Coils rarely go bad, but when they do it's pretty crappy performance. Are there any other modifications to the bike?
If not, I'd look at a couple of things. Since you've changed the plugs already, that's one thing you can rule out.
Check your air filter and snorkel to make sure they are clean and free of obstruction. Improper air fuel mixture will cause the bike to run rich.
Ensure that the gas cap is venting properly. A cap that does not vent properly can cause improper fuel flow, especially when the engine and ambient temps are high. Carb bowls don't fill up properly therefore you get some fuel starvation, even to the point of a shutdown in some cases.
Make sure the fuel level in the tank is adequate. Low fuel levels also will cause fuel to flow slower as the system is gravity feed.
There is a problem with some IC Ignitors (CDI)
, although there is no recall or service bulletin on it. The problem seems to mainly occur when the temps are high outside. This may cause the rpms to read higher than they should but the speed doesn't necessarily change.
If you've got unburnt fuel coming out of the exhaust, you may have a compression issue or a timing problem.
You can also use the Troubleshooting Guide
to help narrow down any other problems you may happen to notice.