The symptoms described sound like the problem common to the EC 200 / 250 / 300. It's covered in the 2 stroke engine and jetting sections in this forum. You have to start with the engine, get the squish to something like 1.2mm either with thinner base gasket or skim the head. Then give it a new set of piston rings. After checking the piston, reeds, con rod bearings, mains and seals, powervalve tension et c. are ok (it is an 11 year old dirt bike), then start on experimenting with jetting. It's almost certainly not as simple as changing one thing.
My EC200 gets nearly 80 miles to a tankfull greenlaning on 13 / 48 gearing and runs fine (has Gnarly pipe, iirc)
It's a bit of work to do all this, but worth it. If you don't have the inclination to do this work on the bike, take it to someone reputable, they will almost certainly be familiar with the squish issue and a professional mechanic should get the work done fast.
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'00 Montesa Cota 315R
'13 Husaberg TE 300
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