What tune-up measurements should I use?
Doign a tune-up on a CT-90 or CT-110 is pretty simple, but should be taken care of when needed, or the bike will let you know pretty quickly it doesn't like you.

Note: these values are from the factory service manual for the 1969 K1/K1b, but they should be applicable to all OHV CTs from 1968 to whenever they changed carbs in the late 1970s. Many of these values still apply to the CT-110. For crucial adjustments or fine tuning, obtain your own service manual or check with a dealer or experienced repair place.

Things you should check fairly often:
  1. Chain tension: chain should have about 1/2" of vertical slack in it. Be sure to keep the chain lubed, preferably with motorcycle chain lube (you can get spray cans of it at any cycle shop). Do not use WD-40 or anything like it unless you want to be buying a new chain soon. If you've been riding where it's really dirty, take the chain off and soak it in some kerosene, let it dry off, then lube it properly. A loose chain will put "cups" in the teeth of your sprockets and then you'll end up replacing it all.
  2. Brake cables: both the brake levers and the brake pedal should have about half an inch of play. Do not be tempted to adjust the play too close, or too tight, or you'll drag the brake shoes and heat the hubs up, as well as lower performance. Check the brake light to make sure it lights on both the pedal and the levers.
  3. Spokes: If you've been riding hard, check for loose spokes and tighten them up. If you break one, plan to replace it soon, or others will break because of the tension imbalance and next thing you know, the wheel will go out of round or even collapse when you hit a good bump.
  4. Nuts and bolts: go over the bike and tighten anything that's vibrated loose. Eventually you will learn which ones are more likely to get loose, and focus on those. CTs tend to shake off nuts and bolts if you don't keep after them. My personal favorites are the four bolts that hold the footpegs onto the bottom of the engine casing, because if you lose or strip those out you've basically trashed the engine case.

Tune-up values:
  1. Compression: (yes, you should own a compression tester) Stick the tester into the spark plug hole solidly. Have somebody kick the bike hard with the throttle wide open and the ignition off. Kick it five or sicx times to make sure it pumps up to the max pressure. Factory spec is about 170psi, but acceptable values for an older bike can be as low as 135, and I've seen CTs run with as little as 90. If you are getting down toward 135, you should probably think about honing the cylinder and relacing the rings, which will improve things greatly. If rings don't help, you make have a leaky valve or the valve clearance is too small. That leads us to...
  2. Valve adjustment: Valve adjustment is critical. Too tight and it leaks compression and you'll burn the rim of a valve, and too loose and things get noisy and don't develop full power. Using a large wrench (not a crescent wrench, you'll trash the cap) remove the valvel tappet caps on the top and bottom of the head (those things with a big bolt head on them). The engine must be COLD for this test. Use a 0.05mm (0.002") flat feeler gauge for both valves, and check to make sure that the clearance between the cam and the tappet is just enough to let you slide the feeler gauge around in there with a little drag. Adjust as needed.
  3. Valve chain tensioner: This is a bolt under the center of the engine, sort of at a weird angle, and basically you just want to loosen this briefly, then tighten it back up. It's spring-loaded and if you think the cam chain tension is wrong the right solution is to replace the big spring in there.
  4. Spark plug gap: 0.6mm, the plug as originally specified is an NGK D8HS, but this may have been replaced. Cross-reference it as needed.
  5. Points: Gap should be 0.3 to 0.4mm at maximum opening. To time the ignition, remove the alternator cover and rotate the flywheel so that the "F" mark is aligned with the little allignment mark nub. At that instant, the points should be just starting to open. Loosen the base screws on the point base and rotate the points until this is so.
  6. Carb settings: adjust this after everything else has been adjusted. Carb problems are usually the LAST thing a CT90 experiences (ignition problems are much more common). This is not intended to be a detailed discussion of setting the carb (it'd take a page and a half -- get a service manual), but basically, set the air screw to about one to one-and-a-quarter turns out from fully closed, then adjust your idle screw to bring the engine speed down to the right level at idle. Make sure the carb is clean and that when you're adjusting idle, the engine is WARM (that's why you have to do this last). Basically, you want to adjust the carb so that it doesn't run too rich or too lean, so that it idles smoothly, AND so that when you nail the gas the engine revs up smoothly and doesn't stumble. If you have flat spots in the performance curve, it's likely carburetion, but if you get misfiring, it's ignition.

Last updated: 05/11/2002