Hopefully somebody can help:

'99 CBR 600 F4- no modifications, all stock

1st-3rd Gear: full power throughout the entire rev range, no problems

4th gear: revs to 9k rpm just fine, then power is jumpy/bike jolts forward for half a second then lets off... this repeats until it finally works through to ~9.8k, and then it is extremely slow to rev up to ~11k, at which point it ceases to rev higher, additionally, the speed increase from 9-11k is very slow, and I cannot get the bike up past ~120 mph

5th and 6th Gear: will not rev past 8k rpm. Even when shifted into 5th or 6th at 120 mph, the bike will not deliver any power until at 8k rpm, at which point it revs up to 8.5k and then fades back down to 8k, also, my speed decreases from 120mph to ~60mph when 5&6 stabalize at 8k rpm

Bike has had this problem since I bought it 2 years ago. Carbs were cleaned by a mechanic immediately before I bought it. I also replaced the fuel pump 1 year ago, tested the flow rate and all was good. Bike has ~11k miles on it and is very well maintained (oil changes, chain lube, etc.)

From what I've read, it could be a partially clogged fuel filter or bad coils, but I really don't know.

Thanks in advance for the help!

 

So you never had anything messed with on the carbs, but the previous owner did? And its done this since day 1?

I'd be thinking carbs...

 

Correct, I've never had anything done to the carbs. Before I bought it the previous owner told me they had it taken into the shop and had the carbs cleaned, and when they tested it immediately after they said it helped a little, but definitely didn't solve the problem.

Also, when you say you're thinking carbs, do you mean they just need cleaned?

 

Yes. Considering the age of the bike. How does it idle? Are you sure the PO did not mess with the carbs? Are the EPA brass caps still on?

 

The PO told me that a their local motorcycle mechanic (licensed) cleaned the carbs 2 weeks before I bought it. I have no idea about the EPA brass caps. I should've mentioned that I haven't taken the bike apart a ton, as I have limited tools and resources to do such. I've raised the tank off the bike and replaced the fuel filter, but haven't personally done much more than that.

I tested the bike again today, and it seems as if each higher gear is worse off. when shifted into gear at 120mph, 4th could barely manage to get to 130mph, then let off to stabilize at ~90mph.... shifting into 5th at 120mph, the bike immediately ceases to rev past 7.5-8k rpm, and can only maintain 7omph....shifting into 6th at 120mph and the bike can only maintain 55-60 mph at ~6-6.5krpm....it's just really strange

 

Sorry, forgot to mention...it always starts up and idles perfectly, since the day I bought it

 

You changed spark plugs? Might be worth doing if not replaced, with Iridium ones if possible... cheaper than fiddling with carbs... tbh, if it's reving to 13k rpm in the other gears... deffo worth getting carbs nicely set though... I'm not 100% convinced it is carbs though, I'd have thought they're be out else where?

 

No, I haven't done anything with the spark plugs.... question though: how would changing the spark plugs be helpful in this situation? do you think they just aren't firing fast enough? Because if that were true it wouldn't rev past a certain rpm in any gear...correct?

 

Concerning your carbs, RPM and gear selection are more of a symptom. To determine what part of the carbs it could be, throttle position is what you want to pay attention to. RPM is a result of advancing the throttle. determine it it happens at 1/4,1/2, 3/4 or full. that will tell you what circuit in the carb may need attention.

 

When I shift into 4th, 5th, and 6th at those high speeds, I shift and fully open the throttle immediately... I haven't experimented with opening the throttle in a different manner, because it simply won't go higher than 8k rpm at full throttle... I will experiment a little bit more, but since it revs perfectly in gears 1-3, I doubt it is a throttle problem

Are you thinking it's an electrical problem?