2018 RX3 misfire

[Juanca]

Hi all

My bike keeps doing misfires from the exhaust, especially when going like 50 mph and then release throttle, then gives a couple of explosions.
I was using regular gas, but I have done at least 5 fill ups with high octane, but keeps doing it.

have you experience this?

[JerryHawk250]

You may have an exhaust leak that allowing air to enter the exhaust when letting up on the throttle. This will cause back firing.

[calvarez]

Is your bike carb or FI? Not sure what made it to South America.

[Juanca]

Jerry, any idea how to find this leak?
Calvarez, mine is EFI

[sqwert]

Leakey exhaust does not usually cause backfires, unless you have an ignition source and a too-rich mixture on the intake side.

Backfires on deceleration are typical of a lean air/fuel mixture on the intake side--too much air, or not enough fuel. Intake vacuum leaks, too small jet(s) or too low needle if carbed, weak fuel pressure or dirty injectors if EFI, and about 300 other things can cause such troubles, but the things listed cover about 99.99% of mixture problems in practice. Note that low fuel pressure, for instance, can be caused by bad pump, weak electrics, kinked fuel line, dirty fuel filter, plugged tank vent, ..., just for starters.

Has the bike always done this or has it just started? With a carb, first thing I'd do is look at the spark plug. Plenty on the internet about reading spark plugs. If the plug reads lean I'd spray a little starter fluid in tiny squirts here and there about the intake and carb. If the engine speeds up, you've found your leak, unless you sprayed in the intake port of the air filter housing. 2nd thing I'd do is pull the fuel line off the carb, stick the line in a clean, clear glass container, and turn on the full. Got fuel? Is it clean? If so, next step would be to check the float level, but since I can rebuild a carb with my eyes closed, I just do a rebuild with a thorough cleaning. I had a brand new Mikuni carb that had a bit of metal flashing about blocking a fuel passage in the body. Picked the flashing off with a fingernail, reassembled the carb, and it worked wonderfully as Mikunis are prone to do.

If the backfire continues after reassembly, then, and only then, would I play with the jets and needle position. I've put many carbs back to stock after fools didn't fix the problem before making jetting and needle changes. Very common stupid thing to do. Especially on older bikes and those sold where anti-pollution measures don't require a lean mixture. Keep in mind that Yamaha uses or used 4 different main jets on the TW200s with the second generation carbs, depending on which market a bike was headed towards. Even California had a different main than the other 49 states. Therefore, jetting isn't really that critical for most people.

[NzBrakelathes]

Reset ecu and I think you’ll find the issue to be like mine wuth the 300 kit and stepper acting too slow
Try that, turn on and off bike 5 time real fast (listen for pump prime) leave off for 15 seconds and ecu wipes all learnt memory
Mine does a bit the same but reduced once I fitted a new stepper
Please don’t say it’s a new stepper or anything till you have done this and gave the ecu time to self learn
AJP PR5 a company changed stepper behavior due to it being not great and no you don’t want to spend 124€ or so to send them your ecu to program
I think this is a minor issue and relates to stepper closing too slow allowing too much air in
I had the same issue and it calmed down after new stepper and ecu reset

[NzBrakelathes]

You stated missfire which is incorrect
It’s a backfire

[NzBrakelathes]

Also you can unplug stepper at idle so it can’t move doesn’t operate
What’s it like then? After leaving it unplugged for testing purposes