Sneezy's beer-carton 2fiddy.
So what are the valve clearances on the Atomik 250 ????at a guess, i would say valve clearances, and possibly running a bit lean if it's popping and backfiring.. if you want, bring it round sometime, we'll have a look at it and see if we can get the valve clearances set properly.
or if your stuck for time and trailer, follow this link, it will show you step by step how to check and adjust the valve clearances on the chinese upright engines. (that's what you have. )
http://www.miniriders.com.au/mini-tutorials-how-s/20425-good-guide-adjusting-valves.html
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