It's taken me much longer than I expected to get my YZ250X dialed for hard enduro, but we've finally figured it out and taken steps in the right direction.
I felt like every step I took was a baby step in the right direction, but never actually fixed the problem I had, which was shifting the power curve lower into the RPM. Some mods went backwards, but for the most part they were all small improvements and I was still wanting more. 9 days before Moab Hard Enduro I about caved after riding my 250FX in the yard and it had that bottom end power I was looking for. I started ordering sponsorship parts to build that bike out for the east coast.
Luckily, I finally chatted with Bob Oliver who was a factory Yamaha mechanic from 1980-2018. He said the only way to get bottom on the YZ was to deck the bottom of the cylinder 1mm. Myself and RK Tek were worried about doing that mod because we would be unable to shorten the powervalve arm unless we cut and rewelded the rod to the correct length. Bob said decking the base of the cylinder 1mm would be no problem at all for the linkage system and powervalve spring to handle the height change.
I also had a few calls with Rob Brown and he verified all the same info as Bob Oliver. Tyler at Applied Technology helped me out and we cut the cylinder on his lathe and ended up cutting 0.04" off the base before Moab, which is a hair over 1mm. Rob Brown stated that I essentially shifted my power curve down 1500-1800 rpm, making the power way more use friendly for hard enduro. Also, I technically increased my stroke as well since the stroke is measured from top dead center to the exhaust port. So changing my port timing and lowering the exhaust port allows the piston to go higher in the cylinder and more path to travel before reaching the exhaust port on it's way back down. RK Tek cut a head to match the squish and compression ratio.
I instantly was impressed with the change and it gave me a ton more confidence in what I could do on the bike. I knew it was obviously a step in the right direction because I was able to take the win at Moab and trust the bike mentally. And btw, it really still is a 250!
As you can tell, I'm still looking for more! So Tyler and I decked 0.06" (roughly 1.5mm) off another cylinder this last weekend before Battle of the Goats. The bike felt phenomenal at 6300' elevation, so I'm excited to see what it's like at sea level. This journey has been a pain in the ass at times, but I personally feel like I'm finally on to something after Bob Oliver was able to help turn my season around. One thing I will mention is that I have now shifted to the standard YZ cylinder and main exhaust valve over the X. The exhaust valve is 0.5mm lower than the X model, which will actually help with bottom as well. Shoutout to Tim King at HITLIS for all the valuable information regarding the different cylinders and exhaust valves. Also want to give a shoutout to Eddie Sanders Racing as well for his valuable insight and sending me some stuff to test as well to help get me in the right direction.
My bike is as close to stock as ever now from where I started. I feel like all the recommendations and renditions I've gone through on the bike were band aids, when in reality I needed stitches. I'm happy to be back on track and in a good mental head space moving forward.
If you want to make your YZ way easier to ride and increase the bottom end. Reach out to Tyler at Applied Technology to send him your cylinder and RK Tek to cut a head to match.
https://www.appliedtechnologyinc.com/
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