Beta 4T Pop in Exhaust

[mcman56]

I have a 2016 300 4T standard model.  It has started to produce a little pop in the exhaust when closing the throttle from small to moderate openings.  The pop is not loud but sort of echos in the exhaust.  It starts, idles and runs well but has become cold blooded so needs to run for several minutes before it will take any throttle when cold started.  I checked valve clearances and cleaned the carb with no affect.  This is with California pump gas.  Has anyone experienced this?  I'm not sure if I should go up on the pilot size or raise the needle.  Carb access is not great on this bike.        

[lemur]

CV carburetor running lean, just like CV carbs were designed to do.  You might try raising the needle if the problem happens at greater throttle opening.  What does the spark plug look like?  & does it smoke at the exhaust &/or is the engine full of carbon ?

[mcman56]

Plug color is a bit uneven but doesn't look too bad.  It doesn't smoke.  

I am running much leaner than stock jetting.  When I got it in 2018, it had the factory jetting of 27.5 pilot and 120 main but ran poorly.  The advice was to go richer but when I went that way it was worse so I went the other way.  It kept running better the leaner it got so I ended up with 17.5 and 115 which is much leaner than all recommendations.  The engine is getting up near 300 run hours so I wonder if it has slightly less vacuum and is less tolerant of lean jetting.