Very noticeable flat-spot/sputtering in power-band between 4-4.5krpm

[Polishblood]

Hello,

When I bought my 2008 xb9sx (6+ months ago), I noticed a flat-spot in power-band around 4-4.5krpm. It's been getting worse and more noticeable ever since.

It happens when I am using normal throttle operation, 50% roll on, to WOT operation, 100% roll on. (With the WOT roll on, the flat-spot can occur for a duration of up to 2 full seconds. Like literally my friends will be accelerating and my bike feels like it is just coasting with no power). As soon as the flat-spot/sputter is gone, by bike instantly kicks back into life with tons of torque. So top end power is very good. Literally the problem occurs only between 4-4.5krpm.

(Under 3 conditions is the symptom less noticeable/avoidable) #1 To avoid the flat-spot or sputter, I have be VERY smooth on the throttle, roll on super slowly, and it won't happen. #2 If the fuel tank is topped off, especially with fresh gas, it is less noticeable. #3 When rolling on (normal to WOT) in first gear, sometimes the symptom is non-existent. The bike seems to power through it due to gearing.

Bike has 12k miles. No check engine lights or codes (I think). Air-Filter was cleaned recently. Did a TPS reset, might do another. Spark plugs are new and gapped (I am using the colder ones). Stock exhaust. Everything is pretty much stock but air filter which is K&N.

Do our xb9 bulls have two sets of injectors? Perhaps a second set of injectors that come on around 4-4.5krpm? Perhaps they are dirty? Bad fuel pressure? Although as I mentioned before, top end power is very good. So is low end power. Maybe bad AMPS to fuel pump?

Would like to heard your thoughts. Thank you!