Valve clearance, inspection and adjustments should be performed with the engine COLD. This just means that the engine should not have been running for at least 3-4 hours prior to starting this process.
Valve Tappet Clearance Inspection
The valve tappet clearance inspection should be completed about every 2,400 miles or if you notice the engine not operating correctly.
NOTE: Many CT'ers recommend that you start with a valve inspection and possible adjustment, if your CT is not operating properly, even if it seems your problem is not related to the valves, start with this! If you just purchased a CT, start with this if the engine doesn't want to start or is not operating correctly. You could also start with a points inspection and possible adjustment, then come back and do the valve inspection and possible adjustment, however, it just seems 'natural' to do the valves, points and then timing (sort of a 'simple to more complex' process).
Remove the valve tappet hole caps, both TOP (INTAKE) and BOTTOM (EXHAUST). The photo below shows the TOP (INTAKE) valve tappet hole cap removed.
Remove generator cover (left side of engine case).
Rotate the generator rotor in the counterclockwise (anti-clockwise) direction using a socket wrench and ratchet on the center nut.
This process rotates the engine in the correct direction (so, no damage occurs) and sets the cylinder at "TDC" (Top Dead Center) of either the COMPRESSION stroke or the EXHAUST stroke. CT's have an engine that is a 4 stroke, thus, the generator rotates twice for each full, 4 stroke cycle (see this for a description of a 4 stroke engine
Align the "T" mark on the rotor with the index mark on the stator (non-moving, outside edge of the generator). The index mark on your CT may look slightly different than the drawing above, however, the index mark is usually on the right side or in the lower right area of the generator.
NOTE: the "T" mark in NOT the "T" itself, but the slim straight mark to the right of the "T".
The next thing you need to do is know if you are on the COMPRESSION or EXHAUST stroke.
This can be accomplished by checking for a "space" between the valve tappet adjusting screw and the valve stem or a "wiggle of the rocker arm" on both the intake and exhaust valve mechanisms.
Testing just the intake valve will not work, because the intake valve is closed on both the compression and exhaust strokes, while the exhaust valve is closed on only the compression stroke, and open on the exhaust stroke.
Testing just the exhaust valve will often be good enough, however, it is absolutely fool proof to test both valves. We want to know when the exhaust valve is closed and we want the intake valve closed as well, just to check that the entire valve train, including the timing chain (and thus the "T" mark's relationship to the valve cam) is assembled correctly (it would be unusual to have the valve train assembled incorrectly, but it is still better to check both valves).
COMPRESSION STROKE TEST. If you have the "T" mark aligned with the index mark and you are on the COMPRESSION stroke, the intake and exhaust valves should be fully CLOSED, providing a "space" between the valve tappet adjusting screw and the valve stem on both valves. Both the intake and the exhaust valves should have this "space" or a "wiggle of the rocker arm". NOTE: There could be an exception to this... if the valves are both closed, yet the valves are adjusted way too tight and no "space" or "wiggle" is present, then check the position of the ignition points because they should be open on the compression stroke and closed on the exhaust stroke
EXHAUST STROKE TEST. If you have the "T" mark aligned with the index mark and you are on the EXHAUST stroke, the exhaust valve should be fully OPEN, providing NO "space" between the valve tappet adjusting screw and the valve stem and NO "wiggle of the rocker arm", while the intake valve is closed and a "space" or "wiggle of the rocker arm" exists.
You want to be on the COMPRESSION stroke, so if you are not, rotate the generator one complete circle in a counterclockwise (anti-clockwise) direction and test for the "space" or the "wiggle of the rocker arm" on both valves again.
Check the clearance of both the intake and the exhaust valves by inserting a feeler gauge between the tappet adjusting screw and the valve stem.
The valve clearance should be;
0.05 mm ± 0.02 mm
0.002 inches ± 0.0008 inches
If your valve clearance is within tolerance, you are good to go!
Valve Tappet Clearance Adjustment
If your valve clearance is NOT within tolerance, adjustment is made by loosening the tappet screw lock nut and turning the adjusting screw until there is a slight "drag" on the feeler gauge. Once the slight "drag" is felt on the feeler gauge, the adjusting screw needs to be held in position while the tappet screw lock nut is tightened. This can be a frustrating job if the adjusting screw moves while trying to tighten the tappet screw lock nut. Just keep trying to get it right, so that after the tappet screw lock nut is tight, the same "drag" is felt on the feeler gauge.
Honda has a specific tool called the Tappet Adjusting Wrench for holding the adjusting screw in position while tightening the tappet screw lock nut.
However, there are several other ways to accomplish this task if you do not have the Honda tool.
Here is an example of using a cresent wrench to hold the adjusting screw in position while tightening the tappet screw lock nut with a box end wrench.
NOTE: Some CT'ers have used a ViseGrip adjustable wrench to hold the adjusting screw in position. They report that if your ViseGrip tool has teeth in its jaw the square end of the adjusting screw may get deformed by the teeth and pressure of the ViseGrip and after repeated use, the adjusting screw needs replacing because the square end is damaged.
A clever CT'er at the 2007 So Cal Tech Day showed the use of a modified "square drive screw" to hold the adjusting screw in position.
Turns out the square drive depression is just the right fit for the square end of the adjusting screw and its pretty easy to hold in the desired position.
These square drive screws are sometime hard to find, so you may be able to post a message over on the Yahoo CT90 Group List
and see if the clever CT'er (or anyone else) can send you one (that's all you'll need, is just one).
Rotate the generator several times, returning to the "T" mark, and do a final check of the valve tappet clearance. If it is not right, restart the process.
Once you have done both the TOP (INTAKE) and the BOTTOM (EXHAUST) valve adjustments, replace the tappet hole caps and the generator cover.
You now have properly adjusted valves.
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