any engine when not set up right and dirty will not run right, no matter what no matter what brand it is.

that missing o-ring is defintely going to let in air and not idle right

Pull the tank, clean it, change the fuel lines, pull the carb and clean it, that requires pulling it and tearing it down. When ever I pull a motor I tear the whole thing down but thats just me. since I'm messin with it anyway I like to see the piston liner and crankshaft/bearing condidtion. I've had just as much problems with super tigers as thunder tigers and tower and OS engines.

Get some red rtv. OS usually don't need it, except the LA's but if it' giving a problem then do it.

Put some RTV on the carb to crankase attach point and back plate lip and muffler. put some moly grease around the high and low end needles, don't get it on the needle, just the threads and o-rings.

Once you clean it and find and fix the air leaks. set the needles to factory starting point. My FX's I open the high end 2.5 turns and the low end even with the barrel and turn in one turn.

One thing no one has touched, How old is the motor? Anew FX is not going to idle until you run at least a half gallon though it, they'er just too tight.

Get it going and open to full throttle and set the high end. Lean to peak rpm then richen till you hear an RPM drop.



lower to idle. if it dies add a couple clicks of throttle and get i going. I like to pinch the test to get the low end close. Pitch the fuel line. If it speeds up and dies, th low end is rich, lean he low end. If it just dies, it's lean richen it. Go in 1/4 turn invervals. Reset the high end.
set the low end, try lowering the idle and repinch it. It should run 3-5 seconds with a good pinch. At this point the low end is close. Advance to full, it should hesitate or die, if it does you'll have to play with it 1/2 turn either way to get a good idle and instant trasistion. just remeber this point and nudge it either way until it coperates, it takes time to get it right.