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Old 01-02-2015, 05:22 PM   #1
Auto-X Fil OP
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Joined: Feb 2013
Location: Montrose, PA.
Oddometer: 703
YZ250 Woods Build

I am currently setting up a 2000 YZ250 for woods use. Since these are pretty popular trail bikes these days, I thought I’d put together a build thread with my findings, to help others with their setups.

Why a YZ250? They are dirt-cheap, and readily available. I was looking for a KTM, but couldn’t find one in good shape for a price I liked. If I wanted a ready-to-ride bike, I would have kept hunting for an EXC, and paid more. It would have been cheaper and less work in the long run. As it was, I wanted a project, and a YZ has all the right basics. Low weight, great power, high reliability, cheap, available parts, etc. There's no reason that a modified YZ can't be every bit as good a bike as a KTM or Husaberg or Beta of the same era.

As-delivered, the YZ lacks in certain areas for woods/trail/enduro use:

The suspension is high-quality, but valved for MX. 1996-2004 are all the same basic shock and forks. 2005+ are on odd year, not compatible with anything else. 2006+ are better-valved SSS forks. However, there’s no reason that well-valved 46mm forks can’t perform wonderfully. It’s just that stock, they have crazy stiff low- and mid-speed compression for woods. It seems like most people even tone them down for MX. I'll go WAY into depth on this in another post.

The motor is great, but was a lot for a C-level rider like me to handle. A great rider might prefer it stock. It chugs well and pulls like a monster when it hits the pipe. It tends to roll onto the pipe pretty smoothly, more than hitting like a 125, which is a great characteristic for this kind of riding.

The first thing I did – the first thing I do on any bike – was to get the controls set up for me. I chopped the bars 2”, put on new levers, cut/bend them to my liking, added handguards and new grips, replaced worn and sticky cables, and lubed everything. I also moved the shifter and rear brake into position for me. Once that was done, I felt more at home, and could begin to feel the bike out.

As I rode around in stock form, the biggest thing I noticed was that the throttle was very touchy. On smooth terrain, I could modulate it. On bumpy, technical stuff, I was working incredibly hard to control the throttle and keep the bike from taking off on its own. I am used to mild four-strokes, but this was still way too hard to handle. There are two aspects to this issue: the control itself, and the way the engine responds to the engine. I tried everything you can do to solve this:

13oz Steahly Flywheel weight. This didn’t really change the engine when the clutch was out. Any difference in acceleration or hit is very minimal. It really changes the feel when slipping the clutch or idiling along, but does nothing to the issue above. I’ll leave it on for stall-resistance, but good clutch control and a strong idle might be a cheaper solution for most riders. It's certainly not a "must-do" mod when converting a YZ.

Steahly 13 oz flywheel weight:


FMF Gnarly pipe. I needed a pipe anyway, so it was worth a shot. It did smooth out the powerband a little bit, but the stock pipe was very good on this bike. Save your money if the stock pipe is in good shape.

It is quieter than the stock pipe, and more dent-resistant as well.



Timing retarded by ~3 degrees. This was HUGE. I was afraid the bike would just run like crap at -3 degrees, but the stator basically works like a power-control on the YZ. I lost no low-end, and almost completely removed the hit. It’s almost EASY to ride now. I’ll dial it back and find the sweet spot between power and controllability. This is so easy, and completely free, so it's worth playing with. The best way to do this is with a dial indicator - see this thread:

http://www.thumpertalk.com/topic/584...timing-thread/

I just did the math and figured out how much to move the stator in order to retard the timing. The said "screw it", made some arbitrary marks on the stator assembly, and eyeballed it. It's very easy to tap it a little bit, ride it, tap it again, ride again, etc. The absolute timing numbers mean way less than engine feel to me, so I am just going by the butt dyno on this one.

See that blob to the top-left of the flywheel weight? That's the pickup for the ignition system. To retard the timing, loosen the bracket that's attached to. It's held on with four big #3 Phillips screws. Mine came loose without much trouble, but some people say you need an impact driver to break them free. Once it's loose (don't remove the screws), you can slide it around. Sliding the ring counter-clockwise retards the timing.



G2 Throttle cam, and loosening the throttle cable. Extra slack in the throttle cable makes it less likely to whiskey-throttle from a closed position. The G2 cam makes the bike less responsive to small throttle inputs during the first portion of throttle-tube rotation, but then gets more and more responsive as you turn, so you reach full power at a similar rotation to stock. I've only ridden it briefly with this, so I'll report back on how it works over the next couple days. I hope that it'll reduce touchiness enough that I can dial some power back in with the timing.

I also added a 13t front sprocket, vs. the stock 14. This made stalling less likely, and crawling easier. No photos of that, hopefully everyone knows how to swap front sprockets.

Once the throttle was controllable, getting the suspension plush became my concern. The stock shock worked OK, but was bouncing me around a bit. The forks were awful, deflecting off everything. I ended up getting a bad, but rideable setup by cranking the fork and shock low-speed compression all the way soft, and leaving everything else alone. Other combinations were so bad as to be almost impossible for me to control. Again, I’m not a great rider, but they were BAD on trails. Between the touchy throttle and harsh suspension, I managed to whisky-throttle myself into a tree. There goes my MCL, and at least the start of my ski season. Ooops. (This was before I messed with the timing and got the throttle cam.)

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Old 01-02-2015, 05:46 PM   #2
Auto-X Fil OP
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Location: Montrose, PA.
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It's suspension time!



I used this how-to when I re-did the shock. It had been recently rebuilt, but with a faulty seal-head from Pivot Works. The dust-seal tore, and jammed itself into the main seal, which made it leak like crazy. I replaced it with an OEM KYB seal head.

http://www.idriders.com/proflex/file...%20service.pdf

I also re-valved the shock. Here are my original and modified shim stacks. The plan was to greatly reduce compression damping, especially in the low- and mid-low-speed ranges, to help suck up trail trash.

I basically swapped the thin shim from the HS stack with the face shim, and pulled 5 shims from the LS stack and put them on the bottom to take up space.

Code:
STOCK				MODIFIED		
Compression	Float:	Clamped		Compression	Float:	Clamped
Dia	Thick	Note		Dia	Thick	Note
40	0.2	x11		40	0.15	
34	0.1	xover		40	0.2	x5
30	0.1	xover		34	0.1	xover
40	0.15			30	0.1	xover
38	0.25			40	0.2	x1
36	0.25			38	0.25	
34	0.25			36	0.25	
32	0.25			34	0.25	
30	0.25			32	0.25	
28	0.25			30	0.25	
24	0.25			28	0.25	
22	0.2			24	0.25	
40	4.5	washer		22	0.2	
				40	0.2	x5
				40	4.5	washer
						
						
Rebound	Float:	Clamped		Rebound	Float:	Clamped
Dia	Thick	Note		Dia	Thick	Note
36	0.2	x6		36	0.2	x6
28	0.1	xover		28	0.1	xover
36	0.3			36	0.3	
34	0.3			34	0.3	
32	0.3			32	0.3	
30	0.3			30	0.3	
28	0.3			28	0.3	
26	0.3			26	0.3	
24	0.3			24	0.3	
23	0.2			23	0.2	
23	0.2			23	0.2	
26	3.1	washer		26	3.1	washer
19	0.7	shim		19	0.7	shim
Here is the stock valve assembly, on the shaft:


The valve itself:


Once I pulled the shims, I laid them out to measure them:


Here's the modified stack:


Assembled shock shaft, ready to go back together!


Assembled shock valve, with the modified compression stack on the bottom:


Now, onto the forks. The manual has a lot of detail on breaking them down, but not re-valving. I'll fill in some of the missing info.

You don't need to separate the inner and outer tubes to revalve. If your seals and sliders are OK, forget that step. To only revalve the base valve, it goes like this:

1) Loosen fork cap.
2) Remove fork tube from the bike
3) Remove fork cap with thin 17mm wrench, per the manual
4) Remove the nut on the damper rod, and the spring and spring guide. Drain fluid.
5) Pull cartridge and base valve apart and out with special tool
6) Remove base valve nut - it's aluminum, so you don't really need to file off the peened end, but running a tap over it after you pull the nut is a good idea.
7) Pull the shim stack apart, re-arrange, and put it back on the base valve.
Then follow the manual to fill the fork fluid and put it back on the bike.

KYB 46mm cartridge removal involves a special tool that fits that aluminum X on the end of the cartidge. You can make one and get to the base valve with a home-made PVC or metal tool - just get a piece of pipe and cut slots in the end.


To break into the cartridge and access the rebound and mid-valve, you really should have the special tool. It requires a ton of force.

1) Clamp it into a vice. I made a tool with a 2x4. Drill out the hole with a hole saw the same size as the cartridge, then slice it apart.
2) Drill out the detents in the cartridge tube
3) Use a torch to heat the end with the seal assembly to kill the loctite.
4) Twist it out with the special tool.


I ended up not messing with the valving on the piston, and I suspect most people can leave it alone and be happy. The compression has a ton (1.25mm) of float, so it's not too important and lower speeds. Rebound should be good unless you changed springs. This will save you a bunch of work.

Base valve out:


Base valve apart:


Shim stack removed:


Shim stack apart:


Base valve ready to reassemble:


Clamping the fork in a vice to fill the fluid:


Here's the stock piston valving, which I ended up leaving alone:

Code:
Mid Reb	Float	Clamped
Dia	Thick	Note
27	0.1	x4
12	0.1	xover
20	0.1	
18	0.1	
16	0.1	
14	0.1	
11	0.25	x3
		
		
		
Mid Comp	Float	1.15mm
Dia	Thick	Note
27	0.1	x7
25	0.1	
20	0.1	
16	0.1	
14	0.3	
14	0.3	
25	0.4
And here's the stock and modifed base valve:

Code:
STOCK				Modified		
Base Valve	Float	Clamped		Base Valve	Float	Clamped
Dia	Thick	Note		Dia	Thick	Note
24	0.1	x10		24	0.1	x5
22	0.15			16	0.15	from the HS stack
20	0.15			22	0.15	
18	0.15			18	0.15	
16	0.15			14	0.15	
14	0.15			13	0.15	
13	0.15			12	0.15	
12	0.15			11	0.3	
11	0.3			18	0.5	x3
18	0.5	x3		24	0.1	x6
24	0.1			20	0.15
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Old 01-02-2015, 05:50 PM   #3
Auto-X Fil OP
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Joined: Feb 2013
Location: Montrose, PA.
Oddometer: 703
This bike was a bit neglected, and I needed to tear into the linkage, as all the bearings were shot. Removing the pivot link bolt that goes through the swingarm was awful - it was completely rusted to the bushing sleeve. It took a couple days of torching it, spraying it with penetrating oil, and smashing it with a hammer. Once things were apart and cleaned up, it was time to get down to business!


Pressing first bearing:


Pressing in second bearing - note the plastic filling the hole. I always jam something in there to prevent from losing a needle. They are usually just held in with grease.


One was to get the right spacing from the end of the link is to use washers or shims of the proper thickness under the bushing. Then you just press down until it hits.

5mm worth of washers:


Or, mark the depth of the bearing on the socket with tape:

Don't press on the outside of the swingarm, it'll flex the arm in. Generally, swingarm bearings are a pretty easy press, so you won't damage the swingarm, but it'll flex enough that it's tough to keep things square. A 1/2" ratchet extension is a great way to access the lower side once you have bearings in place.


Pressing the connecting link bearings in:


Lubricating the bearings with the good stuff:

In general, bearing depth isn't super-critical. But I'm anal.

Ok, sometimes you CAN press on the outside of the swingarm. My biggest impact socket just so happens to be a perfect fit for the gap in the center, to prevent flex or deformation.


The bearings on the outside of the swingarm have thrust washers that go over them, so they can be pressed flush with the face of the recessed hole.

I just pressed them in with a large washer, so that when it bottomed on the face of the hole, I was done.


Thrust bearings:


Polishing a horribly rusty linkage bolt.


Forks back on!


Swingarm back on!


It looks like a bike again! Bonus points if you can spot what's missing:


After a quick test-ride:
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Old 01-02-2015, 06:40 PM   #4
EastBoundAndDown
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Killer!

I decked out an 03 RM250 for woods years ago. It was great once the suspension was revalved. Handled nimbly and always power on tap.
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Old 01-03-2015, 06:28 AM   #5
brucifer
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Excellent! Nice write-up and pictorial.
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Old 01-03-2015, 07:30 AM   #6
Auto-X Fil OP
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Well, I just got a nice hour-long ride in. It's still way too much bike for my novice skills, but it has no evil habits for woods riding anymore. The new suspension valving works very well - it feels quite similar to my WRR, just a bit stiffer. There's no more deflection, though. I got bounced a couple times, so I might need to dial up the rebound, but it was hard to say. There's so much to get used to with this bike that I don't notice subtle changes very easily.

The G2 throttle cam and a little extra slack in the cable got rid of all the touchiness. It was actually a little flat, so I dialed the timing back a degree. I'm having a little trouble popping the front tire up without accelerating too fast - the pilot screw seems OK, but that might be part of the issue. It might just be me needing to use the clutch on that stuff, when I'm used to just cranking the throttle on the four-stroke. It's 20 degrees right now and riding in cold, low-traction conditions is so different than the summer that I'm not going to worry about fine-tuning the engine or suspension, yet.

Here's one of my favorite spots to ride. It's all open woods, where I can just putt around popping off rocks and logs, or string together a 2nd-gear shot through the trees.

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Old 01-03-2015, 09:15 AM   #7
vandiesel
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What a great write up. Thanks for taking all that time to share.


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Old 01-05-2015, 06:03 PM   #8
Auto-X Fil OP
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I did some riding with the G2 throttle cam. It's awesome. As the weather had gotten a lot colder, I needed to tweak my air screw before the bike had good throttle response. Once I did that, the bike was responsive, yet controllable. I backed the timing to within 1 degree of stock, as I don't need to kill the hit so badly with the throttle cam.

In summary, here's my recipie for a YZ trail bike:

Buy a stock, steel-framed YZ.
Re-valve per my specs above ($50 for tools and oil).
G2 throttle cam ($60)
If you're feeling spendy, and ride real slow stuff, add a flywheel weight and lower gearing ($120-200).

That's all it needs! Well, unless you buy a $1000 bike that needs tires, chain, sprockets, bearings, rings...
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Old 01-17-2015, 10:00 PM   #9
BIG ED XT FAN
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YZ 250 Woods bike?

Thanks for the write-up. I am going to redo my forks and shock on my WR400f!
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Old 06-07-2015, 12:43 PM   #10
Barnman
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Quote:
Originally Posted by Auto-X Fil View Post

Base valve ready to reassemble:

Great write up

Do you know if it is possible to remove the compression adjustment needle from the base valve? Mine has started leaking from the adjustment screw, so I am guessing an o ring has failed. However it looks like the adjustment screw and needle are pressed into the base valve.

Barnman screwed with this post 06-08-2015 at 01:29 AM
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Old 06-08-2015, 07:12 AM   #11
Auto-X Fil OP
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Sorry, I didn't mess with it. You might try the tech forums over at Thumper Talk - they know this stuff better.
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Old 06-08-2015, 10:14 AM   #12
Barnman
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Thanks for the reply

For the record, it is pressed in, although it is possible to use a lathe and just trim the punch points off from the bottom and then push it out to get to the seals.
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Old 06-09-2015, 05:03 AM   #13
Auto-X Fil OP
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That's very good to know, thanks for the update. Base valve assemblies are expensive!
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Old 06-09-2015, 06:21 AM   #14
trailer Rails
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Looks like Yamaha saw your build and decided to copy it. It is nice to see the Japanese investing a little in two stroke.
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