I put a post up ages about a problem with my bike when it get to about 7k-7.5k rpm's, once it get passed 8k - 8.5k, its fine.
I always suspected it to be a carbie issue, but could never get to the bottom of it.
It doesn’t happen all the time, but most. The bike will rev OK with the clutch in but while under any load, the bike would cough and splutter when the rpm's get into that range (7-8.5)

I have always used premium 98, but the other day, I used 95 plus Ethanol (95+E10), the bike now run heaps better, and even though the problem is still there, now it happens only about half the time.

Any thoughts on what in the carbies would cause such a phenomena

 

How do the plugs look? Ya ever had the carbs cleaned and sync'd? How about the pilot jets or the main jets? Are the floats set right?

 

hmmm.... actually our bikes should be running on regular gas. The higher octane does not make more power. In fact because of the slower burn rate, it might be the crux of your problem. the slower burn rate means that it doesn't expand in the cumbustion chamber unless it's under more pressure, i.e. higher compression and can hold more oxygen.
Try running regular gas no ethenol for a while and see if it doesn't inprove. I would bet it does.

 

I change the plug about 3 months ago, I took the bike to the shop and got the Muffler repacked, cabies cleaned and they also replace the plugs(wasn’t needed)
after that, the bike sounded a lot better, had more low end grunt, but the problem was no better or worst.
When I pull the plugs, they look good and all look about the same to each other.

Last year, I had a new cam/tensioner put in and carbies balanced plus values clearances done
Also both times, I had the air filter cleaned, I have also check this myself

I have a 4-1 exhaust with a K&N air filter, I have no idea on the jets

 

I'm going to take her out today and fill her up, so I will put some 92 in it, or maybe some 92-E10 and see how it goes

 

MarkR, it sounds to me that you have a leak where the header pipe meets the head.
You will have to retighten the head pipes more evenly and/or replace the seals.

 

Wouldn’t that cause a ticking noise and maybe some discolouration of the affected area

 

Yes, sometimes you do hear the ticking but I haven't seen discoloration before.
If it is #2or3 you may not hear it over the engine noise, especially at higher RPM.

The leaks can be tricky to find.

What they do is to allow cold air into the pipe and it will expand when the next shot of hot gas comes along. The cold air expands and shuts the pipe down. The sign is an upper midrange to high breakdown.

If it runs at full throttle and at idle, my guess is that the jets and fuel delivery are fine but the pipe is giving trouble. That's my guess.

 

I dunno ... sounds more like a fueling issue. Especially since it has good low end grunt but performs badly under a load at higher rpms. The very best way to diagnose is to run it on a dyno with a sniffer and see what your a/f ratio looks like. I would almost bet money that you're running rich at the higher rpms. The bike can't burn all the gas you're throwing at it until it goes over that rpm range. Running better with 10% ethanol supports this theory -- but hey, it's only a guess ... you gotta strap it to a dyno to know for sure.

 

Could this also be caused by a leak in my inlet/carbie ducts, or my air box and/or air-box/carbie ducts
‘ducts’ properly isn’t the right word