Porting info anyone?

Have a spare 09 yz250 cylinder. Has anyone noticed where and what ports were modified for more mid-top power? Any epoxy anywhere? Ports widened , raised or lowered? I know the head has a lot to be desired.

start grinding!  Best way to learn is to hop in.

I think raising the exhaust port 1-2 mm is good for top end. It explained it in a video I was watching on porting a while back.

I would start by blue printing the cylinder making all the intake ports the same size, shape and height.  Polish the power valve, makes it hard for carbon to stick, blend the exhaust port edges a bit. I think the head makes a big difference.

 

With the jug off, and PV installed, blend and smooth any areas which might restrict flow.  Hard to explain, easy to show you.  If you want to deck the base of the jug, it will increase the low and mid hit a bunch.

 

I would optimize what you have before hogging out a good jug.  Easier to take it off than to put it back on.  You can do a lot with reed composition too.

 

Ride hard, take aspirin, repeat as necessary.

If you have Pro Circuit port it, then you can see how the master does it.  Best tutorial you can get for $325.

I ported and polished my 09 yz cylinder myself and decked the head and jug myself aswell. For me i used my drimel tool with a softer wheel and took out .003 out of the intake side but on the exhaust side i only took out about .001. I was trying to get smoother rpm pull than i had with a bit more bottom end grunt. Then used a polishing type wheel to smooth it out real well. Came out nice and cloudy, if you cant tell how smooth it is with your fingers, use a ball point pen to run on the surface youll be able to tell almost any imperfections.

When jetted properly, my bike is pretty strong for a 2stroke right off idle, then the pull gradually gets stronger as the rpms climb and doesnt fall off up top. It is an extremely fun bike to ride.

I mainly ride moto with the occasional trail ride and this bike is a blast in both, hope this helps you. Good luck and take your time do as much research as you need to.

Thanks for all the tips! I read in motorcycle USA yz250 build where pc ported the cylinder. They mentioned widening and raising the exhaust port. I'm going to clean up any inconsistencies with ports and pwr valve and port match. I plan to open the exhaust sub ports up slightly, raise and widen exhaust port at the upper half, look into making the boost port flow more efficient. Not sure if I can create any Venturi in any of the ports but ill see. The plating has some slight pitting. I might price out a 265 bore and replate too.

Thanks for all the tips! I read in motorcycle USA yz250 build where pc ported the cylinder. They mentioned widening and raising the exhaust port. I'm going to clean up any inconsistencies with ports and pwr valve and port match. I plan to open the exhaust sub ports up slightly, raise and widen exhaust port at the upper half, look into making the boost port flow more efficient. Not sure if I can create any Venturi in any of the ports but ill see. The plating has some slight pitting. I might price out a 265 bore and replate too.

 

 

They werent talking about the exhaust duct...

I would start by blue printing the cylinder making all the intake ports the same size, shape and height. Polish the power valve, makes it hard for carbon to stick, blend the exhaust port edges a bit. I think the head makes a big difference.

With the jug off, and PV installed, blend and smooth any areas which might restrict flow. Hard to explain, easy to show you. If you want to deck the base of the jug, it will increase the low and mid hit a bunch.

I would optimize what you have before hogging out a good jug. Easier to take it off than to put it back on. You can do a lot with reed composition too.

Ride hard, take aspirin, repeat as necessary.

I agree with this. Cleaning up what the Japanese did is a great idea. There is a lot of science and geometry in a 2 stoke cylinder and ports and if something is altered too much or incorrectly I could do more damage than good. And yes there is a lot to be had with cylinder head mods (squish clearance, band width, the degree of the squish band) and with the head if you don't like it you are only out a head not an expensive cylinder. I think you have the right idea though with just doing clean up work. Also I was about to match my cases to my cylinder ports and apparently they are designed with a step for a reason. So that shows that cleaning up what is already there may be the best and safest improvement. Edited by Bremer120

There is a a fair amount of power to be had if Done correctly?? I would start with blue printing the cylinder. Meaning you measure the height of evey port, Check the directional flow of evey angle towards the center of the cylinder, Include the angle of the boost port on the intake side aslo. Then check the roof port angles. Port angles along with heights make or break a 2T engine big time!!  Now there is also a lot of power in the triple port exhuast as well. I build Prostock drag race snowmobile engines and there is power to be had but it must be done correctly or you will end up with less power than you started with. Pollishing the exhaust and power valve is a very good thing to do and that is some thing all 2T owners should do. Now with triple port's just making them bigger lenght wise (front to back) will increase your power but you also need to change the angle where it sucks the exhuast from if you enlargin it.

 

FYI, Most engine builder's use a degree wheel to figure out port timing for the heights.

Edited by Zrt1200

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