8nisd10a

So it's a bit of a story what has happened I will try to summarise briefly. Grinding noise in engine, took it to a mechanic he told me the problem is in the bottom end. I stripped the engine down completely and put it back together, put it in the bike but it didn't run. Had a look at the cam gear and realised that somehow I had set the timing wrong (I don't know how I did this after putting the whole engine back together correctly!). The inlet pushrod bent and there was a black mark on the piston from the valve hitting it but the cam gear looks undamaged and crankshaft is still silky smooth. I did a compression test and found with a bit of oil the compression jumped from 110 to 125 so I changed the piston. Fired it up and it sounded good. Jumped on and rode it it around but as soon as I open it up at all it chokes. It doesn't stall but it won't produce any more power. It feels a lot like a problem I had doing high altitude driving and just like then the spark plug gets black very quickly. I have done everything I can think of to diagnose and fix it. Ran it without air filter in, cleaned the carb, put a new spark plug in, changed the fuel, adjusted the carb floats, changed the exhaust gasket, nothing makes a blind bit of difference, it just can't handle any kind of load on the engine. I will change the head gasket but I don't see how it could cause this problem. Any ideas? I'm really stuck as to what to do next but I love the bike