engine won"t rev up past 1/2 max rpm, valve adjustment, please help

[Bull]

HI,

I am new, looked on this forum but did not see a definite answer, I have a zongshen Bull 250 and one day I was riding and suddenly the bike lost all power and now will not rev more than half of max rpm, it acts like it has a rev limiter so low, and it pops and maybe smokes little black, i replaced the carb so it is not an issue there and then noticed after a day that it has low compression on the kickstarter and there is a sound in the air filter/box when i kick it over so i figured maybe a cam chain skipped a tooth and now the timing is off and intake is maybe open slightly so i am trying to adjust the valves but I cannot find the T mark, there are random letters like SM, and then there are 2 lines that match up with the marking on the crank case but no letter T for Top dead center.. How are the markings on the cam sprocket supposed to sit relative to the T mark for Top dead center? I don"t know anymore where to start and what to do, can someone guide me through valve adjustment and what the clearences should be? I think the engine is CB250, it is air cooled and it is SOHC. THANK YOU!

update: I don"t know if my timing chain has skipped a tooth, so I need to know where the marks on the sprocket are supposed to sit relative to the top dead center. I also noticed that my timing chain is loose on the timing sprocket. Could that be just because I took the head cover of? Is there a way to tighten the timing chain?