you previously said this misfire was off idle - main jet only really affects 3/4 to full throttle so, while 160-165 would be right for many exhausts, that's unlikely to be the problem. is it misfiring under load or when revving it at the side of the road? is it when opening or closing the throttle.
the number of turns on the airscrew is only indicative. are you clear on how to set it properly? thoroughly warm the engine and then, with it still running, wind the screw in until the revs fall, then wind it out as the revs rise and then fall again. the right setting is halfway between the positions at which revs fall when winding in and out. if this is 3 turns, fine. remember to set the idle speed once the mixture is set rather than to use idle speed to compensate for bad mixture.
despite all that, I'm not clear that it'll help you much because I'm not really clear what the symptoms are. one XR400 I had came with a high float so the fuel level was high. this caused an intermittent hiccough under load which pointed to a large main jet, but was actually resolvced by adjusting the float height
good luck and try describing the symptoms more fully
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