Help 2008 crf450 popping and backfiring

Hi guys, I just bought a 2008 crf450r. It had been sitting for a while wouldn't idle with the choke off and it sounded like when I gave it throttle the the rpms would stay high for a minute then come back down. I decided to clean the carb and check the jetting. I left all the Jets the same except I seated the pilot screw then backed it out 1 5/8 turns. Started it up and now it's popping and backfiring, after about 8 minutes of idling it puked up antifreeze. My buddy thinks it is lean. Any advice?

Put in a NEW pilot jet

Clear the passage above the pilot jet with fishing line and spray cleaner

Inspect the fuel screw oring

Inspect the hot start for coorsion (not sealing, and leaking)

It's all jetted the same and it wasn't backfiring until after I cleaned the carb

Then you put the slide plate in upside down (hole goes on the bottom)

I'm new to this. Where is the slide plate

On the slide.....assmuming you removed the slide.

The exhaust wasn't tightened enough backfiring has stopped. The slide facing the air box is slightly open and opens all the way when you give it throttle. I didn't see a hole on it. it still pops and crackles a bit when it's on the stand and I rev it but riding it I didn't notice it happening.

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