2021 YZ250 Flywheel weight

It will help a little. It's not life changing but it does help smooth out the power delivery a little and provide more tractable power. I personally run the GYTR one, think it's a little over 8oz. You could also try retarding the timing a degree or two. It will smooth out the hit just a touch and pull a bit further on top.

At 65 and being off two stroke for a dozen or more years I thought the same thing about the 07 RM250 I bought. Tried reeds, gearing, etc. but thought the 10 oz FWW I added made the most difference in rideability. As long as you aren't doing jumps with no approach I would even go heavier. They are still a challenge but sure are thrilling to ride.

Here's a glimpse into a serious attempt at reducing arm pump on a YZ250 2T.

More than you asked for AND a bit of a tangent, but here's what I ended up w/ on a 2006 YZ250 two-stroke that made it a 3-stroke. Hopefully this is helpful and thought provoking.

My primary *unplanned* solution ended up being the Rekluse Core EXP 3.0 clutch system. For some reason that EXP clutch was perfect for me and the way I like to ride a 250 2-stroke. I also changed the gearing to 15/45 - and it was still awesome (did it for desert riding, ending up loving it for nearly everything), no kidding. A steering damper paired with Flexxbars helped me a lot, too. I also installed MotoT VForce reeds and a ZipTy Racing exhaust - which was the Factory Yamaha system (Dubach Racing was making the same exhaust for a while) to produce tractor-like power. Additionally, the following combo was a surprise arm pump reducer, too, that a guy on ThumperTalk pointed me to years ago:
Magura HYMEC clutch with MME clutch lever (the M2C model: https://www.midwestme.com/find-your-lever/midwest-moutain-engineering/f…). And lastly, I also installed a Steahly 9 oz flywheel weight ... figured I better add that part to "sort of" stay on topic and bring things full circle (I was sensitive to anything over 9 oz, didn't like the handling changes).

Oh - 2 last arm-pump reducing "gems":
(1) I discovered that the most low profile grips I could find (Renthal full diamond) resulted in the least tendency to pump up. I equate it to trying to do pull-up on a bar that's larger diameter and you can barely hold on to vs. the perfect diameter that allows you to do the most pull-ups.
(2) I've also been playing with Stegz Pegz lately. I think there's something here for me, but I need more hours to better understand/learn how to use them. Here's a look:
https://www.rockymountainatvmc.com/parts/stegz-steg-pegz-p

The above setup ^^^ resulted in one of the most versatile bike's I've ever owned. And I was an idiot for selling it.

Have fun on that YZ250!

GYTR FFW & STIC Metering block & it will pull off bottom end smooth like a 450..........

I’ve been a YZ250 fanatic for 15 years or so now. I’ve tinkered with them for years, trying to make them a little more manageable and user friendly in the woods, yet still feel good on a MX track.

The flywheel weight is good. Even MXA runs one. anything bigger then the GYTR or the Steahly 9oz is a bit much for moto use. Especially when you start getting into deeper soil or sand.

Also, as mentioned above, retarding the timing has a big effect on these things, too. Get a dial indicator and experiment with 1* and 2*. I didn’t like anything more than 2*.

The last thing… I refused and resisted it as long as I could, thinking it was all internet hype… but the Apex head is the single best modification I’ve done to a YZ250. They offer MX, SX and XC domes, and I bought and tried all 3. The MX dome is the best all around setup. It has strong, controllable bottom end, and then pulls hard all the way through the rev range. Best way to describe it, it fills in any of the low points in the power curve, making it more broad and tractable. Bike is also much easier to jet now. I’d start with the head (either buy the complete billet head with dome, or send them yours to get cut) and flywheel weight. It’ll be a completely different motorcycle.

Flywheel weight(gytr is 8.4oz, steathy has 9 & 11oz options), retard timing, Yz250x pv set up, Yz250x head(lower compression ratio) that right there will drastically smooth the power delivery. Can also use a Yz250x Cdi.

I’ve had serval yz250’s and prefer the smoother yz250x motor traits.

Just my preference.

I've set a few YZ's up with the "X" CDI ( better than retarding the timing, IMO ) and "X" power valve spring mod, along with lowering the compression with a head cut. Really smooths out the power and adds some rev's. I also love the DEP pipe and shorty silencer. With this set up the FWW is sort of optional for MX, but doable for woods racing. It pulls a gear high off the corners a little better without it. At 60 smoother is faster.

Digging the tech info in this thread! Thank you!

+2 on the apex head with the MX insert, and the X Power Valve Spring mod, I also did a 11oz stealthy FWW but didn't notice it as much as the head and PV mod. I did both of these and the bike is MUCH easier to ride. The power delivery on mine is much closer to a 4stroke now. This guy does a great video on how to do the power valve mod. https://www.youtube.com/watch?v=Le3chZSKXng

The stock squish clearance on the current YZ250's runs about .074". Simply getting down to under .055" makes a huge improvement, if you put the volume back. I like about .042" to .045". And yes, you can run pump gas that tight, especially if you ease up on the compression and maintain the squish angle. Getting the squish right and backing of the compression a little just makes a huge improvement in smoothing the power and how the bike jets in. They just run so much better when the squish is right.

We have a couple spreadsheets you guys might find useful on our resources page:
(engine-timing-change-calculator.xlsx and 2-stroke-cylinder-head-squish-cc-calculator.xlsx)

https://www.luxonmx.com/resources.html


I made these up when I had a YZ250, so all the numbers are relevant to that bike and the changes I made at the time. I honestly don't remember the specifics of the what and why as it was about 6 years ago, but you can see what I did and it might be helpful. I do remember that I settled on 2 degrees of timing retard and the lighter GYTR flywheel weight (still heavier than stock) to smooth out the power.

I downloaded your spread sheet a while back and it's pretty cool. Thanks.

Funny thing is ( if I remember right ) some of the manuals differ in what they say the stock timing spec should be. I'm thinking some of them spec like .0070" and others are like .011". Anybody else seen this, or am I having a senior moment. Regardless, with my set up I like mine set at .011".