Bike won't start
If the CT-series won't start, there are only three major areas to check:

Spark (and timing)
Fuel
Compression

There's a reason they're listed in this order, because this is the most common order in which things fail. No-spark conditions account for the vast majority of no-start situations. Fuel accounts for a few more, and the cost to repair these types of failures is higher. Lastly, low compression usually means big failures and big repairs, and happens much less often.

Useful tip: the spark plug on the CT-110 (and probably the 90) is not a normal automotive size. I use an 18mm deep-well socket on my 110. The real size is probably something odd.

If you check this	and you encounter this	Do this
Pull the spark plug, connect it to the plug wire and lay the tip against the block. Crank the engine.	Decent blue spark at the plug gap	You have a fuel, timing or compression problem
Same as above	Weak spark (yellowish, hard to see, tiny, or intermittent)	Try a new plug and set the gap to spec. Check the point gap, the plug wire (for cracks or shorts) and the coil.
Same as above	No spark at all	Check the points. If you have spark at the points but none at the plug, either the plug is bad or the coil is bad. If you have no spark at the points, either the points are bad or there's a broken wire somewhere.
Check the points	No spark across point gap	Check the wire leading to the points. Find a 6v battery and connect the points directly to it, and connect the - terminal to the cylinder head surface. If you still get no spark, the points are bad. Replace them.
Check the points	Spark across point gap, but no spark at plug	Bad plug, bad plug wire or bad coil

Assuming ignition has now been ruled out and you have good spark, check fuel problems:

If you check this	and you encounter this	Do this
Spray some starting fluid (ether) directly into the carb intake, kick the engine	Nothing at all, not a puff	You could have a bad timing problem or a compression loss so bad the engine can't run.
Same as above	Engine lights up but dies when the ether runs out after a couple seconds	You have no fuel going into the cylinder. Check for blocked fuel lines, a stuck float or a plugged jet.

If you have fuel problems, move on. Compression problems can sometimes be diagnosed by trying to push-start the bike. Put it in first gear and start pushing. The bike should drag the rear tire. If it instead starts spinning the engine over easily, you probably have compression so low the engine won't function. This could mean a bad set of rings, a cracked compression ring, or sometimes a broken, bent or burnt valve. Any of these involves tearing the engine down to check.

If you check this	and you encounter this	Do this
Open the carburetor drain when there's fuel in the gas tank and the fuel valve is open	Nothing comes out	You probably have a badly clogged fuel intake in the tank... sludge built up so high that both intake points are blocked. Also, the fuel hoses could be pinched, or the carb float is stuck closed
Disconnect the fuel lines from the fuel valve (carefully)	Nothing comes out	You definitely have a clogged fuel intake or pinched fuel hoses.
Same as top	Fuel comes out, but the engine still won't start	Probably a clogged main jet... take it out and clean it, and blow the carb out with compressed air if you have it.

If you have spark, have decent compression, and have fuel, particularly if the fuel is wetting the spark plug but not getting burned, you have a timing problem. Find a service manual and set the timing correctly. Set the point gap correctly.

One last possibility: once, I encountered a situation where a CT90 was very hard to start and ran with little power. What had happened was that the timing chain had jumped a couple of teeth, and while the ignition timing was OK, the valve timing was all wrong. This could happen on older bikes where the timing chain tension hasn't been checked. Fixing it requires retiming the valves, covered in most manuals.

Last updated: 08/04/2001