Motorcycle Repair: honda ct200 sputter, stock air filter, spark plug wires

Question
I am baffled Bill, I have rebuilt a 1964-65 Honda ct200 and I am having trouble with it sputtering/cutting-out/misfiring at high RPM. I have cleaned the carb multiple times and replaced the needles and jets as well as adjusted the float up and down but sticking with a height of 20-19mm. I had the cylinder bored with new piston and rings, the valves reseated, the clutch inspected and cleaned. I have also cleaned (point filed) the points and adjusted them to specs as well as timed the bike with a timing gun. The bike has a new fully charged battery. I have tried switching spark plug wires and condensers, rectifiers, and spark plugs. Nothing has caused a significant change. Every time I rev the bike up or run it down the street at higher RPM it starts cutting out almost like a misfire. It never reaches full throttle and stays there. It feels like there isn't a spark or it is missing on the advanced timing. I have inspected and switched timing advancers with still no resolve. Any suggestions? I could really use an expert for such a simple bike

Answer
Aaron,

Well, my usually helpful Honda tuning book has little in the way of carb setting information on the C200 and CT200 Trail 90s. It does list the float level at 19.5mm. Idle jets in the microfiche illustrations are from #35 to #42 and #75 through #85 for main jets. I don't know what size main jet you have, but given the alcohol gasoline that we have to run now, I have found that increasing the main jet about one size is usually helpful. Try applying a little choke ever so slightly and slowly when it misfires. If the engine starts to pick up revs when the choke is partially ON, then it is running lean. CT90s from the following era have main jets from the low 60s up #78. Do be aware that running anything but the stock air filter and muffler system will alter your jetting setup.

If you have re-adjusted your valves to .002" then your compression readings should be in the 160-175 psi range. If that is the case, then the mechanical side of it can be mostly ruled out.

The point gap should be about .014"-.016". There are rare cases where the point bushing can bind up on the pivot pin at high speed/temperatures so be sure the points have good tension on the closed position. After 50 years, there are cases where the point tension springs "relax" and don't keep the point arm in constant contact with the point cam. If you are using a timing light, are you seeing full spark advance at about 36-38 degrees?

Do be aware that there are a couple of brands of ignition system components.. Kokusan and Hitachi and mixing parts from both brands may not be suitable for perfect running.

Sometimes the simple things can present themselves as something else. Check your gas cap to see if it is venting properly. The hole is about 1/16" but must be clear for the tank to vent correctly. Also, if you have old fuel lines they may be partially clogged with old varnish and scale on the insides, which impedes fuel flow. I guess you have checked and cleaned the fuel screen on the bottom of the carb body, already.

All you need is compression, spark, fuel and air.....

Bill Silver