but i dont think that it moves too much up and down, but im sure it does...So im thinking i have to split the case and replace maybe crank bearings/seals and rod bearing.

took off the powervalve covers and the one on the left was filled with tar like oil, coated the whole inside. was all dried up but you can put a fingerprint in it or fingernail a scratch in it. Whats the story on that. On the left side cover there is what appears to be a drain screw, whats the story on this?

I dont have parts at the moment but am going to get top end kit first (get a good deal from a friend). and then have someone over to see whats wrong with my bottom end and fix that before i put everything together.

Rcannon, ill check on the powervalve later today. Anyone have suggestions on what else to check while i have the bike apart, it should be apart for awhile cause i cant buy everything at once.

thanks everyone.

 

[dk_250]

delete

 

[Rcannon]

Side to side movement is ok...
Up and down is bad! The powervalve ara on these bikes is filthy. The black tar appears to be what is left over after the gas oil mixture burns. Yamaha uses this crud to lubricate teh powervalve as it moves. I have tried to drain mine before, but nothing has ever dripped out. I clean it out every few months for the heck of it.

You can also rub some of it on the mailbox (where the letter carrier grabs it) amd watch him/her get very angry from being filthy.

If you look at the ceneter of your piston, the exhaust side, you can see if the powervalve has been hitting. There will be a polished stripe there.