Motorcycle Repair: CT 90 carb, honda ct90, rotor stator

Question
Bill,

I am trying to get a 1977 Honda CT90 running.  I have thoroughly cleaned and rebuilt the carb, however it will not advance off of idle.  When the engine is rev'd and released or you come to a stop when riding, the idle remains high. It idles very smooth at low rpm's and runs well above 1/4 throttle.  I am at 7000' altitude and the Keihin carb has a high altitude compensator pull out valve.  Also, after it runs and gets warm it is hard to start.  The spark seems good.  Any ideas?

Answer
Deni... check valve clearances first.  COLD .002" on both sides.

Second, take the spark advancer off and mark the cam location to the advancer base. Remove the cam, gently... clean, lube and reassemble it in the correct orientation. Clean point faces and set point gap at .014" and move the plate until they open at the F mark alignment on the rotor/stator. Always use a fully charged battery for testing. This is a battery operated ignition system.

Part throttle mixtures are controlled by the needle position in the slide, mostly. Part of the transition from idle to part throttle also has to do with the idle jet size and air mixture screw adjustment. Float level is 20mm, with a stock 62 main jet and #35 idle jet. Mixture screw setting is 1 turn out.. at sea level. Compression at sea level is around 170 psi.

At 7k altitude, you may still have to jet it down, even with the compensator, if you are running rich most of the time. If you are always going to be at that altitude and it is usually cold, you might need to drop down one heat range on the plug, too.

Bill Silver