No air is coming out of exhaust

#1
I have a ct200uex with a stock engine with a vm22 mikuni on it and I just changed out the whole head because I bent a valve and I put a 212 head on it and put 16 lb valve springs and was running fine but when I was going down the road on it all of a sudden the throttle stopped working and the bike would barely move and I had to go full throttle to make it move any. Then I adjusted the valve lash and checked everything and got it working again but now when I try to pull the pull start I noticed It was hard to pul on the exhaust stroke and it wasn’t blowing out air on the exhaust stroke and I could hear the air slowly being squeezed out. I’ve already adjusted the lash and it still is doing that only on the exhaust stroke. What do I do to fix this?
 
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