[dropcap]T[/dropcap]he single cylinder engine that Honda debuted with the CBR250R is also the basis for the CBR300R power plant as well as the CRF250L. This engine is a DOHC type with shim type valve clearance adjustment. Unlike many engines of this type it has a handy feature that allows for quick shim changes without having to remove the camshafts.
1. Turn the crankshaft counterclockwise with a 17 mm socket and align the “T” mark with the indicator in the left crankcase cover. The “IN” and “EX” marks should face out and be flush with the cylinder head as shown to indicate TDC on the compression stroke. Turn the crankshaft one full rotation and realign the “T” mark if the camshafts are 180 degrees out of position (TDC on the exhaust stroke).
2. Check the clearance between the rocker arm and shim with a feeler gauge. Record the clearance for each valve and compare this to the specification.
The camshafts do not need to be removed to adjust the valves. Remove the rocker arm shaft cover with a 6 mm Allen socket. The sealing washer should be replaced with a new one on installation.
3. Thread a 6 mm bolt into the rocker arm shaft and pull the shaft out to free the rocker arm. This will allow you to access the valve shims.
4. Slide the rocker arm back and remove the shims with a magnet. Be very careful not to drop the shims into the engine.
5. The shims should be marked to indicate the thickness. There are 69 different shims ranging from 120 (1.200 mm) to 290 (2.900 mm) in 0.025 mm increments.
If the shim marking has worn away, measure the shim thickness with a micrometer.
Calculate the proper shim size needed to put the valve clearance within specification using the following formula:
A = (B – C) + D
A = new shim size
B = measured valve clearance
C = valve clearance specification
D = present shim size
Example: The measured exhaust valve clearance is tight at 0.145 mm with a present shim of 2.60 mm. This results in the need of a 2.475 mm shim. This new shim will be labeled 248. (0.145 – 0.27) + 2.60= 2.475.
2.40, 2.425, 2.45 and 2.475
240, 242, 245 and 248
Install the rocker arm shaft covers with new sealing washers. Tighten the rocker arm covers securely with a 6 mm Allen socket. Rotate the crankshaft counterclockwise several times and recheck the valve clearance.
Replace the cap O-rings with new items. Apply a light coat of engine oil to the new O-rings.
Install the timing and crankshaft hole inspection caps with a large flat blade screwdriver.