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57
Joined
11/14/2023
Location
Tollesboro, KY, USA
I picked up a 2019 250xc with only 7hr on it. Has this bad pipe bang/decel surge. I put a keihin 38 and rad valves and went up and down on jetting. It’s very aggrevating. I don’t remember my early 2000 250’s having this. My 2020 and 2023 300xc’s didnt have it. Currently I’m on a 2024 yz 250 and it just rips and purrs. Its compression is 225psi, this 250xc is about 155-170. I plan on putting a 300 kit on it but want to understand this phenomenon and make sure it can be tuned out. Ive scoured and searched…rich, lean, low compression/squish, timing....no consistent cause. Maybe someone here has the magic cure.
I have this issue when I ran 93 pump gas. Ethanol free 90 and the problem went away. You can also mix it 50/50 with race gas.
I had this a little when my 2023 MC250 was a 250 and it could be tuned out with jetting for the most part. You have to remember, the 300 kit for that bike is quite a bit different from the 250 stuff. It changes from a domed piston to a flat piston and matching head shape. The 300 cylinder also has a compression release bleed hole from the combustion chamber to the exhaust port. My bike has no pipe bang as a 300. I'm running Avgas.
9 times out of 10 your needle is lean.
So far new carb and new reeds, up and down with jetting. No change in pipe bang. My intention when I bought this was to put a 300 kit on it, but would hate to deal with this after a $1200 300 kit, cause this engine sucks. No excitement, terrible sound/tone, and this crappy pipe bang/decel pop surge. I’ve had 2 t’s in the past with this to some level….but after a 2020 and 2023 300xc with an incredible power band motor and now a yz250 rocket, this thing is a dud. I am leaning towards raising compression with base gaskets if deck height allows.
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Why would this 2019 be any different than your 2020 bike was? Same bike, right? Sounds like something else is going on or not right .
No, the 2020 300xc was tpi, first year for it. I think ktm started putting the “b” team on production 2t’s in 2018ish and the “a” team was on the new fuel injected motor. 2017-2019 carbed ktms dominate the jetting/pipe bang internet search results. The xcw was fuel injected for 2019 fyi. The sx didn’t go FI until 2023.
And for the record, the tpi was good, decently great running bike with no jetting issues. The 2023 300 tbi was my favorite dirtbike I’ve ever owned. The motor was incredible, more than incredible. It was a rocket, or it was a moped. Just depended on what you wanted at the time. Unfortunately a bog I could not get rid of has made me swear off FI 2t’s. But they are awesome.
From the peanut gallery... 😁
What exactly is pipe bang? And is it a result of less than optimum jetting that somehow manifests itself on some engines more than others?
I had this same issue on a 2019 300XC, you already have the "good" carb with the Keihin, I used JD jetting setup. The best thing I did was replace the head with one from RK-Tek, made the bike awesome. As good as my 2025 300XC with TBI, I have ridden them back-to-back. I assume this would help the 250 as well but personally only experience is with 300. Also strange to me was the bike seemed to run best with 93 pump gas (with ethanol) compared to my 50/50 mix of race fuel with non-ethanol.
From everything I’ve gathered is excess unburnt fuel/gasses in the pipe. The pipes actually feed fuel back into the chamber and if enough is there it will “ring ding” with a slightly elevated rpm til it’s burnt up. Here in this vid you hear me rev it, then as it goes to idle it almost sounds like I barely open throttle up then it will return to normal idle.
https://youtube.com/shorts/uPtuquU68Fs?si=1x0MVFS-obYwb3Vc
So your saying the head, which I assume tighten squish up and raised compression took care of the pipe bang/decel surge? Cause that’s where I’m leaning.
yep, at first I went the cheaper route and got the 300SXS head from KTM and it was an improvement, but didn't totally solve the pipe-bang 100%. The RK-Tek head fixed the issue, one of the best mods I have done to the bike
Thanks. Higher compression and better dome burn better not leaving combustible exhaust left over.
Got it, thanks. Though others stated the needle is a little lean as being the cause. But it would seem like a rich mixture would cause this? Please don't misunderstand, I'm not trying to debate. I'm trying to get a better understanding of what this is and how it works and how to get around it. (I love these tech nerd discussions.) Is it possible that simply some cylinders are more prone to it due to the way the ports are laid out or whatever?
yeah, like these guys said, it seems to be excess combustion in the pipe, hence the bang. It gives you the sense almost like a hanging idle where you let off the gas for a corner and the bike kind of still diesels or chugs or "surges" as boolzi said. Could be a combo of things causing it... jetting, head shape, porting, pipe shape, etc. Also possibly the PV not returning to closed quick enough? If you've ever had a PV stick open, you know it causes lots of nasty pipe bang noise. Like I said, on my 250 I got it pretty much eliminated with jetting, but an improved head shape seems like a more direct remedy.
It's not un-burnt fuel. Un-burnt fuel causes a backfire. It's lack of fuel. It's not head or pipe or porting design. If it was they all would do it. I can tell when you twist it your needle is lean even more so than when you let off. It will like be more pronounce when ridden.
Move you clip and come back and tell me i am wrong. lol.
Now you may have to drop the pilot 1 but we won't know until you do.
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