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Detune a YZ250 even more

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Forgive the long explanation but I want to be thorough and not cover ground already gone over in the FAQ about detuning.

 

When I was doing a cross country trip on my Stratoliner I was camped at a campground in Alabama and ran across a very interesting fellow and his equally interesting YZ250.  He had set it up for nasty, rocky, woods riding and when he let me take a ride on it, it was brilliant.  It had the usual suspects like a flywheel weight, mellow timing and a progressive throttle tube but he said he also had the head machined for a lower compression ratio and had done something to the power valve but I can't remember what.

 

The power was so smooth, with almost no hit at all, very torquey and could find traction in the nastiest of conditions.  Think KDX200 but with more torque.  

 

I would very much like to replicate this bike (currently looking on Craigslist for a 99+) but have found very little in the way of dished pistons or head machining to actually decrease compression as most people want to increase it.  The KDX200 has a 9.6-1 ratio and the WR200 has a 6.3-1 so I am trying to split the difference and get it to somewhere around 8-1.  This is obviously not possible with just a thick head gasket.

Also I've never torn into the powervalve on a YZ...is it similar to a KTM where a different spring can be used to change the opening rate or maybe a Beta that can be adjusted with just a few twists of a screw?

I know this is possible, I've ridden it...just not sure of the specifics of how to accomplish it.  Permanent engine changes are OK, I can't imagine a reason to go back.

 

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I think the guy was talking about messing with the shims and spring in the governor. There is a huge thread in here about it. The spring is not like a KTM spring. 

As far as the head, he probably had his squish cut (plenty of threads on here about that) and maybe the dome cut to match that of the 250X.

Edited by AnimalMother85
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Before you start throwing money at the bike, try this first--install various base gasket thicknesses and see how she runs, and don't be afraid to go thick.  I once had a CR250 (01) that had been damaged by the previous owner, who pried the cylinder off with a screwdriver.  In an effort to make the bike ride-able while searching for parts, I cut myself a base gasket out of a sheet of automotive gasket paper (which is really thick, but unfortunately I did not keep any measurements).  I was amazed at how well the bike ran with that really thick gasket.  Ultimately I went with a JD kit for jetting (01 CRs were  notoriously hard to jet), but I wound up with a brilliant woods/single track motor--no hit, just smooth 250 power.  I kept using that base gasket material as long as I had the bike (even after repairs).

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4 hours ago, rpt50 said:

Before you start throwing money at the bike, try this first--install various base gasket thicknesses and see how she runs, and don't be afraid to go thick.  I once had a CR250 (01) that had been damaged by the previous owner, who pried the cylinder off with a screwdriver.  In an effort to make the bike ride-able while searching for parts, I cut myself a base gasket out of a sheet of automotive gasket paper (which is really thick, but unfortunately I did not keep any measurements).  I was amazed at how well the bike ran with that really thick gasket.  Ultimately I went with a JD kit for jetting (01 CRs were  notoriously hard to jet), but I wound up with a brilliant woods/single track motor--no hit, just smooth 250 power.  I kept using that base gasket material as long as I had the bike (even after repairs).

Thanks for that, it has been ages since I've owned a 2 stroke, and I'd completely forgot that you can lower the compression and change the port timing like that.  Bonus points for being such a cheap mod too!

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8 hours ago, AnimalMother85 said:

I think the guy was talking about messing with the shims and spring in the governor. There is a huge thread in here about it. The spring is not like a KTM spring. 

As far as the head, he probably had his squish cut (plenty of threads on here about that) and maybe the dome cut to match that of the 250X.

Searching now, thanks for a direction to look.

5 hours ago, markmc2 said:

RD Designs will cut th head for ya. Yz250x cdi box and power valve springs. Maybe 18" rear wheel too.

Researching those YZX changes, those would fit on any 99+ YZ correct?

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13 hours ago, Jimmy the Heater said:

I would very much like to replicate this bike (currently looking on Craigslist for a 99+) but have found very little in the way of dished pistons or head machining to actually decrease compression as most people want to increase it.  The KDX200 has a 9.6-1 ratio and the WR200 has a 6.3-1 so I am trying to split the difference and get it to somewhere around 8-1.  This is obviously not possible with just a thick head gasket.

YZs don't have head gaskets, they have o-rings. You must machine the head if you want to adjust squish clearance and/or combustion chamber volume. Unless you raise the cylinder above the piston with a thicker base gasket... but that would change the porting in the opposite direction of your desired power.

 

I think you can get thinner head gaskets to lower the ports, but then you'd have to adjust the head, which would be closer to the piston. You might as well give the cylinder, piston, and head to an engine builder and say, "I want a low-mid, butter smooth, electric torque engine that will climb wet rock garden hills without wheelspin. Get cracking."

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If you want to do it with bolt on parts, run 2 base gaskets then get a 250x head, cdi, power valve springs and washers.

 

If you want to do it cheaper/mod what you have, get the dome cut deeper/wider in the head without decking it to tighten up the squish like you normally do when modding the head and back the timing off a mm or so. Plus run the power valve washers the X has.

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Seeing as this will hopefully be my forever bike, sending it to a builder like has been mentioned will be money well spent.   Thank you all for the very very good tips.  Now my turn to get cracking on finding the best YZ in my area.

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If you want a tractor motor AND with good economy then don't lower the comp much, if at all.

Instead you can lower the ports (shave the cyl base) and delay the PV open timing.

When you shave the cyl base to lower the ports you should get the head dome vol increased to maintain stock comp or go lower comp. Also get the squish clearance reset to 1.3mm.  Don't lower the comp too much or economy is lost.

You can delay the PV open time with more washers in the governor device. Or you can delay it completely by getting some JB-Weld and gluing all three PV parts into the closed position. Then you can remove some PV parts. Just leave the rotating shaft in there.

What ever you do, you obviously need good jetting and a healthy reed valve. You'll need to choose a pipe too.

When you raise the cylinder, you raise the transfer and exhaust ports which (alone) would boost top end power and kill bottom end. But you lower the comp a lot and that newters the motor at any RPM.  With the stock head you'll also go from stock wide squish (ordinary) to no squish which is ok. Just lower efficiency.

Edited by numroe
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short summary to me it looks like there are a few options.

Head mod - larger dome, correct squish- RB or Eric gorr is good too

throttle cams -  g2 400 or 500

flywheel weight -  11 or 13 oz

pv washers added to governor, I tried 3 and it really keeps the bike from hitting the powervalve,  2 may be the sweetspot for my conditions.

smooth jetting  -  went with lectron, didn't like the keihin setups I tried as much.

retard timing - just a smidge on 250x

250x CDI ( I have a new spare one if you contact me via pm)

Gearing to suit your terrain.

 

This should get you close   :)

 

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Thanks again for the responses, I've got a couple of pages of notes so far on what to do once I find a suitable bike on CL.  Finding one with a mostly stock engine is a bit more difficult than I had thought.

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On 2017-05-15 at 1:45 PM, biglake said:

If you want to do it with bolt on parts, run 2 base gaskets then get a 250x head, cdi, power valve springs and washers.

 

If you want to do it cheaper/mod what you have, get the dome cut deeper/wider in the head without decking it to tighten up the squish like you normally do when modding the head and back the timing off a mm or so. Plus run the power valve washers the X has.

I agree about the yz250x head, cdi and power valve spring. Just to add to that, I have read from the guys that have tested different pv settings ad nauseum that the best pv setup on a moto yz250 is the x spring with just three of the four stock pv 1 mm washers installed.  Remove the washer closest to the spring, if you switch to the yz250x pv spring.

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11 hours ago, numroe said:

 

 

You can delay the PV open time with more washers in the governor device. Or you can delay it completely by getting some JB-Weld and gluing all three PV parts into the closed position. Then you can remove some PV parts. Just leave the rotating shaft in there.

 

just out of curiosity , do you have any more info on this ?

have you ever ridden or seen this done to a bike ?

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8 hours ago, kxrob said:

just out of curiosity , do you have any more info on this ?

have you ever ridden or seen this done to a bike ?

Yeah. In my current 295cc top end - the three PV "slides" are glued in the closed position. I did it after I decided the cylinder was at the end of it's life. It needed another plating but worse was that the main PV slide cavity was worn out.  Rather than discarding it I thought I'd try a no PV 295cc setup with a fresh piston. I've put another 60 hours on it since. With a Wossner piston.  The motor is very smooth and easy to ride. It's a 295, so it pulls really well from down low, and with the Fatty pipe it still pulls surprisingly well up top. It just takes a while to rev out. No need for a flywheel weight with the PV fixed shut.   Less power up top than a stock 250 but so much width/range in the power that it's still very quick around a track. Almost as little gear shifting as a 450F.

I first started experimenting with the PV fixed closed when I broke some PV link parts and wanted to ride the bike before I had the new parts. Actually it broke at a track, so I removed all three PV covers and stuffed rags over the valves to hold them shut and went back out and rode. I rode for hours. The motor was so easy to use. Great traction and control on approach to choppy hardpack dirt jumps.

Today I have a new 295cc kit to go on with a Wiseco piston. The new setup has the main/middle valve functioning normally and the side exh ports are welded+plated over. So I have no thin exhaust bridges.  I expect it'll have a little more top end power than a stock 250 and much more bottom end. With a very linear easy to use power curve.

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interesting ,

how do you think it would go on a 250 ?

i wonder if this is what Scorpa did with there trials bikes up until 2008 when they used yamaha motors ?

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I've not tried it on 250 (less torque, so needs to rev more), but the effect should be the same.

On any 1999-2017 YZ250 cylinder, to try the PV fixed closed, all you need to do is remove the left side linkage cover and install a 50c bolt into the hole used to lock the linkage crank. Cut the bolt to the required length so that it almost touches the installed cover so it wont fall out. Also wrap plumbers teflon tape around the bolt thread so it wont rattle. Then go ride it.  It's also a simple way to check the low RPM effect of stiffening the PV gov spring or adding more washer spacers if you are undecided about that mod.

 

Edited by numroe

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