Motorcycle Repair: piston hard to cycle, yamaha 175, intake ports

Question
i have a 1973 yamaha 175 ct3. It didnt run when i got it. I adjusted the timing, new fluids, new plugs, cleaned the carb, and to install a new key for the flywheel. It fired right up. As this bikes new to me I wnet to ride it to get a feel for it. I slipped the clutch and it dided on me after about 3 miles, and then was hard to kick over. I figured I had seized the piston even though i had checked the oil pump and had added oil to the gas for extra caution. Once i pulled the spark plug you could then cycle the piston with the kickstarter with ease. Install the plug and you can barely kick it over. So I broke the engine down bit by bit. The only thing i found was possible bad crank bearings, and a broken flywheel key again! I replaced ALL the bearings for crank and tranny. The crank checked out good for play, but i didnt get the truness checked,  because it ran i figured it was good. The cylinder and piston all measure within tolerances. I put everything back together except the flywheel on,  it is still hard to kick over. The trany is in neutral. It seems as if the piston has to overcome a lot of force. This opposition starts as the piston closes the intake ports, and releases when it opens the exhaust port. From the moment the piston goes from btdc, to aftdc, there is a jerk as the piston travels down. The wrist pin and bearing show no signs of step wear. There is discolorment from the areas that touch the pin and those that dont.. ie.. bright to dull. It has been bored over .75. Any thoughts?

Thanks for any help, noone so far can understand whats happening.
Neil

Answer
Hi Oneil,


I would need to inspect and measure parts to accurately diagnose the problem. The truth lies within the wear and measurements.

I recommend having a local machinist check the suspected parts for you.

Respectfully,
Mark Shively