Kymco 450i MXU backfire when deaccelerating / weak low end power

[Kamil]

Just bought myself a 2021 Kymco 450i MXU and it backfires slightly (baby backfire) via muffler only on sudden deacceleration / letting go off throutle.  This one is fuel injected so I have no idea what is wrong. Maybe thats normal couse I never drove one, but it also seems too weak for 450cc power output. I was expecting that it will grind the grass off dead stop, but it has very weak low torque. I wish I could know how to rev up idle rpms so it would had more kick, but I feel that the CVT is already catching the sheave, so not realy an option. Power seems normal once on higher rpm. Anyone had that issue? Maybe I am just used to my china 150cc GY6 but that little thing would mow the lawn from dead stop. This atv has 27miles on it. Belt is brand new. I will start with investigating gas quality, but the backfire seems to do it more when it gets hot. On cold start it seems to be little better, but not great. Also deley in throtle response is longer then I am used to. Please help. If that "power" is normal for a 450 I will have to mod it somewhat couse there is no way I can slip the rear tires even on gravel. Its very weak, no way to fishtail. And yes, I did verify - its in 2wd mode. Sugestions?

[Ruffus]

@Kamil welcome here!
-I'm not really into 450s, but I would  check airintake (airways inclusive filter) for backfire when hot could mean lean mixture.
-then sparkplug, renew it
-accelerator cable position (too much leeway)
-fuel and vacuum lines all tight

With this injection models ECUs you cannot adjust anything without an KYMCO digitool.
Another possibility would be a HONDA or similar dealer close to you with an analyzer.

[Kamil]

Thats actuly great advice, and I have not checked for vacuum leaks. Time to review the service manual. It would be very kind if anyone would let me know if their 350/400/450 or 500 is able to fishtail on gravel from dead stop. I am surprised that my Kamco has so little torque on the low end. I never had driven one so I have no clue, but I am dissapointed about the low end torque and delay in revup/ throttle response. This actuly conflicts with my research about the 450i mxu before purchasing. Like I said, my china "raptor" replica with GY6 150cc engine would cut grass from dead stop like there is no tomorrow. And thats on grass, I think it would dig a hole on gravel.

[Bratman2]

I have a 2014 Kymco MXU 450i I feel it is pretty awesome for 450. Way faster out the hole than our Yamaha Grizzly 450 when both were stock. This is a You Tube video of it accelerating from a stop to 39mph maybe. It does have exhuast and CVT mods in this video. But like I said it was way faster than the Yamaha stock.

Just copy this into your search bar.

[Bratman2]

Had one more thought? Did you buy this new? If not, maybe the throttle screw has been turned in some. Possibly due to a younger driver that the previous owner wanted to reduce the acceleration and top speed some. Right at the throttle thumb lever on the front side you can see the adjustment screw sticking out. Try a couple of turns out and see if that makes a difference. If not just screw it back to where it was. If that is it post back and I can measure how far mine is out. You could then set it the same. I have never touched mine so it should be factory setting.

[Kamil]

Solved, used high octane frash shell gas with ignitors cleaner liquid and octane stabilizer. Shes getting healthier. I still have some old gas left in there but its much better. Also, I had limit screw all the way in. Still shouldnt couse the "backfire" and random shot off when letting off thrutle but shes torque now.

[Bratman2]

Seems mine backfired some when new. Got better after a couple of hundred miles. Awesome to hear it is running better. I don't know where you are located but you may check around for Shell marine grade 89.  Some of our local Shell stations have this and it is nonethanol. See if you can find one and you will have found very good fuel for it!