Please carefully pull the air filter and look into your intake tube that goes to the carb with a light the tube should look like brand new shiny black rubber, then take a white paper towel and wipe the inside of the tube see if you get in dust.
That sand is a killer even with a fresh clean well oiled filter in place its also why we recommend you installing pre filters on your dual intake mod, you will love the dual intake mod BTW.
Hopefully it does not look like this.

Please consider removing the top end of your
Engine and inspecting the piston sometime soon, 350's are notorious for breaking the piston skirts then the broken skirt getting wedges between the crank shaft and crank cases and punching a hole in the bottom of the Engine, it pretty much takes a 200.00 replace the piston job and turns it into a find another Engine or have the cases welded up job, replacing all the bearings, seals, gaskets pretty much 800-1000 dollar job
Cool thing about a 2 stroke Engine is it will run like brand new right up until seconds before it it locks up and needs hundreds of dollars in repairs its very rare you will get any indications its about to poofkaboom this is why Honda says in the manual to take it apart and inspect it every 30 hrs of run time.
Not trying to scare you or make you paranoid just share important info with you before not after
Prefilter mod on other machines might be useful for ideas
viewtopic.php?f=1&t=8929More filter info
viewtopic.php?f=9&t=5491