A Wicked Fast YZ125 big bore stroker

We just built what I consider to be the fastest yz125 in the country.  And guess what - it's not your everyday yz144.  Instead, we stroked it 2mm, bored it 2mm, and made a 140cc mean machine.  While not the cheapest route due the cost of precisely stroking a crank - the additional displacement and port window height given by stroking go hand in hand.  Due to the math of it all, when going big bore - every time you add bore size your displacement goes up by half the bore size squared.  However, the port area only goes up linearly.  This means all things being equal - when going big bore - you loose potential power per cc because the window area is not the same per CC of displacement.  In addition to this, OEM castings do not leave huge room of excess material to relocate ports, or properly add back in some pockets and "hooks" that create the idealized scavenging flow pattern.   When stroking the engine, the added displacement is accompanied with added window area, making it a great option.  Instead of going our normal 144 route of no stroke and 4mm of overbore, we chose to limit boring to an additional 2mm for 9cc added to the engine coupled with the 2mm stroke for 4cc's - and you have a 140cc of fun that gives you almost the same port window area per cc as you started with at 125cc's, but also allows some of the critical hooks that normally get bored complete out and must be ground back in without going to far and hitting a water jacket, to still exist properly when only 2mm overbored.

So the real question is how did she run!  And the answer is astonishingly well!  See the attached dyno for all you could need to know.  We've compared it to the VERY respected full JMS YZ125 race package with vortex, fmf, and vforce that is heavily utilized at all amateur nationals.   We've made 144's and 150s with more hp, but the spread of power, huge torque production, and incredible crispness of this 140 makes it my personal pick for the best combo to date.