I’m with Swoop, Schu and Rt325 on the worn nozzle and the needle. I have a 350 Alpina I got with 17 miles on the odometer. After many years it was running rich like you all describe. Egg shaped hole in the needle jet. I replaced the needle and jet and it ran great.

Someone pointed out before on this forum that large displacement two strokes can have dramatic pulses in the intake tract that hammers the needle back and forth in the jet, wearing it in an oval or egg shaped pattern pretty quickly compared to smaller displacement machines..

I replaced the needle jet/emulsion tube and the jet setter o-ring (item #7) and it's running much better now. Thanks to all who contributed. It's a great forum!

Good to hear , but why not go the whole hog , and replace the needle as well .
Then you will have removed all the wearable components , for the best result .

 

Update-I still had a little burble over 5000 RPM but since I don't ride much in that range I wasn't too worried about it... but it wouldn't leave me alone.   I purchased a new NOS needle and that cleared up the remaining burble.   

As a recap, replacing the emulsion tube/needle jet, jet needle, and jet setter o-ring solved the blubbering problem.  

Interesting topic. I had my MX360 out at El Mirage OHV in So Cal a few weeks ago and got down to the dry lake bed. I got it going in 5th wide open and it started to sputter and bog until I let off the gas a little. I never ride it that fast. Heck I think i have only got it in 5th 3-4 times in 25 years...