Thanks Pe, So when would you suggest going to a colder or hotter plug? I am converting a 40cc, 4-stroke and using a carb that has no adj.needles. How do these carbs work without being able to make adjustments or should I say how do these engines get the proper fuel mixture from a carb that cannot be adjusted? I am using a Walbro WYJ-220 that has a venturi of 10.5 (1.5 bigger than the stock carb.) and a .370mm jet. This jet is smaller than the one in the stock carb. so I will drill it out to .380 for starters or maybe .390. The WYJ will be getting more air than the stock carb so doesn't it make sense to be able to get more fuel. To burn it wouldn't I need a hotter plug? Thanks for your help.