I know it is recommended to use "resistor" plugs but I never see anyone talking about "heat range" and I think this would be important when trying different fuel mixtures. Can any of you motor heads comment on this and also what would be the heat range of a RDJ7Y plug. Would it be considered a hot or cold plug? Thanks for your consideration.

It's a medium hot plug, the 6 is medium, and in some engines is slightly too cold.
See http://www.prme.nl/forum/viewtopic.php?t=10 if you want to study plug heat range and mixture settings.

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.

I would not tamper with the carburettor or muffler system to start with. This engine is a matched system. Once you change that, it might be better to use a standard WA or WT carb so you can adjust idle and High needles. Or you might contact Walbro for spare fuel jets of different size. These jets are precision drilled. Once you start drilling them out yourself, you know where you came from, but loose track of where you go.

Stick with the heat range of the plug that is fitted. If it is a "7", that should be OK.

Okay Pe, I ust poped upon the stock carb (WYL) and it has a tappered spring just below the jet. The WYJ does not have this spring. Can you tell me why? While I was at it I swapped the jets out so now I have the bigger jet in the bigger carb. Thanks for all your advise. I had read that someone tried a WT carb on one of these 4-strokes and didn't like the way it worked. Can't control the butterfly to open and close smooth enough.

NGK bigger number colder plug, Champ is reversed

RDJx is Champion (Y is long reach electrodes)
I would love the day that plug numbering systems be standardized.

Ya mean like servo arms???

No, More like the Walbro throttle stop screws and fuel jet needles [:@]