Moto Grand Prix
Main Motorcycle: 1973 Triumph TR7RV
Join Date: Oct 2013
Location: Pleasant Hill, CA USA
Other Motorcycle: 1964 Triumph Mountain Cub
Hi Ploughboy, Hmmm.... That is interesting. I don't have the knowledge off the top of my head to say if the valve is open to far or not from the photo. I'd have to compare to my bike. I know you can see the valves pretty well when open as I've looked at them many times.
A few thoughts. When motor is stuck, put a rod, screw driver or dowel down cyl. Mark it & compare with other side to see if piston is coming up too far. Coming up too far is another subject we'll only get into if that is found.
If you've not changed valve adjustment yet, valve clearance can give some clues.
I gather from what you say the valve is still moving & not stuck fully or more open?
If.... you can get the right intake fully open what is valve clearance on left? It should be at least close to normal clearance. If clearance is very loose that can indicate a bent valve. Like .020-.030+". If clearance is zero valve may be held open some or a lot. You say it looks normal closed, but is it?
If push rod jumps out of the tappet or rocker balls & sets on edge of ball(s) it will over lift the valve & might interfere with piston. But if it's still stuck on top of ball ends the rocker/valve should look pushed down. You can look with a torch inside the rocker box & visually see top of push rods & see if they are socketed into rockers as they should be. I don't know why push rod would come out, all of a sudden during starting other than the valve stuck in it's guide, then came free & push rod is unseated somewhere.
When you back piston down slightly, can you see witness mark on piston where valve contacted it.
You could try backing off rocker adjusting screw & see if motor moves farther compared to the marks you made on test rod earlier.
BUT!!! Keep upward tension on rocker arm to keep the push rod notched in. If you let rocker get too much clearance with screw removed or backed out too far, it might let push rod jump out.
I've been crazy busy with elder care lately so I'm even more fuzzy minded that usual & my time is consumed by that. It might be a few days before I can pull my plugs & check look at my valve/piston clearance.
At this point you are into serious diagnosis. So go slow & easy. We want to find root problem & correct it or it can happen again.