There's no rev limit on this engines...but of course...if it's full throttle all the time, that will reduce the life span of the engine...but that's what they're made for anyways...plus...it's fairly cheap to rebuild those engines...
On my first engine I ran almost a gallon of fuel...but that was because the air filter fell off and it suck tons of dirt...So I rebuild it install a hose clamp in the filter, and I have ran like 2 gal of fuel so far...mostly full throttle though...
And regarding on how to measure the temp...you're doing it right...it's right in the plug
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