TECH: MISSION REFRESH, BREATHING SOME NEW LIFE INTO OUR 2008 RM-Z250
Four-strokes have the reputation of being time bombs. With neglect and abuse they most certainly are, but with love and care you can keep that motor pumping out horsepower and avoid costly repair bills. At 30 hours our 2008 RM-Z250 had provided plenty of fun, but was feeling a little sluggish, and the suspension needed some new oil and stiffer front springs. With the help of Scot Gustafson we freshened up our RM-Z motor and brought some life back into the suspension.
Out on the track the difference was huge; it was like having a new bike! The engine and suspension work was provided by Scot Gustafson of Team Johnny Campbell Racing.
The first stop was the dyno to discover what the machine was putting out after 30 hours of use. We were curious to find out how much power we would gain with a top-end rebuild and the addition of an aftermarket pipe. Before we turned a wrench, peak horsepower was 32. After cool-down we replaced the piston and rings with new stock Suzuki parts and the cylinder was deglazed using a ball hone. After careful inspection of the valves and valve seats it was determined they were still in good condition and didn’t need replacing, just adjusting. The valves were readjusted to factory specs. The exhaust valves were both at 0.007 and readjusted to 0.008 inch. Intake valves were at 0.003 and 0.004 inch and readjusted to 0.005 inch. After 30 hours of hammering, the clutch was toast! The fibers were worn out and the plates were discolored from heat and abuse. We used all stock Suzuki parts including fibers, plates and clutch springs.
With a fresh piston/ring and valves properly adjusted, the bike went back to the dyno where the new peak horsepower was 33.5. We gained 1.5 horsepower, just from a new piston/ring and properly adjusted valves.
Our 2008 RM-Z250 didn’t have a ridiculous amount of hours, but after just thirty hours we had lost horsepower. The multiple trips to the dyno were provided by Keith Mitterrer of Santa Barbara Motorsports.
With 30 hours on the stock muffler, the packing was beyond blown out. It sounded horrible and we called upon the doctor—Doctor Dubach, that is—to get a full system for the RM-Z. With system installed it was once again back to the dyno where the mighty RM-Z put out 34.5 horsepower, another horsepower gain, and it sounded much better. Throughout the dyno testing we stayed with stock jetting.
The RM-Z250 is a great bike in stock trim and did well in our 2008 250F shootout, but our biggest complaint was that the suspension felt like it lost its spunk within a few hours, especially feeling a little soft in the front. Since we were freshening up the motor, we went to work on the suspension, swapping out the stock fork springs (0.44kg) for a set of stiffer (0.46kg) springs and set the oil level at 360cc (370cc stock). With the new springs and new oil level we found the best settings on the clickers were C-14 and R-11. We like the way the rear shock works and we left it alone, running about 100mm of sag. After the work our 2008 feels like new and we like the stiffer front springs. The bike still turns like a Suzuki, which is excellent, and doesn’t have a tendency to dive or bottom out the forks. Mission refresh a success.