It's either too rich mixture and or injecting/premixed too much oil if it's really bad or you're just expecting too much if you're leaving it for ages, it is after all a stroker, they don't like steady revs at any level.
I don't leave mine sat there ticking over but it would die inside 5 mins if I did..
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Please do not PM me technical questions, if you can't find it on the Forum start a thread