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sm converted yz 250 02`only does 60mph

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Hi everyone! I`ve got a 2002 yz 250 thats converted to supermoto that I just finished now. I had a crash last year that I think caused the damage to the garbox by landing on the gear lever, (i`m not sure how it could happen) and when i was finished building it to a supermoto a month ago I found out that the 1st gear was shot, the bike almost stopped when it was in 1st and it locked itself in 1st at the end of the test run. I realised that I hadnt any other choice than open up the gearbox, there I found a destroyed shift fork, 1st and 5th gear had a bit wear also, so I changed these three parts.

 

I did my first run with the bike today with the supermoto wheels and the bike did max. 60mph with 15-44 sprockets, when I drove in 50mph I felt I was chasing the bike too much. I actually had to pull over on a parking lot to see if it didnt go in 5th gear or something..

 

I`ve saw that alot of people has converted with the yz/wr/400/426/450 gearbox. 3,4 and 5th. But how much is the difference between those 5th gear actually? how much more speed could I expect?

 

Hope everyone understands me:)

 

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hmm no? lower in front will gain acceleration?

Why you running a 15 front sprocket? Drop it to a 12 or 13. it will loose some torque but increase the speed.

GoYamaha

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Why you running a 15 front sprocket? Drop it to a 12 or 13. it will loose some torque but increase the speed.

GoYamaha

 

No, dropping the front sprocket will give you more torque 

 

OP, I hit 65 on a stock YZ250 with 13-49 gears. Something is going on with your bike.

 

Can you count all 5 gears as you shift? Maybe it is stopping at 4th gear?

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Yes as I wrote, I had to pull over on a parking lot (was a bit traffic around me) to check if it was stuck in 4th, something it didnt.. It cant be possible to install some gears wrong, everything worked fine.. I have no clue at all. I`m not any experienced on dirtbikes.

No, dropping the front sprocket will give you more torque 

 

OP, I hit 65 on a stock YZ250 with 13-49 gears. Something is going on with your bike.

 

Can you count all 5 gears as you shift? Maybe it is stopping at 4th gear?

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Yeah I know, I've got a 150/60/17 rear tire.. How about a yz426 3,4 and 5th gear conversion and a 38t rear sprocket?

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You would have to add teeth to the front and drop teeth in the rear to gain speed. Remember that this will make you lose torque.

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I used to run 15/40 with a 19" rear, WR tranny, and Gnarly pipe.

That's the only gearing/pipe that I would run out of power before maxing out the engine RPMs.

 

The top speed for me was in the low 90's.

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something is wrong for sure, was the new 5th gear the exact same size and teeth as the old one?

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I used to run 15/40 with a 19" rear, WR tranny, and Gnarly pipe.

That's the only gearing/pipe that I would run out of power before maxing out the engine RPMs.

 

The top speed for me was in the low 90's.

Thanks, I dont know what to do.. I'm afraid that if I change to a wr/yz tranny and put on a 38 teeth in the rear, it still wont be enough top speed for me.

something is wrong for sure, was the new 5th gear the exact same size and teeth as the old one?

Yes, I just changed to gears, so it isnt possible that I installed something wrong, I'm pretty sure that I wouldnt be able to shift gears then.

I'm so frustrated over this, in worst case I have to change motor.. Dont know the gear ratio on a 125 with 6 speed but this 250 motor with this tranny is useless for me now.

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You could also research swapping the crank gear and clutch basket gear for more top speed. There was a land speed guy on a YZ250 a while back and I think that was one of his mods.

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You could also research swapping the crank gear and clutch basket gear for more top speed. There was a land speed guy on a YZ250 a while back and I think that was one of his mod

Thanks bg10459, I`ll check it out:)

 

If anyone have any clues or other solutions or anything at all, I appreciate it!

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Something definitely seems amiss. 

 

A 150-60-17 should be 24.09" tall, which is quite a bit shorter than stock (about 26.8"). But 15/44 gearing should more than make up for it. 

 

In a gearing calculator I have (send me an email address and I will send you a copy, Excel) it shows the following.

  • Your tire and 15/44 gearing should run 60 mph in 5th at only 7,010 rpm, a far cry from the 9,000 rpm or so it should be able to pull in overrev. Is the power valve functioning? I believe it opens around 6000-6500 rpm, the engine would run out of juice pretty quickly after that if it were stuck closed. 
  • If your bike could pull to 9,000 rpm in 5th then it should do 77 mph with your mods.  
  • Vortex shows they make sprockets down to 36 tooth that fit the YZ hub. That would get you to 94 mph at 9K with stock trans. 

 

 

Here is the thread about the primary gearing swap. Not straight forward, he talks about numerous machining steps that needed to happen to make things work together. 

http://www.thumpertalk.com/topic/913019-primary-gear-swap-2005-yz250/

 

 

He ran 16/40 sprockets and 101.7 mph with stock YZ 5th gear.

Never gave an update with the primary swap.

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Thank you adam728, I've been thinking alot last few days and I'm going to purchase a new tranny/gears.

I've also read what jessechop has done to his. I filed a bit of the case and installed a 16t front sprocket and buy a 36t rear from vortex. And I'm afraid thats not enough so I'm looking for a wr400/426/450 but i want to gamble on the wr250 as jessechop has mentioned, 5th gear ratio on the wr250 is 0.814 vs 0.84 on the 400/450. We go all in!;)

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Need pics of the SM converted YZ!!!

Here ya go:)

image.jpgimage.jpg

Edited by _Niclas_
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I feel like you still need to diagnose why the revs peak so low right now.

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