Do not set valves the way the manual says to do it.
Use this procedure.
It works every time and is easy!
The official Honda Service Manual recommends valve clearance maintenance adjustment for the 1985 model year only at 4000 miles and all other years inspection and adjustment if necessary (Maintenance, 3-1).
Why would that be? The 1985-1988 engine design is the same and the valve bodies similar.
No big design change made in one year.
But before I tell you what I learned, I'll discuss the two methods recommended to adjust your valves.
What The Maintenance Book Says To Do: Adjust the valve clearance while the engine is COLD.
Remove the inspection hole cap from the left side of the cylinder head.
Remove the left crankcase cover, rotate the drive pulley counterclockwise and align the index line on the camshaft with the index mark on the cylinder head cover to bring the piston to Top Dead Center (TDC) on the compression stroke.
Loosen the valve adjuster lock bolts which are located on the left side of the cylinder head.
Move the intake and exhaust adjusters outward (away from each other) fully, until resistance is felt.
Then, move them inward (toward each other) one graduation. Tighten the lock bolts.
What Honda Told Their Mechanics To Do: Adjust the valve clearance while the engine is WARM and running. With the side access panel removed to allow access to valve adjusters, run engine to normal operating temperature.
With machine on center stand or supported and idling, (increase idle speed slightly to prevent stalling) work on each adjuster in turn, loosen the adjuster quadrant clamp bolt and move the quadrant inwards a little (the motor will start to sound "tappety" ((valve clearance increasing)) followed by slowly moving the quadrant outward whereby you will notice the engine will become quieter (valve clearance decreasing).
When you reach the point that there is no tappet noise, hold quadrant in place and re-tighten clamp bolt.
If you go too far normally a misfire will appear or the engine stalls, the reason for increasing the idle speed.
A slightly loose tappet is less harmful than a tight one. (courtesy, Steve, Honda mechanic, Honda UK, post 2041, Twist-n-Go Scoot)
So, back to the original question--why the discrepancy in the intervals of valve maintenance of similar engines?
When the service manuals were written production of the Honda CH250 and Helix CN250, which share the same basic driveline, had not commenced.
Honda used relevant sections from earlier manuals to piece together the manuals of those bikes.
Additionally, clearances in prototype models were different than tolerances in production machines.
What mechanics discovered was that the second method was more successful being that the lifters are mechanical, not hydraulic.
Several Honda mechanics I talked with favor adjusting the valve clearance if the lifters are noisy and leaving them alone if they are quiet regardless of the mileage. Randy Pozzi (Rev. 08/2005)