Hai,
When the bike is not under load (reving), there is much less (actually no) demand on the engine. Therefor the fuel/air-mixture is different (very lean) and ignite's easy. To keep it in high rev's (not wise) you don't need to open the throtle all the way.. it will just go there.
When you put the bike "under load" there is a real demand for power, mixture changes, become's richer and more difficult to ignite so every "mis-fire" is noticable. It just needs a better spark to ignite the fuel (on time).
Even if the spark looks good (when you take out the spark-plug) and the coils measure fine, when things get rough.. it's a diferent storry.
I hope this explains it a bit.
Maarten
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