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Old 17-10-2017
Scootersbikes Scootersbikes is offline
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Back on a 2 Stroke Need Mods

Brand new on this site. Hello everyone.

So I have owned a 2003 YZ450F for the past 7 years or so. Has been a solid bike, no real complaints at all besides it being a heavy pig to ride in the bush and in the harescrambles I race. I am not a fast rider and am 45 years old but I have a blast racing.

On a local FB group I am in a guy posted up a 2003 YZ250 with a transmission problem that he wanted to sell. I offered my 450 up as a straight up trade.
After seeing the 250 and hearing it run I figured worst case scenario I would have to buy some gears etc.

Made the deal happen and the bike did not shift into 1st or past 3rd. I split the cases on the 250 and found a burr in the shift fork shaft port. I smoothed that over and put it back together. Bike works great.

I am going to probably tear the whole thing down again and do a proper rebuild of replating the cylinder and all new trans bearings.
While its apart is there anything else I should do inside? Porting, wide ratio gears, update the forks etc?

Not overly familiar with the YZ250 but I love the 2 stroke hit under me once again.
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Old 29-10-2017
numroe numroe is offline
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Quote:
Originally Posted by Scootersbikes View Post
I am going to probably tear the whole thing down again and do a proper rebuild of replating the cylinder and all new trans bearings.
While its apart is there anything else I should do inside? Porting, wide ratio gears, update the forks etc?
For just about any type of riding you'll probably enjoy broader power from the motor. Common effective mods are jetting, pipe, head and porting.

Pipe: FMF Fatty gives nice broad power. FMF Gnarly is tougher metal and gives more low RPM power, but you'll lose a lot of power in the higher RPMs.

Jetting: Replace your stock Yam N3EJ needle with the Suzuki NECK on the same clip position. With the zook needle, you must go 3 sizes smaller on the main jet and 2 sizes smaller on the pilot. Result is a more linear pull with no delayed response. Also no black oily spooge out the exhaust.

Head: The stock squish gap is too wide. Dave at TSP can fix this for you. You'll then get more consistent throttle response, increased economy and your jetting will be less sensitive to changes in temp, altitude of humidity.

Porting: Stock YZ250 1999 onward is very good. If you want to shift the power slightly lower in the RPMs then try a thinner base gasket. Stock is 0.6mm thick. If you try a 0.3mm gasket that lowers the ports that amount is the change to the porting is very noticeable. This is assuming you get the head volume adjusted/increased so the comp doesn't change. If you want more high RPM power then you can try a thicker base gasket, and you get the head shaved to get correct squish gap.

Fork updates: If you are riding offroad or rough MX tracks, then inside your 46mm OC forks remove on face shim off the mid comp stack. In the base valves you can drill the piston ports just 0.5mm larger to reduce the spike on high speed strokes. The stock base shim stack is probably fine. Disclaimer: I did this on my 48mm OC forks and I'm guessing it'd work well on your 46s.
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