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Glad you solved your issue! The only time I use a pressure washer is at the races, and I aim low with the high pressure. Mainly use my garden hose to clean bike….and never spray water at switches on handlebars…or connectors for TBI…. I have never had an issue with both of my 300SX”s….stock these bikes are fun, with a tune and gearing for your riding….these bikes are unbelievable….I go from high rpm Mx track to tight woods riding with just a pipe swap and selecting the a different map…

Could be the reflash, contact motox and see if they can restore it back to stock...as a test. The mapping might be too aggressive for your conditions, most of these bikes run different from each other right out of the crate. A reflash doesn't always work for every bike across the board, yes it ran good for a short time but then it caused adverse affects....hard to know for sure. starting back from stock eliminates the reflash, then you'll know. You might be better off to go with a tsp erm, you can control mapping, build your own maps, download...

Throwing any codes? Try another ECU…check wiring for pinch and/ or connections…I have had 2 300SX”s…a total of 200hrs between the 2 of them…. Never had any issues….I replace in line fuel filter every 20hrs, OEM spark plug and powervalve reset every 25-30hrs….top ends at 100hrs…have never had reed block apart on either machine….

Sounds like the perfect time for a total overhaul crank up before it goes boom.

Sorry, late to the party but agree with your findings. Probably fuel injector. I had similar issues, would back off throttle or at idle and bike would die but run ok under load. I cleaned it and it would get better for a little bit but not long. A new injector was the fix. Fuel pump is almost the exact opposite, will idle on a stand and run ok under light throttle but dies under a strong load. You can rev it on the stand as well and since there isn’t resistance on the drivetrain it doesn’t kill the motor...