Brand new 250f is oddly slow?

I'm not 100% sure. But the Yamaha doesn't have a preset "aggressive map". You have been on the tuner and changed the timing and fuel? To what some1 has posted online.

Maybe you fucked that up or some1 posted a dodgey map. Reset it to all 0 on the app. And go ride it. 

That would be the 1st thing I do.

Back firing would be a sign of bad fuel/ air mixture.

I hate to be THAT guy here, but you're saying local average B rider....and hitting a jump wide open in 4th? You understand guys are hitting jumps like Larocco's Leap probably at the top of 3rd or maybe clicking 4th on the ramp.

You say you're experienced, and I believe you, but if the whole wide open in 4th thing is true, then yeah I'd say something is wrong with that bike. 

I read something about that in MXA last month...I think both maps are the same from the factory....one of MXAs complaints.    

Take the shop rag out of the air box, or you’re just not as fast as you think you are 🤷‍♂️

Don’t quote me on Yamaha specifically, but engines are typically dyno’ed at the factory and have to be within a certain parameter before they can move on. With that said, my buddy and I both bought ktm 450’s at the same time and his hit like a bear off the bottom and signed off early and mine was smooth off the bottom and revved out far so there are differences in new bikes 

Take it to the dealer and tell them your KX250 easily clears the jump and the YZ won't make it at all. I think they do get dyno'ed at the factory so it should have pulled power there but something may have gone wrong in ECU, injector (could be anywhere in the fuel system), coil, maybe crank position sensor, etc.

I bought into all the yz250f hype and was extremely disappointed, it may be the way you ride. Yamahas fall off hard up top. I like to ring bikes out and ride them like 2 strokes. I rode an Austrian 250 and it felt a ton faster, and after looking at the Dyno charts and seeing it puts out 2rwhp more and over 1ftlb of torque aswell it made sense. Try one of those maybe =\ .The yz250f best motor nonsense is just something people say

If you’re actually on the limiter in 4th, then something is wrong with that bike. I’ve hit some big jumps in my time, and while I’ve been wide open in 4th, never been on the limiter. 

Have a friend that is faster than you ride it and see what they think.

Clutch slipping?

Long shot here but have you checked for a banana in the tailpipe?

I was going to ask the same question…the backfire is weird though…

Let the dude jumping the jump (or some other fast guy) ride it and see what he says about it….

If it were my bike, I’d check the obvious (clutch slipping, return map setting to default etc) and pay to have it dyno’d. You’re in So Cal so there are plenty of reputable tuners who have dyno history of your bike. It would bug the crap out of me if the bike was a turd out of the box. I do believe you that something is off. Just my two cents. 

Have you tried 5th gear?

Dyno it, or you will constantly question it forever.  The bike is now in your head.  

I'm 190 pounds and the top of 3rd on my CRF gets me over mostly anything I want to jump. 

What fuel are you using?  My buddies YZ250F backfired/popped a lot until he switched from VP to Sunoco 50/50 mix with pump.

Didn’t I just read how new Yamaha’s come with the exact same map in each switch position.  Until you update it, both maps are the exact same

Could be gearing too short ? 

I've had a bike jump a tooth on the cam timing a couple times. I run manual tensioners now. 

Blown out clutch makes sense, but not given the age of the bike and the decel popping. 

“and I'm literally riding the throttle stop screaming all the way up the face “

I’m Not sure if he is saying bogging in a higher gear going up the hill but throttle is WFO or if he is actually at higher revs.

Found it in the online version of March 2023 MXA....

I'll be the a-hole again here, something isn't adding up man.

If there was something THAT dramatically wrong with that bike, where you can ride your other bike back-to-back and still be iffy enough to be "wondering what's wrong" there's some misinformation here. 

In my area local B riders are legit dudes, not guys talking about hitting jumps wide open in 4th gear.

I'm not trying to be a jerk here, I'm just saying if you are looking for legitimate help, you're probably not going to get it with this explanation of how things are going down.

I've jumped the Fly 150 (it's gone, sadly) on a practice day....on a 250F (not mine)....in 3rd gear. If you're used to hitting some jump wide open in 3rd and you aren't making it, I find it hard to buy the whole "so I just hit it wide in 4th"

Jumps that big ruin lives man lol. In my experience it's usually not a litmus test for how a bike is running. But whatever I'm just an old washed up guy.