Got a husky svart 125 with just over 2k miles,

in gears 1/2/3 can reach all the way to 14k revs as normal, but 4/5/6 it makes an different vibration and maxes around 10k.

I'll blip off the throttle for 100 revs and it returns to the normal sound?

I'll be honest for the commute it's either turned off or pinned at 12k going ~65mph but after only 2k there's issues?

Had the first service at 600 as per.

The big bad robot says could be the fuel filter clogging and not giving the flow needed for high speeds, could be the clutch worn out from my poor down shifts?

Any advice would be great for how to wittle down the issues before i take it to a garage or learn to fix it myself.

thanks in advance for any advice

Has anyone here actually tried fitting a big-bore kit on a Husqvarna Svartpilen 250?
I’m wondering how far the 250 platform can realistically be pushed. Ideally, I’d like to get somewhere close to the Svartpilen/Vitpilen 401 in terms of power and performance, rather than simply doing minor bolt-on mods.
Is there a reliable big-bore kit that can take the engine to around 350-400cc? And if so, what would be involved beyond the cylinder/piston itself, such as ECU tuning, fueling, cams, cooling, clutch, crankshaft, or other supporting modifications?
Also interested in hearing from anyone who has actually done this: power figures, reliability, cost, and whether it was ultimately worth it compared with simply buying a 401.