400ex no start air through exhaust

I’m new to forums so I’m not sure if this is in the right place or not but I have a 1999 400ex that I am rebuilding it was a big bore 440 with a web 450/451 grind cam blew a head gasket took it back to a stock 85mm cylinder with a wiseco 11:1 compression piston cosmetic gasket kit and stage 2 hotcam i took apart the carb and sprayed everything out and put it back together I have got the bike back together piston and hotcam in and put it in time and when I tried to start it the motor was rolling over but not firing it would back fire if it was cranked long enough their is air coming through the exhaust and blowing out the intake so initially I thought it was 180 out of time how ever I took the valve cover off found TDC lined up the hash pulled the plug and can feel the piston at the top of the cylinder but the lobes were facing up so I took the chain of the sprocket and rolled it 180 lined up the horizontal hashes put the tensioner in checked TDC again put it together and it’s doing the same thing as before air sucked through the exhaust and put the intake the first time around I had gas leaking from the header pipe and also spraying out the intake boot towards the air box