I was the proud owner of a 2014 JTG 280cc, best looking bike I've had I think.
Agree with the post above and advise from Paul Arnott - Hell team. To add, your spacer engine side of the reed block will also make this flow WORSE, emphasising the already worser flow rates of the larger kehin carb. Anyone that thinks these k carbs perform better down low must have had the dellorto performing not at it's best.
I"ve never been concerned with the fuel I ran in my last 15 or so trials bikes, until this JTG, certainly did not like 98 shell... fouled plug every 20mins, jetting seemed perfect till ''it went'. E100 United sorted this. E100 a slightly leaner fuel so had to go back to std or richer jets.
Glad you've got it sorted, often pays to ring the importers, they all want their brand of bikes running the best, not good marketing for any of their bikes to be running like ****... Selling different heads might be, temporarily. On heads, I had both std and super high comp on mine, squat difference to jetting.
Glad you got it sorted, mine certainly sounded sweet with std carby, didn't change the plug for 1 and half years once had it sorted.
happy days