Bike starts "every second" time only....

[Black Warrior]

Here is a deal - my friend got the same bike as we have quite recently (2000, 7000km). I helped him to clean a carb, tune it a bit etc. So everything was fine until he decided to get a new muffler ... I thought I opened the screw (2.75 turns out) enough to have a bike running nicely (btw shim mod is done there too - 130 jet), but right now his bike behaves actually even better than mine during all regimes, but has a problem to start After you crank it for 2-3 sec, let it stay for another 5 and it starts a nothing happened (with an open choke though). I thought that the mixture was too rich and it needed some extra air, so going down to 2 or 1.75 did not fix it, the other way around I went to 3.25 and still the same story with an exception that bike is just being SOOOO LOUD, but it's not a biggy.... Please, could you share any ideas on what it could be! It's kinda unusual for me, since, thanx God, I have not had that problem and my bike is only 5000 kms older.

P.S. Plug is new and working, when the bike is warm it starts as nothing ever disturbs her.