QuestionI have a '81 Honda ct70 which idles great and will run good at low rpm and mid range, but when I fully open the throttle up fast it will bog. I have pulled the carb and cleaned it well. Adjusting the fuel/air mixture screw has absolutely no effect I can turn it all the way in or out and doesnt change anything. thanks for your help.
AnswerGary,
http://www.bikebandit.com/houseofmotorcycles/honda-motorcycle-ct70-1981/o/m2293
to look through the parts illustrations.
If you cleaned the carb and main jet circuit, inspected and cleaned the slide needle and noted the 10.7mm float level, the there are a few other areas to investigate.
Adjust the valves: clearance is only .002" cold on both intake and exhaust valves, set at TDC (T mark on flywheel) COMPRESSION STROKE. If you set the clearances on the OVERLAP stroke (T mark, again, just different cycle), then the valves will be VERY loose and noisy when you try to start it up again.
Check the air filter! Carburetors are calibrated for stock air filter element and stock exhaust system. If either parts are changed or damaged, then the calibration of the jets will need to be changed.
Check the ignition timing! The appropriate point gap (look through the slots on the flywheel, the points are mounted on the backing plate) will be about .014", but what you need to watch and adjust is the moment when the points just OPEN, which should coincide to the F mark alignment. Ignition fires on every stroke, wasting the spark on the exhaust stroke.
If the points are opening at the T instead of the F mark, then the spark timing will be too late for good high-speed running. There is no spark advancer in this system, so initial spark timing is set around 27 degrees before TDC, all the time. When the points close up from wear and lack of maintenance, the timing retards back towards 20-22 degrees and the little engines run poorly.
The air mixture screw is only for idle mixture adjustment. It should make a difference when the engine is idling. If not, then there is an air leak or some internal metering passages are not open.
A 70cc bike with automatic clutch isn't going to be a rocketship, but you should get smooth power delivery all the way up to redline in each gear, when tuned properly.
Bill Silver
