The Basic Sequence and Process to Set or Adjust your Honda CT90 Timing

The primary goal in setting the timing on your CT90 is to have a spark occur at the spark plug due to the points opening up when the piston is at the ideal position before top dead center with both the exhaust and intake valves closed. In the write up below I will share how I go about adjusting the timing on a CT90 and I will be doing this on an engine setting on my bench, but everything I share below is equally applicable for an engine mounted on a bike.

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To set the timing on a CT90 there are four main steps:
1. Making sure that the timing chain sprocket that drives the overhead cam is installed at the correct position on the timing chain relative to the crank/piston assembly

2. That the valves are adjusted correctly after the the timing chain sprocket is installed on the cam and the crank/piston is at top dead center.  This step isn't really required to set the timing, but this is just the logical time to adjust the valves since you will have the crank in the correct position.

3. That the points are gapped correctly

4. That the points plate is adjusted correctly so that the points open at the correct crank/piston position in advance of top dead center

In making sure the timing on your CT90 is correct, it is very important that step one has been completed correctly or has been verified to be correct before ever moving on to steps two, three, or four, otherwise you may be just wasting your time because if step one is not correct ,you'll never get the timing on your CT90 correct.

Focusing on step one, there are three key features you need to be aware of in making sure everything is installed correctly.

The first features are the timing marks on the rotor that is installed on the crank and the corresponding pointer installed on the stator next to the rotor.  These features are key and will be used throughout the timing process and are labeled in the photo below.

The next key feature is the timing mark on the timing chain sprocket that is driven by the timing chain that is driven by the main crank.  This mark is what gets aligned to a corresponding mark on the cylinder head.

The key feature on the cylinder head is the timing mark shown in the picture below that the timing chain sprocket is aligned too to make sure the position of the crank/piston is properly synchronized with the timing sprocket and cam shaft.

The first step in making sure these timing marks will be in the correct position is with making sure the timing chain sprocket is positioned correctly when it is assembled to the timing chain. without going into detail on doing a top end rebuild, when you assemble the sprocket to the timing chain the top dead center mark on the rotor needs to be aligned with the pointer on the stator and then you assemble the timing chain sprocket to the chain with the timing mark on the sprocket pointing away from the engine with the two treaded holes on  the sprocket in a horizontal position like in the pictures below.

If you keep the timing sprocket installed on the timing chain while you build your top end then the mark on the timing sprocket should be correctly aligned with the notch in the cylinder head when you go to install your cam.

I will sometimes hang a wrench on the bolt that holds on the rotor to help keep the crank at top dead center as sometimes the crank will want to roll away from top dead center and the weight of the wrench acts as a counter balance to hold it in position.


If you partially disassembled your engine just to check to see if the mark and notch were aligned and found that the weren't, you can remove your cam and then there is enough room to disengage the sprocket from the chain to rotate it a link or two to get the two mark and notch aligned.  If the mark on the sprocket never seem to align with the notch and is always a half a tooth off one way or the other, then your timing chain is worn and needs to be replaced.
As a side note, the Honda engineers did a nice thing when they designed the interface between cam and the timing chain sprocket in that they offset the two threaded holes on the sprocket so that you can't screw up installing the cam to the timing sprocket as it can only be assembled one way.

When the cam is installed to the timing chain sprocket it can be difficult to see the timing mark on the sprocket as it is partially covered by both the timing chain and the flange on the cam.

But if you look closely (it can be a pain if there is oil and other crud) you should be able to see the timing mark and check that it is aligned with the notch on the head when the crank is at top dead center.

Once you have verified that the timing marks are all correctly aligned and you still have the engine at top dead center, its a good time to check that both the intake and exhaust valve are correctly adjusted which I won't go into here.

As a side note I wanted to point out something I ran into one time in case someone else happens to run into this in the future if they happen to be working on this area of the cylinder head.  One time I had a CT90 that wouldn't start so I was disassembling this area to check if the timing mark was aligned to the notch in the housing and found a funny coiled spring on the shaft that drives the spark advancer, like what is shown in the picture below.

I was an engineer before I retired, so I quickly recognized this as the garter spring from the oil seal that is installed in the cover for this part of the assembly.  The purpose of the garter spring is to energize the lip of the seal when it is installed on the spark advancer shaft on the cam.  So if you do happen to find this spring don't throw it away and just reinstall it back into the lip of the seal like is shown in the photos below.  When you go to install the cover remember to lubricate the lip of the seal and that should prevent it from rolling under during installation which is what allows the garter spring to pop out.  While the bike not running was unrelated to this spring, I did learn something and at least avoided a leaky seal at this location.

With the alignment of the timing chain sprocket mark and housing notch verified and the cover with the seal installed, you should next check that your spark advancer operates freely as that is key for your engine to operate correctly at any rpms above idle.  If your spark advancer seems sticky or gummy  spray it down with your favorite penetrating oil to free it up.  If it still doesn't seems quite right, pull it off, disassemble and clean it.  If you need help getting it back together I made another post here on how to build back up your spark advancer.

The next step is to focus on making sure the point gap is set correctly before going on to the final step of setting the actual timing.

Before we discuss setting the point gap I wanted to share what I learned from working on one of my CT90's where no matter what I did, I could never get the points set correctly because the static timing light I had connected would never turn off, even when the points were open. In the end what I found the problem to be was that I was getting electrically conductive contact between the forked end of the wire that comes to the points and the point bracket itself which resulted in the wire always being shorted to ground. The problem had to do with the fibre washers shown in the picture below that are supposed to electrically isolate the wire from the bracket.  The fibre washers had been incorrectly installed allowing the wire always to be in contact with the bracket.  Once I got the fibre washers correctly installed as shown in the photo below, the points worked correctly.

Getting back to adjusting the point gap, the picture below points out the screws and features involved with adjusting the gap on the points.

If I'm not installing new points I'll generally take my points file and run it between the contacts on the points to make sure the contact faces are flat and clean.

Prior to the actual adjustment of the point gap, you first need to rotate the crank counter clockwise using a 14 mm wrench and watch the point gap and once you see the points at their widest opening then stop, as this is the point where you need to set the final point gap. 

To adjust the point gap you would first loosen the two screws that hold down the points assembly just enough so that it can be adjusted but still hold everything in position when you stop providing input. Then using a wide blade screwdriver like shown in the picture below, rotate the screwdriver counterclockwise to reduce the point gap and rotate clockwise to open the point gap.

The point gap will be set correctly when the gap is within .012 to .016 inches using a feeler gauge which should slide through the point gap with just a little drag, but without opening the gap to allow for insertion of the feeler gauge.  I always adjust the gap close to .012 inches which allows for more time to charge the coil and therefore more energy to generate a spark at the spark plug.  Once you have the gap set to the correct dimension, tighten the two adjustment screws to lock down the point assembly.  It doesn't hurt to check the point gap again after this step to make sure nothing moved when you were tightening down the two adjustment screws.

With the point gap correctly adjusted, we now can complete the final step which is to set the actual timing.  The picture below shows the features on the points plate involved with setting the timing.  Setting the timing also requires that you have a static timing light available and if you don't have one  I have a post at the this link where I show you how to build your own from a simple LED light.

While there are alignment marks on both the points plate and housing, they are really only important when first installing the points plate to get it in the rough ballpark of where it needs to be, and are not critical for the final adjustment.

The first step in setting your timing is to connect your static timing light which is shown in the picture below.  One lead from the light needs to be connected to the end of the wire coming from the points and the other lead from the static timing light needs to be clipped to the housing.  When the points close the light should come on and when the points open the light should be off.

With the static timing light correctly hooked up the following are the steps to adjust your timing:

1. You first would loosen the two screws shown in the picture above that retain the points plate to the housing just enough so the the plate can rotate with some resistance, but will stay in position when it is not being held.

2. Using a 14 mm wrench connected to the rotor and the crank/rotor in a position slightly below top dead center, rotate the rotor counter clockwise.  As the "F" indicator mark (not the F itself) on the rotor approaches the pointer, the static timing light should first be on and then turn off just as the indicator mark comes into alignment with the center of the pointer.

3. If the static timing light turns off before the indicator mark reaches the pointer then your timing is too advanced and you need to rotate the points plate counter clockwise slightly to retard the timing.

4. If the static timing light turns off after the indication mark passes the pointer then your timing is too retarded and you need to rotate the points plate clockwise slightly to advance the timing.

5. Once the static timing light turns off just as the indicator mark reaches the center of the pointer your timing is set correctly and you can now tighten the two screws that retain the points plate.  It is good practice to go back and recheck the timing once the screws are locked down to make sure nothing moved when you were tightening down the screws.


Sometimes it is easier to show something then trying to describe it, so I included the video below to show what the static timing light should look like for a CT90 where the timing has been correctly adjusted.  While turning the rotor counter clockwise the light should initially be on and then as the "F" indicator mark just reaches the pointer on the stator, the light should turn off.

While the best guidance is always to adjust your timing to the the standard setting as described above, there are times when yo may want to consider setting your timing so that it is just slightly advanced. These cases would be if you had a Big Bore Kit as you have more fuel to burn or if your timing chain is older and slightly stretched which would tend to retard your timing.  Also, bikes with the timing a little advanced may be easier to start.  In the end, timing is just another adjustment in tuning your CT90 and once your are comfortable with doing it, it enables you try things to help get your CT90 running at its best.

I hope you found this post helpful and that you're successful in setting the timing of your CT90!

Related Posts
My Process to get a CT90 that won't Start to Start
Adjusting and Jetting a CT90 Carb
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Building a static timing light from a free Harbor Freight LED Light

Helpful Links (Shop Manuals, Wire Diagram, Model Information, etc.)

Link to page with listing of CT90 parts available on Amazon