CRF450R 2003. All in stock. Low jet # 38. Valve clearance OK! Easy (one or two kicks) cold engine start with chock pulled and twice or triple shift of accelerator grip. Without grip fuel pumping starts after 10-15 kicks ever hot (no chock) with backfire. Oxygen sensor installed in head of exhaust pipe (about 5’ from exhaust port). Gauge show slightly reach on idle with or without air filter. No A/F meter gauge reaction on removing or installing air filter on idle The same with completely turned on pilot (low jet) screw. Idle is going down just a little. Get lean on idle when hot start lever is shifting. RPM is go up. What is going on?