Power Sports Tuning: Yamaha Yz250 Rebuild - Power Sports Tuning

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Yamaha Yz250 Rebuild Suspension and engine rebuild

#1 User is offline   Ziggy

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Posted 25 September 2004 - 11:46 PM

2002 Yamaha YZ250

Engine Re-build and Suspension Repair


First off lets get some specs on this model,..

In 2002 Yamaha corporation made some changes to the previous YZ250 model, these included engine and chassis changes. It was “bike of the year” the previous year and in my opinion they made the MX bike better with a smoother power band and improved handling with the frame changes, All together that won them the title of “MX Bike Of the Year” in 2002.

Model Name YZ250PI (USA CDN ZA)
Model code 5NX1 (USA CDN ZA)
Basic weight 105.0 kg (231 lbs)
Engine Type Liquid cooled 2-stroke,gasoline
Cylinder arrangement Single cylinder, forward inclined
Displacement 249cm3 (8.76 imp oz, 8.42 US oz)
Bore X Stroke 66.4 x 72mm (2.614 x 2.835 in)
Compression ratio 9.1~10.6:1
Cylinder head
Combustion chamber capacity 21.5cm3 (.757 IMP oz, .727 US oz)
Warp Limit 03mm (0012in)
Bore size 66.400~6.414mm (2.6142~2.6147in)
Bore size Limit 66.400~6.414mm (2.6142~2.6147in)
Taper limit 66.5mm (2.618in)
Out of round limit .05mm (0.002in)
Piston .01mm (.0004in)
Piston size 66.352~66.367mm (2.6120~2.6129in)
Measuring point 17.5mm (.69in)
Piston Clr .045~.05mm (.0018~.0020) Limit .01mm
Piston offset 1.5mm (.059in)/ex-side
Piston Pin OD 17.995~18.000mm (.7085~.7087in)
Piston Ring Plain
Thickness 1.0mm (.0395in)
Width 2.55mm (.100in)
End gap installed .04~.050mm (.016~.022in) limit .95mm
Side clr installed 1st .03~.065mm (.0012~.0026in) limit .1mm
2nd .03~.065mm (.0012~.0026in) limit .1mm
Crank width outside lobes 59.95~60.00mm (2.36~2.362in)
Runout TIR bearing fits 3mm (.0012in) limit .05mm (.002in)
ConRod big end side clr .25~.75mm (.010~.03in)
Small end free play .4~1.0mm (.016~.039in)
Carburetor USA   Europe
Type/Manufacturer PWK38S/KEIHIN
ID mark 5NX1 00 5NX2 10
Main Jet #178   #175
Main Air jet #200   <
Jet needle/clip pos N3EJ/2   N3CW/3
Main nozzle 2.9   <
Cutaway #7 <
Pilot jet #50 #52
Pilot air screw turns out 1 1 3/8
Valve seat size 3.8mm (.15in) <
Starter Jet #85 <
Power Jet #50 <
Float arm Height 5.5-7.5mm (.22~.03)   <
Reed Valve
Thickness at the tips .42mm (.017in)
Valve stopper height 10.3~10.7mm (.406~.421in)
Valve bending limit .2mm (.008in)
Spark Plug
Type/Manufacturer BR8EG/NGK
Gap 0.5~0.6 MM (.020~0.024in)
Ignition System
Ignition timing (BTDC) .18mm (.007in)
Advancer type Electrical
Magneto model stator/manufac 5CU-01/Yamaha
Source 1 resistance (color) S720~1,08ohms @20deg C (68deg F) (Black-black/red)
Source 2 resistance (color) 44~66ohms @20deg C (68degF) (White/Blue-White/Red)
CDI unit/Manufacturer 5NX-00/Yamaha
Ignition coil
Model/ Manufacturer 4mx-00/Yamaha
Min spark gap 6mm (.024in)
Primary winding resistance .20~.30ohms @20deg C (68deg F)
Secondary winding Resistance 9.5~14.3ohms @20degC (60degF)
Clutch Type: Wet, Multi-disk
Friction plates (8) thickness 2.9~3.1mm (.114~.122in) limit 2.8mm
Clutch plate (7) thickness 1.5~1.7mm (.059~.067in)
Warp limit .2mm (.008in)
Clutch spring free length 50mm (1.969in) limit 48mm
Transmission USA, ZA, AUZ, NZ EUROPE, CDN
Primary reduction System Gear
Primary reduction ratio 63/21 (3.00)
Secondary reduction System Chain  
Secondary Reduction ratio 50/14 (3.71)   14/49 (3.500)
Transmission Type 5-speed
Operation left foot  
Gear ratio 1st 27/14 (1.929)
2nd 25/16 (1.563)  
3rd 23/18 (1.278)  
4th 24/22 (1.091)  
5th 20/21(.0952)
Chain Type/manufacturer DID520DMA2 SDH/DAIDO
Number of links 113links + joint
Chain Slack 40-50mm (1.6~2.0in)
Frame type Semi-double cradle
Caster angle 27.0deg   26.8deg
Trail 118mm (4.65in)   114mm (4.49in)
Type with tube
Size (front) 80/100/-21 51M
Size (rear) 110/90-19 62M
Tire pressure (front and rear) 100kPa (1.0kgf/cm, 15 psi)
Front brake type single disc brake
Operation right hand operation
Rear brake type single disc brake
Operation right foot operation
Front shock absorber Telescopic fork
Rear shock absorber Swingarm (link type monocross suspension)
Shock absorber
Front shock absorber Coil spring/oil damper
Rear shock absorber Coil spring/gas, oil damper
Wheel travel
Front wheel travel 300MM (11.8IN)
Rear wheel travel 315MM (12.4IN)

Ok now that we know the Bike, let’s get a little history on it. I bought the bike brand new in the spring of 2002, It was then sold spring of 2004 to a friend of mine. He is the current owner. The bike has had the exhaust port reshaped and some casting flaws have been removed from the barrel, it has a fmf fatty pipe and also a 49 tooth rear sprocket, these are about the only modifications that have been done to it. The reason it came to me is that Matt, the current owner seems to like to jump it. Large jumps. While jumping it he noticed that the suspension was having some troubles. Seems that one of the front forks is bent and or has a piston seizing problem. But we will not speculate. We will find and fix the issues. At first glance the bike is very dirty and has been landed rough a couple times. But when you jump 80ft tables all the time, that’s bound to happen. The bike also had a mysterious oil leak. Well lets get to it…

Phase 1 (Motor removal)

First thing you want to do is make sure you have the correct tools for this job, Then get the bike up off the floor so you can work on it comfortably. The stand that bike is on is here. It’s functional and cheap.

Here is a tool you can make for removing the exhaust springs on the two stroke equipped power sports vehicles.

All it is a piece of .125 (1/8 inch stainless wire) wrapped around a handle. I guarantee every time you need to remove a pipe you will wish you had one of these.

Drain the fluids; remove all plastic and anything that will get in the way of the motor coming out. On the YZ models most of them, the motor has to come out the right side, (when sitting on the bike), so the brake lever and brake fluid reservoir have to be removed. On the reservoir you can just remove the screw and move it out of the way. Remove the throttle and clutch cables. Disconnect the two quick connectors for the carb electronics. Unplug the CDI unit cable from the Magneto. Remove the chain by splitting the link. You may have a continuous chain (no Link), if so, a quick trick for this is to remove the front sprocket shaft nut and remove the washer. Once you get the motor loose you can have a buddy hold the sprocket while you remove the engine. But if your rebuilding it would be good to have a chain breaker to split the chain. You will probably ready for a new chain. Once you have all the cables, hoses, and wires disconnected you are ready to remove the engine bolts. The rear one is last because we have to support the bike while you remove the Swingarm pivot bolt. You will need to use a stable jack under the bike to hold it up while removing this bolt. Once you have the motor out you can put the bolt back in so the rolling chassis is rollable.

When your removing parts label them or at least put them in little zip lock bags, you do not want to loose any vital parts. These bikes seem to loose nuts and bolt on their own we do not need to help. Also watch out for warn or broken parts that you will want to replace when putting the bike back together. We found that on this Bike the oil plug bolt hex was pretty chewed up and needs to be replaced.

The first thing I look at, when I pull apart a bike is the air filter. I am looking for your ability to service the bike when it needs it. Most MX bike riders do not care or take enough time to replace the parts that have to be maintained after every ride. If nothing else, Do yourself a favor and install a fuel filter and maintain your air filter after every ride. This is the very min. of maintenance but can save you huge repair bills. This filter needs the be throw out. It has a filter skin on it but is covered with sand and mud. I was riding with the owner when this occurred and it was at the end of your riding weekend when we ran into some water. ;)  so his bike was going up on the rack anyways. But your filter should never look like this.

The exhaust has some serious carbon built-up and you can see from the pic above, needs to be cleaned.

I normally will wash the bike meticulously before I take it apart, but on this build it wasn’t my job, Do your mechanic or builder a favor and clean your own stuff!! Here are some pictures I took of items that seem to be a concern. In this next picture the Phillips head screws in the throttle cap must be replace and should be the first time the bike is taken apart. If you jet your bike like you should be, these screws will get some serious abuse. They need to be changed to a Allen type screw to really hold up reliably. Use stainless steel. On the subject, Yamaha is pretty good now days with bolts, but any Phillips head screws should be replaced with SS Allen heads. They will not corrode due aluminum’s oxidization issues and will not strip threads as easy when removed.

This front sprocket is toast and the chain and rear sprocket aren’t looking so good either.

and that’s pretty much it. Phase two will involve the motor work. I will try to get this in the next couple day so stay tuned…..

To invent, you need a good imagination and a pile of junk.
Thomas A. Edison (1847 - 1931)

#2 User is offline   Ziggy

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Posted 04 October 2004 - 02:06 AM

<body link=blue vlink=purple>
2002 Yamaha YZ250 (Matt's)
INCH MM Degrees
Deck Height 0.01 0.254
All dimensions are from top of barrel!
All widths are straight across measurement! 0
Exhaust Port 0
Powervavle Up 1.884 47.8536 101deg
Powervavle Down 1.575 40.005 88deg
Exhaust port width (widest point) 1.73 43.942 N/A
Exhaust port Bottom 2.845 72.263 180deg
Aux Exhaust Top 1.605 40.767 89deg
Aux Exhaust Bottom 2.155 54.737 113.5deg
Aux width (widest point) 0.625 15.875
Tranfers 0
Front Transfers 2.31 58.674 122deg
Rear Transfers 2.33 59.182 123deg
Transfer bottom (Both) 2.93 74.422 Below BDC
Rear Width (widest point) 0.96 24.384 N/A
Frant Width (widest point) 1.01 25.654 N/A
Intake 0
Boost port Top 2.33 59.182 123deg
Boost port Bottom 2.94 74.676 Below BDC
Boost port Width (widest point) 0.8 20.32 N/A
Main window Top 3.4 86.36 Below BDC
Main window Bottom 4.865 123.571 Below BDC
Main window Width (widest point) 1.74 44.196 N/A

To invent, you need a good imagination and a pile of junk.
Thomas A. Edison (1847 - 1931)

#3 User is offline   Ziggy

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Posted 04 October 2004 - 02:31 AM

Ok here are some pics of the Vinamold casts taken from the Barrel. they show that the port layout is in good order but they do show some casting flaws that need to be removed.

First the transfer, note the taper from top to bottom, It's all about stuffing as much in as quickly as possible.
Attached Image
Attached Image

Now the intake window. this intake has HUGE Boysen ports the size of your thumb.
Attached Image
Attached Image

Finally the exhaust. This exhaust is nice and straight and evenly sized, the aux exhaust post gently merge with the main exhaust .
Attached Image
Attached Image

Now for some pictures of the Barrel

The tranfers from the bottom
Attached Image

The Intake
Attached Image

The exhaust
Attached Image

This is all for now. Once i run all the figures I will be able to let you know what we are going to do with it..

Stay tuned.
To invent, you need a good imagination and a pile of junk.
Thomas A. Edison (1847 - 1931)

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