low end hesitation

Hi all. New to the site, so please excuse poor placement or phrasing of questions.

Having some issues with my ZZR 600, 2003 model. Completely standard. To my knowledge has had a minor drop prior to my buying it at 3500 km and repaired but for a few scratches on the exhaust. Ran perfectly for the first 8 months. Then, soon after filling up with fuel, the bike began to hesitate in the 1500 to 5500 rpm range. Very sudden change. Runs perfectly above 5500 rpm. For what it may be worth, I also noticed that the rpm:speed ratio had changed - in 6th gear the rpm would be almost spot on half of the speed, eg at 100kph the rpm would be 5000, 120 kpm 6000rpm. Now rpm is higher for the same speed.

Initially tried some fuel system cleaner with the fuel with some effect, thinking I had picked up some bad fuel. Now that makes no difference at all. Had the valve clearances checked, all good there. Had the carbs synced, and despite some significant adjustments, the bike does not run any better. The fuel consumption has increased and exhaust smells really rich.

To my knowledge the carbs have never been disassembled, so shouldn't have lost any parts or diaphragm leaks. Any ideas? Anything electrical that might be going on? New plugs made no difference.

Thanks.

Doesnt sound likely, but fuel filter?? Sounds like a fueling issue to me more than an electrical one seeing as you have no ECU.

just out of curiosity...did you drain the carbs?

i'm actually wondering if it's a weird battery - ignitor thing...not getting enough juice 'till the generator's spinnin' hard

Hey, thanks for the suggestions.

Unfortunately, have already tried draining the float chambers and changed the fuel filter. Missed out those bits.

I'm sure I'll get there in the end.

Thanks.

What about blocked jets, worn needles?