09 yz250 more bottom to mid power

Looking to get a little more bottom to mid power out of my bike currently I have a fmf fatty pipe on the bike and a rekluse clutch, I was thinking either a pipe or a porting. Please give me some suggestions. 

A Gnarly pipe really helps, but it'll cost you on the top end.

For what type of riding?

I found the Fatty can work ok at any revs.


What is your squish clearance, and carb setup, and typical air temp, altitude and humidity?

For what type of riding?

Hairs rambles on supercross style track with a woods section and also I also ride a lot of arena cross style tracks in the winter. I am not really using all of the top end most

The hard question is how low of an RPM are we talking...

Some interesting gains and changes can be had on this engine - but it requires a lot of tuning to get the ultra low end better.

The dyno shown in here is with some porting changes, and most importantly a powervalve spring change - but as you can see it's better WAAAAy down low - then a little worse - then better through the meat - then signs off a little earlier...

But most people ride the bike 6k on up...4k is practically idle.


I have found really interesting and bad changes to the powercurve from many of the porting set ups out there - because the rest of the package isn't dialed in.  If more transfer timing is added - the already lacking blow down with the PV in the down position KILLS power.  See the second attachment to what a MINOR (and popularly done) transfer timing change can do to the powercurve.

Screenshot (12).png

Screenshot (11).png

Here's what I've done to my 05 YZ250 for SOME actual power gain,but also to spread the power where I like it.....1 tooth larger on the rear sprocket,9 oz. GYTR heavier flywheel(which makes your bike FEEL like it has gained bottom and mid,but actually is just giving you better traction,and doesn't take away any power),Fatty pipe,VForce 3 reed assembly(known for bottom and mid gain,as opposed to the Boyesen,which is more top end),and a Boyesen X-Wing(which I thought might just be gimmicky,but REALLY gives you better throttle response.I've had dedicated CR250 guys and KX250 guys ride my bike(I'm a moto track rider only,Old Timer novice),and they are blown away by the torque and power of my set up.A KX250 Intermediate rider said,"I didn't know a 2-stroke 250 could feel like this". Of course you can port and go big bore and whatever else you want,but I wanted to do it with bolt on stuff that did not mess with reliability(potentially).That's my very happy and satisfied 2 cents!And I can lug my bike in a crawl in 3rd gear and it doesn't stall.....100_0957.JPG

I would have the bottom the jug milled down.  Someone here has a link to a guy who does that.  By lowering the transfer ports, you get that monstrous bottom end that strokers often loose.  Get the squish on the head done too.  Different reeds can help a little as well.

Machining the base IMHO isn't what you are after...

Easier to bolt on a gnarly pipe and get the same effect - lowering the ports together is the same as running a longer tuned length pipe.


The comments about the head are correct - if you can afford race fuel (or av gas) the adding compression and tightening the squish is excellent modification along with adding ignition advance.


Im not personally a fan of the divider wings - just jet it correctly!  

I'll remember that you're not a fan of divider wings when I'm sending out 8 x 10 autographed glossy's of my X-wing,but it does something that even spot on jetting does not,and the channeling of the air INTO the carb,gave my bike a noticable boost.(Yes,you've insulted my bike,my carb,my jetting ability,my $140.00,the Tassinari family,all of Italy,and probably parts of Sicily as well.Take it back or some kneecaps may get busted up).

harrperf, in your second plot, with the sane transfer port timing, how did that alternate PV spring compare to the stock part?

harrperf, in your second plot, with the sane transfer port timing, how did that alternate PV spring compare to the stock part?


See the first dyno chart.

See the first dyno chart.

I literally meant what I wrote - how does that PV spring compare to the stock spring? Rate, length, etc.

I literally meant what I wrote - how does that PV spring compare to the stock spring? Rate, length, etc.



Sorry, when you referred to transfer timing I figured you were referring to the Pv spring's influence on power.

The spring rate is a secret ;)

Edited by harrperf


bottom end power build, worth the watch

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now