YZ250X Mod Comparison

[Familyman6000]

So I used to have a 2018 YZ250x that I had done some mods to (#40 power jet, RK Tek head, and GYTR flywheel), but I didn’t do them one at a time. So though I was extremely happy with the final package I didn’t know the effect of each. So I stripped them off before selling the ‘18 and today I had time to ride my track with my ‘19. I started out riding stock, then did the jetting mod and rode that, then put the RK Tek and rode that. I will get to the flywheel tomorrow.

 

Here are my back to back impressions:

 

#40 power jet changer per Klinger-

Bike was noticeably crisper. Bike felt better pretty much everywhere.

 

RK Tek head-

So this is where things went weird. I actually felt like the bike ran WORSE with the head. I found that accelerating out of corners was more sluggish and I had more trouble making the doubles without serious seat bouncing. The mid and up felt about the same.

 

I’m going to swap the heads back and forth one more time to see if my original impression was correct, and also have my riding buddy who races the pro class give a second opinion. For information I’m at about 1500’ elevation and 65 degrees F.

[Brian Morrison]

   What is the volume of that rk insert your using?

    I had to lean my fuel out a touch when the squish was corrected. Do you think this played any role?

 

Edited March 23, 2019 by Brian Morrison

[Kinger317]

Amazing how lost we can get with all the changes made to our bikes. You’re lucky you’re only dealing with those three things! I just did the same thing with my 2019 (MX version) and it’s amazing how many things didn’t make the cut. Amazing how good these bikes really are when kept close to original form.

[Clutch44]

1 hour ago, Familyman6000 said:

 

So I used to have a 2018 YZ250x that I had done some mods to (#40 power jet, RK Tek head, and GYTR flywheel), but I didn’t do them one at a time. So though I was extremely happy with the final package I didn’t know the effect of each. So I stripped them off before selling the ‘18 and today I had time to ride my track with my ‘19. I started out riding stock, then did the jetting mod and rode that, then put the RK Tek and rode that. I will get to the flywheel tomorrow.

 

Here are my back to back impressions:

 

#40 power jet changer per Klinger-

Bike was noticeably crisper. Bike felt better pretty much everywhere.

 

RK Tek head-

So this is where things went weird. I actually felt like the bike ran WORSE with the head. I found that accelerating out of corners was more sluggish and I had more trouble making the doubles without serious seat bouncing. The mid and up felt about the same.

 

I’m going to swap the heads back and forth one more time to see if my original impression was correct, and also have my riding buddy who races the pro class give a second opinion. For information I’m at about 1500’ elevation and 65 degrees F.

 

that's interesting. Was the jetting off after you changed to the rk tek? The way I feel my motor works with the modified head is the low gets replaced by the mid. Like as soon as you crack the throttle it wants to go. There is no working through a sluggish low end.

[dazza1968]

If you change the head the you have got to do the jetting at the same time. The head will make jetting alot more precise and that's why it wasn't great... Clean up the jetting now to suit your new head and the rest will fall into place.... Bet it was even a bit gurgly too.
Regards Dazza

[Familyman6000]

   What is the volume of that rk insert your using?
    I had to lean my fuel out a touch when the squish was corrected. Do you think this played any role?
 

Not sure what the volume is. I just told them the fuel type I use, riding type, and elevation. I’ve never measured squish either. I thought my ‘18 ran phenomenal so I just rode it.

[Familyman6000]

If you change the head the you have got to do the jetting at the same time. The head will make jetting alot more precise and that's why it wasn't great... Clean up the jetting now to suit your new head and the rest will fall into place.... Bet it was even a bit gurgly too.
Regards Dazza

It wasn’t gurgle at all. It still sounded like it was running clean, it just felt like I was trying to accelerate through the bottom end while pushing the back brake a little. That is the easiest way to describe it. I don’t remember my ‘18 feeling this way, but I never rode it with just the jetting mod.
I actually thought the RK Tek head would be less finicky with jetting? I actually bought it so when I ride single track at 5000’ my bike would run cleaner.

[mlatour]

Apart from altering the squish band height and the dome volume cc,

the shape of the whole combustion chamber (taper of the squish band into dome) will have an effect on the power characteristics 

(more hit or more linear)

[dazza1968]

It wasn’t gurgle at all. It still sounded like it was running clean, it just felt like I was trying to accelerate through the bottom end while pushing the back brake a little. That is the easiest way to describe it. I don’t remember my ‘18 feeling this way, but I never rode it with just the jetting mod.
I actually thought the RK Tek head would be less finicky with jetting? I actually bought it so when I ride single track at 5000’ my bike would run cleaner.

Yes it is alot easier to jet and to do it precisely.

[dcdr]

On 3/23/2019 at 12:23 AM, Familyman6000 said:

 

So I used to have a 2018 YZ250x that I had done some mods to (#40 power jet, RK Tek head, and GYTR flywheel), but I didn’t do them one at a time. So though I was extremely happy with the final package I didn’t know the effect of each. So I stripped them off before selling the ‘18 and today I had time to ride my track with my ‘19. I started out riding stock, then did the jetting mod and rode that, then put the RK Tek and rode that. I will get to the flywheel tomorrow.

 

Here are my back to back impressions:

 

#40 power jet changer per Klinger-

Bike was noticeably crisper. Bike felt better pretty much everywhere.

 

RK Tek head-

So this is where things went weird. I actually felt like the bike ran WORSE with the head. I found that accelerating out of corners was more sluggish and I had more trouble making the doubles without serious seat bouncing. The mid and up felt about the same.

 

I’m going to swap the heads back and forth one more time to see if my original impression was correct, and also have my riding buddy who races the pro class give a second opinion. For information I’m at about 1500’ elevation and 65 degrees F.

 

Did you try the heads back to back again?  What about the fww?

On 3/23/2019 at 12:58 AM, Kinger317 said:

Amazing how lost we can get with all the changes made to our bikes. You’re lucky you’re only dealing with those three things! I just did the same thing with my 2019 (MX version) and it’s amazing how many things didn’t make the cut. Amazing how good these bikes really are when kept close to original form.

What mods made the cut?  Haha

[Tahoe Gator]

What’s interesting is that many people over on the Kinger317 powerjet 40 thread are running RK Tek heads or squish band adjusted stock heads done by a machine shop such as RB, and most (like me) are reporting good results on their YZ250X. So, if the jetting needs to be tweaked for the RK head differently than the “K317” setup (sorry, growing tired of typing Kinger317 powerjet 40 in post after post), that knowledge would probably be valued and/or probably also refuted by many.

[Familyman6000]

I think I may have solved this issue. I realized the next day that I had forgotten to adjust the air screw. So it was still at the stock setting (about 1-3/4 out). Once I turned it in to 1 out it seems to have fixed the low end issue. I’m going to try and run it back to back this weekend with the airscrew at both positions just to confirm.
I haven’t installed the flywheel yet.

[rzr900]

What gains if any does the gytr tc2 give the x with a stock pipe ?