Yamaha YZ125s are great bikes but have been basically the same since 2005. For this build we thought we would see how good we could get this 2011 YZ125 that was saved from a premature death. The bike had been rode hard and was pretty beat. The cylinder was in bad need of full repair from Millennium and the bottom end was no better.
Jay Clark thinks it’s good to build some of these Craigslist finds as cheaply as possible (the bike was only $800), however, for this build Jay thought why not go all out and build a full-on race ripper motor with the help for two-stroke guru Tom Morgan. Tom worked for factory teams and top racers for years and has long been the choice of top two-stroke engines.
So we shipped off our engine to Tom and we tackled the suspension and chassis part of the build.
The YZ134cc still sounds like a 125cc and produces more mid-top HP while maintaining a normal type power band. This power works well for use on longer, faster tracks. The YZ134cc is a standard bore stroker; TMR strokes the crank and leaves the bore stock size. The cylinder port timing and head dome shape and volume are modified to achieve the desired performance with the stroked crankshaft. The piston is modified to add a boost port of the correct open duration to further improve performance. The crankcases are also modified for clearance to the stroked crankshaft and other changes to produce the correct power output. The motor also has a balanced crankshaft from CWI to help on vibration and of course to change the stroke they have to modify the pin placement.
After throwing way too much money at this thing we have the ultimate fun machine that is great for a young rider or an older guy who wants to relive his past glory days.