Can't keep idle and poor mpg

I have a puch freespirif wit a 15mm bing, 50cc kstar and hiflo air filter. I've been trying to figure out y I cAn keep an idle when I first start it but can't after I run it. I've just put it together that I also get extremely poor gas milage. I think they are related. Any thoughts on wat it could be.? I have no airleaks and plug chop came up good.also timingis decent. All help will be apprciated

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