Question about rotating engine...

[TheImp]

Why would it happen when I tried to rotate the engine to TDC on compression stroke I reached the "C" mark for that cylinder and as I got close it just moved right passed the mark? It's as if the crankshaft had no tension as I cranked it with the socket wrench and passed it about 20 degrees or so. The only way I could get it to stay was to crank the small distance counterclockwise, but I don't think I should do that too much.

I ask the damnest questions sometimes but googling something like this isn't easy.

[jmillheiser]

my CX500 does the same thing. it will rotate past TDC on the right cylinder easily, even with the spark plugs removed which should eliminate any compression

[TheImp]

Do you think having either the intake or exhaust valves not sealing properly (since its on the compression stroke) that perhaps that happens? I mean its virtually impossible to hold it at that position once I'm approaching it, it just zooms right past it as the socket wrench brings the mark into the window with barely any force.

[jmillheiser]

try pulling your spark plugs. the zooming past the TDC point is more likely the compression pushing the piston down on what would be the power stroke (that happens right after TDC)

[TheImp]

ok, will try it out. Appreciate the help.

[DRam]

If this is happening with the spark plugs out (as it does on my bike) it is likely that one or more of the valves are part way open. The valve springs will tend to push the cam forward (or backward, depending on position) and that in turn moves the crank. If the spark plugs are in, it's very possible that compression is moving the piston and taking them out might solve the problem. Sneaking up on TDC helps, as does using a breaker bar, which doesn't ratchet, to turn the crank and hold it in place.