joe wiseguy intake

[xschris]

Plugs can still spark but miss under a load. If the bike has done a lot of idle time put in a fresh set or soda blast the ones you have to remove ALL the deposits on them.

 

[sproutroot]

Or the spark could be significantly underpowered. Check resistances for the coils. It could be a fault in the plug caps, the wires (although you've said those have been replaced), or the coils.
Also, if the wire-to-plug-cap connection isn't right it can result in extremely varied performance.