Part seven of the CR250 engine build. Now that the bottom end is done, we can turn our attention to the top end. Top end segments include, cylinder and cylinder head prep, power valve installation and set-up, piston and rings, reed valve and carburetor.
This segment will cover power (or exhaust) valve installation.
The exhaust valve or HPP (Honda Power Port) on these engines was very effective and comparatively low maintenance.
Tools needed: circlip pliers, torque wrench that will measure down to 4 lb.ft. and standard metric tool assortment.
Thoroughly decarbon and clean the power valve components. I have found that a common oven cleaner aerosol does a good job of removing the carbon buildup on these components. Once you remove the carbon, clean all the parts in a solvent based cleaner and dry with compressed air
Power valve components. The two shafts are identical. Most of these parts came out of a high time CR. They were cleaned, NDIed, decarboned and checked to be in spec. Items out of spec were replaced, the rest will function as good as new.
Apply moly 60 grease to the flap valve shaft and install the flap valve through the exhaust outlet with the ridge facing up. Line up the round hole in the flap valve and insert the flap valve shaft. Check that the flap valve moves up and down freely. Coat the left sub exhaust valve with your favorite two stroke oil and set it aside. Apply moly 60 grease to the sub exhaust valve shaft and insert it into the left sub exhaust valve you just oiled. Insert the sub exhaust valve assembly into the cylinder passing the shaft through the elongated cutout on the flap valve. Apply an anti seize compound to the threads of the stopper bolt and install it along with the flat washer into the top of the cylinder and torque to 4 lb.ft.
Coat the drive shaft and bushing with multi- purpose grease and install through the bottom of the cylinder and seat the end of the drive shaft in the drive pinion. Snug the socket bolt on the drive pinion but do not tighten yet. Install the spring clip.
Socket bolt on drive pinion.
Operate the exhaust valve lever by hand and check that the flap valve opens and closes. Now check that the mark on the left sub exhaust valve points to “L” when the valve lever is fully counterclockwise and the flap valve is closed.
Turn the valve lever fully clockwise and check that the left sub exhaust valve point to “H” and the flap valve is open.
Once you have completed the functional check, tighten the socket bolt on the drive pinion to 4 lb.ft.