Hesitation around 5k
 

Hoping that the collective wisdom can help.

2000 CBR600F starts great, idles smooth, revs clean when stationary.

Take it for a run and open her up - there is a hesitation / flat spot at around 5k to 7k rpm before it clears its throat and shoots off up the rev counter.

Checked the battery and the charging system - all good.

Carb balance is spot on using a morgan carbtune balancer.

Had a good look at all the jets.
Low speed jets are #40 all across like they should be and are all spotlessly clean.
Pilot screw is at 3 turns out as per the book setting.

Mains are #132 outer and #135 inner
The book says that the UK model should be on #130 inner and #132 outer

Bike is on stock filter and stock exhaust

Would the oversize jets cause the hesitation at around 5 to 7k due to overfueling ?

Any other suggestions welcome

Hi Dean0,

Sorry I can't help on this as you are way more advanced technically than me
;)

Was the bike fully warmed up when you where having the hesitation? Did you clean the carbs and now your having the problem?

Yes bike was fully warm ( fans cut in and out as expected ) - starts well, idle is smooth and even, runs great up to 5k

Carbs were cleaned due to rough idle and stalling. ( idle jets were almost blocked )

Pickup is smooth from idle to 5/5.5k - then its like hitting a wall between 5.5k and 7k - an after 7k it takes off like a banshee to the redline.

EDIT - just been out on the bike again - all warmed up - no air filter in air box.
Brick wall gone but still hesitating between 5.5k and 7k
If I back off and re-apply throttle at the hesitation area it goes through it.
However now it can be taken in lower gears with part throttle through the 5.5k to 7k band and on up to red line.

I am now convinced it is too rich at WOT and the mains need to come down a size.

Air filter clean? You could set the pilot out a bit more (say half a turn) that should help. Going back to stock mains wont to do you wrong either.

Air filter brand new

How will pilot out a bit more help if it is running rich ? - I would expect to turn it in for leaner mixture.

Am I missing something ?

Pilots are responsible for idling, start-up and the low-mid range of RPMs, mains control the fuel delivery at mid-high RPMs. The reason why I said to adjust your pilots is to smooth out the fuel delivery curve, yes you bike might be running a bit rich, the flat spot at 5k could be where the bike is transitioning from the pilots to the mains. Just a thought mind you. You could pull your plugs, they will tell you if your running rich or lean.

Edit: Just thought about it and the fuel needle also in-charge of the mid range so you can play with that too.

OK - no probs - cheers.

I was basing my assumptions on the old school CV carb theory of :
Idle & pilot jet cover 0 to 1/4 throttle
Needle shape & height cover 1/4 to 3/4 throttle
main jet covers 3/4 to full throttle

Because the bike only seems to have issues at WOT i assumed it was all main jet based.
The needle would apear to be correct based on the fact I can ease it through from 2 to 10k without hesitation.
The idle jet & pilot screw seem ok cos it starts & idles perfectly

This on the forum some where but this might help

http://www.factorypro.com/tech_tunin...m_engines.html

Thanks - some good reading there