cleaning carb on 06 yz450f - found few things...questions

I pulled the subframe/shock and removed the carb to give it a good cleaning and found the following (bike is 100% stock, prior owner had aft mkt pipe,,said put everything to stock )

start jet was 72 and that is stock

pilot jet was 48 and stock is 42

main is 175 and stock is 165

leak 55 and stock is 55

clip in 4th slot

the needle valve tip is rubber not metal???

I am at sea level, just trail ridding, stock exhaust

i was getting some backfiring could this be from the over rich condition on decel?

How much a difference (going back to 100% stock settings) will a 165 make over the 175 and 42 over the 48?

and lastly I thought needle valves were all metal not rubber coated (some type of flexible polymer)

thanks in advance

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