Notice:
All information in this article is not for the amateur or person who is inexperienced at motor work. This is not a tuning manual. Motor work has many kinds of risk and motorcycles can cause you serious injure or death. Take your own risk and responsibility if you attempt to work or to ride. We hope we can leave this article open public for enthusiast. Thank you.
KFR-TW
Yamaha TW200 is street use bike. Nothing special, nothing high performance, but people like this because they can enjoy modifying many ways.
In picture bike looks like normal except Super Trapp down exhaust pipe but It has 249 cc tune up by Yamaha genuine parts.
Modified point:
XT250 piston
TTR230 crankshaft
Webcam 89 cam with valve spring
KFR Big valve (Exhaust)
WPC treatment
Ported cylinder head
2001~ model ECU and electric wiring etc.
This bike came here with extra used motor. It wasn't running. We picked and put in good parts together and rode for while. But we knew that motor needed rebuilt. One day, bike start leaked motor oil. Now it's time, we decided split motor and rebuild with a little more fun.
This is the first motor condition. Amazed this motor was running.
Of course, sleeve looks same way.
Transmission. Cavity tooth all over.
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Here is some picture and detail about modified.
Bore x Stroke 74.0 x 58.0 mm as XT250.
After re-sleeved
First what we needed was cylinder re-sleeve. XT250 piston is bigger than stock. This work we've ordered and done by LA sleeve. They could put nikasil aluminum sleeve and that will be great combination with aluminum forged piston but we didn't put this time by running purpose and cost.
We need bored out crank case hole caused by bigger cylinder sleeve.
Before (stock)
After
Still crank case needs grind work little bit for cylinder sleeve install.
Grind needed both sides little..
Fit in finally.
Cleaned off crank case and repainted.
Now it's time for assemble TTR230 crankshaft and other all parts.
All bearings, transmission parts are new of course.
Assemble is nothing special, just make sure every piece is set right position.
stock 200cc motor off.
We clean off and repainted swing arm at once. Looks nice!
Let's see TW stock piston and new XT piston.
Left is stock TW, Right is XT250 (forged) piston.
Piston rings are much thinner.
Newer design piston has thinner wall thickness also. And more holes.
Sophisticated design.
TW stock piston with rings 184.7g
XT piston with rings 191.9g
New piston and rings are treated by WPC treatment. (Yamaha stock XT piston has not same color shown as pictures.)
We forgot measure and take picture with piston pin! Pin is also shorter design than TW's. We can expect total lighter weight but again, XT piston is bigger and forged.
Installed piston.
Mount cylinder head and check piston-valve clearance.
Put too much clay. Closest clearance In 1.9mm Ex 2.0mm at least, good enough.
Things don't go smooth usually, cranking by hand feels something not right.
It happen when piston position around bottom dead center.
We found piston skirt has interference with balancer. NG.
We decide to cut off piston skirt this time. (Sorry, don't have pictures.) That work made piston weight 0.3g lighter, well done.
Back to assemble again with custom head gasket.
Cam chain will stretch easily than stock after changed to strong valve spring so, better keep watch tensioner adjustment. You can think of cam chain is like "Drive train" as well. It is very important for this motor.
27% bigger displacement from stock motor pulls so much torque and HP. Gearing is 15-50 now, and you can still release clutch from idle for start.
We can recommend this modified to TW200 or TTR230 user however, better remind always, engine modified has effect and side effect. Good effect is nothing but more torque and power. Side effect is you have to always pay attention to motor health and operation. It is tuning up motor even if built by Yamaha genuine parts. It will be short overhaul cycle than stock for sure. Cam chain stretching is common problem for this motor. And running will not have the same handling feel as 200cc motor, it is bigger bike. Someone like, someone maybe not.
Currently, we made 1000 miles test ride for custom cylinder head (with KFR original big exhaust valve) and 500 miles for whole motor without problem. What surprise thing that runs fine without any carburetor jetting or adjustment. Possible reason are large piston making more vacuum suction and all stock (regulated) intake system. We can think about this motor possible to make more high output. But this bike is for street use mostly. High power performance is not everything for street stage. This motor making 72~73MPG actual gas on city ride. Stock 200cc was made about 74~75MPG. Do you like or not?
Our best recommend is up to 223cc by Yamaha TTR parts. You can tell that is still "TW". This 250 motor is 'not racing horse but not work horse' as stock motor. Better not take serious ride.
You can make what you like. Hopefully you enjoyed this example.
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KFR wants to send thank you very much to these nice people:
Three "camshaft magicians" from WEBCAM
