CR250 Engine build part 7

[dogger315]

Now that the bottom end is done, we will turn our attention to
the top end. Top end topics will 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 on the 92 -01 CR250 engine was very effective
and relatively low maintenance. This system was the last of the
true "power" valves before Honda stepped backward with the ill
conceived case reed, electric power valve motors starting in 2002.

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 bearing with two stroke oil and install.


Apply two stroke oil to the right sub exhaust valve and install it into the right
sub exhaust valve port lining it up with the sub exhaust valve shaft previously
installed.


Coat the bushing with two stroke oil and install it.


Now install the washer.


Finally, install the circlip making sure it is seated.


Mix up another batch of molybdenum disulfide grease and transmission oil
at 50/50. Aplly the mixture to the joint areas of the valve link. Assemble
the valve link components.

Apply grease to areas indicated.


closeup showing spring orientation.

Install the valve link assembly into the cylinder, attach the valve lever to the
right sub exhaust valve with the socket bolt 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.

Left sub exhaust valve pointing to "L".

valve lever shown fully counterclockwise.

Flap valve shown in closed position

Turn the valve lever fully clockwise and check that the left sub exhaust
valve point to "H" and the flap valve is open.

Image of valve lever fully clockwise

Image of the flap valve open.

Once you have completed the functional check, tighten the socket bolt on
the drive pinion to 4 lb.ft.

[FFRacing11x]

I would like to add that the drive pinion and spring should be preloaded before tightening. This will prevent valve flutter. No it doesn't state this in the manual. Tdub

[Woody_393]

Great work