Small bore 250?

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#1
To Eric, Rich and the likes- Is it possible to down size a 250 motor to a 200 or 220? I ride in nasty terrian. Nasty rock and root sections. The races are 3 hr harescrambles. Now that I have finally got the suspension working right, I want to tackle the motor issue.
I have a 05 YZ 250 two stroke with low hours on it. I did a powervalve mod and added a washer to smooth the bike out. Put a 13oz fww on it and the bike is still too much. I just rode a 08 KTM200 and liked the motor. Now I know Eric does a ton of big bores, ever done a small bore :)
The KTM is 193cc and shows a bore and stroke of 64 x 60mm (2.52 x 2.36").
The YZ is 249cc and shows a bore and stroke of 66.4 x 72.0mm (2.6 x 2.8")
 
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#2
What exactly do you want to gain by reducing the size of the bore? If you want to smooth out the power, have it ported for smoother power.
 
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#3
Dan105 said:
To Eric, Rich and the likes- Is it possible to down size a 250 motor to a 200 or 220? I ride in nasty terrian. Nasty rock and root sections. The races are 3 hr harescrambles. Now that I have finally got the suspension working right, I want to tackle the motor issue.
I have a 05 YZ 250 two stroke with low hours on it. I did a powervalve mod and added a washer to smooth the bike out. Put a 13oz fww on it and the bike is still too much. I just rode a 08 KTM200 and liked the motor. Now I know Eric does a ton of big bores, ever done a small bore :)
The KTM is 193cc and shows a bore and stroke of 64 x 60mm (2.52 x 2.36").
The YZ is 249cc and shows a bore and stroke of 66.4 x 72.0mm (2.6 x 2.8")
You won't be able to get the same power characteristics out of that long stroke 250 as you would with the short stroke 200. If you like the 200's power, you should get the KTM. :nod: :cool: :)

Rotor
 
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#4
Rotorranch said:
You won't be able to get the same power characteristics out of that long stroke 250 as you would with the short stroke 200. If you like the 200's power, you should get the KTM. :nod: :cool: :)

Rotor
I was hoping someone wouldnt say that. Just wanted to throw it out there and see if it was possible to get the same characteristics out of the yz motor.
 
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#5
76GMC1500 said:
What exactly do you want to gain by reducing the size of the bore? If you want to smooth out the power, have it ported for smoother power.
2hours into a harescramble i dont want to aciddently blip the throttle and see god. The bike is great when I am full of energy but when you get tired it can get hairy in tight races. Maybe the porting will be enough but I doubt it.
 
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#6
Dan105 said:
I was hoping someone wouldnt say that. Just wanted to throw it out there and see if it was possible to get the same characteristics out of the yz motor.
Sorry dude! :)

There's ways to make the YZ better for the woods. Eric Gore would be a good place to start. Big bore, ported for grunt. :nod:

But I've seen the KTM200 turn faster laps than the bigger bikes in South Eastern hare scrambles. They are nice bikes.

Rotor
 
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#7
Rotorranch, well at least you corrected youself. Its like telling you or myself to buy a new bike. Just not happening.

Anyways try a 265 big bore with bottom end porting and some taller gearing. You could also make the top end rich with jetting. Sounds dumb but its the only way to ride the 465 in tight situations.
 

SpDyKen

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#8
I would suggest that you send an e-mail directly to Eric. I suspect that he will be able to advise you of a good alternative; perhaps one not suggested here.

BTW, exactly what did you do to modify your power valve, and what were the results you experienced?

Please keep us posted here! KW
 
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#9
2 strokes for life said:
Rotorranch, well at least you corrected youself. Its like telling you or myself to buy a new bike. Just not happening.
:) :rotfl:

That hurts, dude! :whoa:

We actually have newer stuff in the garage. Currently, beside the antiques I ride, there is an '04 KXF250, '02 KDX220, '99 DS80, 01 XR100, 06 PitsterPro. But The Kid won't let me ride them. :laugh:
I'm really looking forward to getting myself a bike from this century. I think I want a white smoker. :ohmy:

Rotor
 

Lorin

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#10
I had my 98 yz bored to 265 with the "mo-better" all-around porting done by Gore. That, along with a 11 oz flywheel weight made the bike very smooth. Power was smooth like a kdx with 10 extra horsepower when needed. Worked well for me.
 
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#11
Lorin said:
I had my 98 yz bored to 265 with the "mo-better" all-around porting done by Gore. That, along with a 11 oz flywheel weight made the bike very smooth. Power was smooth like a kdx with 10 extra horsepower when needed. Worked well for me.
What type of conditions were you riding it in? Might have to give the 265 a try.
 
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#12
SpDyKen said:
I would suggest that you send an e-mail directly to Eric. I suspect that he will be able to advise you of a good alternative; perhaps one not suggested here.

BTW, exactly what did you do to modify your power valve, and what were the results you experienced?

Please keep us posted here! KW
I added a spacer (thick smooth washer) to the powervalve. The mod is from Team Green and how they make their KX more tracable. The powervalve in the kx and yz are pretty much the same. The mod worked well and smoothed things out a bit. I would reccomend it
 

truespode

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#13
On my 93 WR250 I had the low end Eric Gorr porting and the bike was great in the woods power wise. Very tractable too.

You can always get a flywheel weight as well.

Ivan
 

Lorin

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#14
I typically rode woods, trails, and enduro's (tighter the better). The 265 and porting worked very well on my 98 yz250. Very smooth.
 

BSWIFT

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#15
Dan105 said:
I added a spacer (thick smooth washer) to the powervalve. The mod is from Team Green and how they make their KX more tracable. The powervalve in the kx and yz are pretty much the same. The mod worked well and smoothed things out a bit. I would reccomend it
Got a pic or diagram?