CT90 Timing

Question:

The bike suddenly died on road. It has spark, compression, fuel. Bike will not start now. Could it be timing chain? If not, what might it be? Also, is there any free online specs or troubleshooting guides? If so, where? {Dale G., CA, USA - 1972 Honda CT90K4}

Answer:

We receive many questions similar to yours. We have found that in most cases folks start by saying that they have fuel, spark and compression, but in reality, they do not.

Fuel

Just because you can smell gasoline or the plugs are wet, does not mean there is fuel getting to the carb. Try a little spray of carb clean in the carburetor with the throttle open, then crank it. Any response? This will guarantee fuel delivery. If you receive more of an engine response by spraying it, you may not be getting as much fuel as previously thought.

Spark

Are you testing for spark with the spark plug from the engine in the wire resting on the head? Using a second plug will test the electrical system, but will not tell you if the plug in the engine is bad.

On older motorcycles the spark timing is set by the points. Your point gap should be .012 to .016 thousandths of an inch. The plate that the points are mounted on can be timed, just as the engine can. There should be a notch and two lines on the plate for adjusting it with a screwdriver.

Compression

Most people test for compression with a thumb over the plug hole. Although that is instinctively correct, it is difficult to determine what is 90 lbs and less, which will not run be enough for the bike to run, and 175lbs, which is the optimum operating compression with your thumb.

It is best to test with a compression gauge. Check this page for more information on testing for compression.

If your timing chain were broken, you would not have compression or cam movement.

If it had jumped time you would also not have compression. But you will have valve, piston and cam movement which may give your thumb a sense of having compression.
You will need to remove the left hand engine (alternator) cover and the points cover on the cylinder head. The flywheel needs to be at TDC. Rotate the crankshaft until at TDC. Look for your cam mark. It is usually a small circle. The small circle should line up with the indicator on the head.

When lined up this way, both the intake and exhaust valves should have a small amount of clearance. If they do not, rotate the engine to TDC again and check the valves for clearance.

If you can not align the flywheel and the cam gear marks at the same time you may have jumped timing. This can result in bent valves.

As for your quest for free resources:

  • Here is a Official Shop Manual for Honda CT90, CL90, CD90, C90 and S90 motorcycles manufactured from 1966 to 1977 downloadable PDF file.

  • MotorEra has some good photos and interesting info about the CT90K models.

  • The World of PowerSports site has the best online Honda microfiche we have found. They show the actual OEM part number so you can compare prices or search on eBay for spares.

  • Here is another Trail 90 question we answered.

  • This is by no means all there is out there. New information is being added all the time.

    Search for Vintage Honda CT90K2 parts on eBay.


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