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02 250 RC valve mod?

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Has anyone tried modifying the RC valve stop so that it lets the valve close more? I read about this on 125s, but not for 250s. It wouldbe easy to make a adjustable stop it was a valid modification. This could possibly pull a little more low end grunt out of the motor?

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On the '05-'07 250 you have two choices- you can modify the "butterfly" arm to allow fuller closure of the RC valve, or you can modify the steel stopper pin that is pressed into the cylinder which limits the RC valve movement. I modified the "butterfly" arm in order to easily and cheaply return to original spec in case the mod did not work out well. Not sure if the '02-'04 arrangement is identical.

You will need to re-adjust your RC valve cables after the mod in order to maintain proper cable tension.

With the modified butterfly, the RC valve closes at least a full millimeter farther and results in better low end and a smoother low-mid transition. Mid range to top end power is not changed.

As stated, the beauty of this mod is that you can easily return to original spec by purchasing a new butterfly for under $20 if you do not like the resuts.

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On the '05-'07 250 you have two choices- you can modify the "butterfly" arm to allow fuller closure of the RC valve, or you can modify the steel stopper pin that is pressed into the cylinder which limits the RC valve movement. I modified the "butterfly" arm in order to easily and cheaply return to original spec in case the mod did not work out well. Not sure if the '02-'04 arrangement is identical.

You will need to re-adjust your RC valve cables after the mod in order to maintain proper cable tension.

With the modified butterfly, the RC valve closes at least a full millimeter farther and results in better low end and a smoother low-mid transition. Mid range to top end power is not changed.

As stated, the beauty of this mod is that you can easily return to original spec by purchasing a new butterfly for under $20 if you do not like the resuts.

Interesting mod 1st time I've heard or read about it. My 07 has plenty down low but might try it as you mentioned to smooth out the low to mid transition. Do you have pics or a link to the mod ?

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how much did your remove from the arm where it touches the pin?

I would like info too on this as I like the way mine is but always looking for more.

off topic...ulmanb.....I wonder how many of us orange kool aid drinkers are sneaking a sip of Cherry? I really like it!:lol:

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how much did your remove from the arm where it touches the pin?

Enough to allow full and complete closure of the powervalve without the arm being stopped by the pin. It required about 1/16 inch of grinding to the arm where it contacts the pin, maybe slightly more. You will definitely need to re-adjust your RC valve cables afterward. Phil Hannah has also performed this mod to his '06 engine. You might want to PM him for his opinion too.

The best part is that this mod is easily reversible if you do not like it as long as you do not grind the pin. Just get a new arm from Honda for about $20 and you will be back to square one.

No jetting changes will be required. There will be no change to mid-top power.

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Has anyone tried this on an 02? I hope I'm not skipping over anything, but it seems like it would work the exact same way. Thanks for the interesting discussion guys - always lookin to get more low end!

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^noticeable difference in low end power due to raised compression?

This brings up another aspect to the discussion. If you look very closely at the powervalve you will observe two things:

First, since the powervalve operates essentially in a guillontine type arrangement (rotates predominantly in the vertical axis) the distance between the powervalve face and the piston is relatively large. This provdes for relatively poor exhaust gas sealing.

Second, look very closely at the outboard sides of the cylinder where the powervalve resides and is seated. You will observe "half moon" shaped cutouts on each side. This is to allow an engineered amount of additional exhaust gas leakage around the sides of the valve even when it is fully closed. The powervalve does NOT provide a completely air tight seal. The side leakage amount may be small, but Honda deliberately designed the valve to function this way.

This brings us to the obvious conclusion that Honda really did not intend the RC valve to be predominantly a compression raising mechanism (probably because of concerns about poor pump gas quality in the US being unable to tolerate large compression changes without damaging detonation occurring). The valve functions more of an exhaust pipe tuning device than an actual compression changing one.

Edited by APBT

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Bump.

Anyone with an opinion on trying this mod on the 02-04 cr250s? According to the model year changes thread, in 05 Honda introduced: "New cylinder design features RC exhaust valve housed in removable case for improved sealing."

Seems like the main concept of the modification should still apply to the 02-04's right? Am I missing something here? Just trying to make sure I'm not overlooking something simple here.

Kerry

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