One thing to maybe check, is make sure the valves are opening when they should be. Be sure the timing chain hasn't skipped a tooth or anything, because if this is an interference motor, meaning the valves can contact the piston when at TDC if not timed right, that could be what you're feeling. It could be the valves coming in contact with the top of the piston, resulting in what feels like compression.
How does a cam chain "skip a tooth" in the first place? If the chain is loose enough to skip a tooth, it has been making noise and running poorly for quite some time (unless it was being run under extreme conditions causing the automatic cam chain tensioner to fail in which case it would suffer catastrophic engine failure).
If the valve contacts the piston and you are turning the crankshaft over by hand, as soon as the valve contacts the piston it comes to a sudden stop.
It's easy enough to check the cam timing, but if it's off a tooth, then you are looking at major engine service without any further diagnosis. On most 'interference' engines if the cam timing is off by one tooth, the engine will still run, but performance suffers drastically. If the chain jumps a tooth, the valve timing becomes late in relation to the position of the piston. If the camshaft timing is retarded, the result is that high RPM breathing is improved, but an unstable idle, reduced power at low RPM and hard starting. As the cam chain wears it has the effect of getting 'longer' which results in the cam timing being retarded a few degrees. A cam chain is complete worn out when the timing is off by less than 1/2 tooth.
On twin overhead cam engines, the lift and duration is set by the 'grind', but the point at which the valve begins to open can be adjusted by slotting the cam sprocket bolt holes. Using a degree wheel attached to the crankshaft and a dial indicator on the valve, the opening of the valve can be adjusted to achieve better performance.
On single cam engines, the relation between the opening of the intake and exhaust valves is set by the cam manufacturer and the only variable available to the mechanic is setting the opening of the intake or exhaust valve. Depending on the design of the cam, the drive sprocket can be modified to allow minor adjustment for improvement of either low or high RPM performance, but not both.
The natural wear of a cam chain results in later valve opening, which results in unstable idle, reduced power at low RPM and harder starting.