In what instance would you want to go hotter or cooler with your spark plugs? In all the fiddling with my bike this week, I put new plugs in my bike. The old ones still looked fine, but I had a new set from a parts lot I bought and put 'em in. They were NGK's for the GS, but were a different heat range than what was in it. They were gapped correctly, but the bike had problems starting with the new plugs. I'm not sure what the heat range reflects, why you'd want to change it, and how you know when to change it. Thanks.
Check out this page from the NGK web site:
Tech Info - Spark Plugs Overview (page 2) (http://www.ngksparkplugs.com/techinfo/spark_plugs/overviewp2.asp?nav=31000&country=US)
Page 3 has a additional info about tip "temperature".
On NGK plugs, the higher the number, the colder the plug, therefor the higher the speed range in which it works best. Remember it this way: High number = high speed. Running too hot a plug (too low a number) at high RPMs can damage the engine. Running too cold a plug (too high a number) may make the engine difficult to start & it may not run too well at low RPMs. Sounds like maybe you ended up with a colder set of plugs. I think the OEM heat range is an "8" so one heat range colder would be a "9"