A gas engine is not at all like a glow engine, and different rules apply. Once you [link=http://www.prme.nl/forum/viewtopic.php?t=10]have the carb set about right[/link], and for safety that is the rich side, you go by plug colour readings alone, not rpm.
The plugs tell the story concerning plug heat range, fuel jetting and ignition timing, depending on what part of the plug you examine.
Want to know more? Google for plug reading, and you'll find a wealth of information.