Power loss in upper gears

[lostinflyz]

G'day fellers.

Anyway got me a NC30 race bike that ive been rebuilding for ages and currently its hitting a brick wall in 5th and 6th. To fully explain

-bikes got full snot in 1st and 2nd
-loses about 10% of power up through 3rd and 4th
-and cant accellerate down the straight in 5th or 6th.

Bikes been dynod at 60 at the rear wheel so its not a power or tuning thing, i dont think, could use a carb synch. but surely this wouldn't cause random high gear death. Any ideas or anyone had this sorta problem. Bike takes an age to get warmed up and running properly so im wondering if its a stuck carby slide or something similar.

Help.

[CMSMJ1]

Has the airbox been fiddled with or is it standard?

I'll stick my 20p on one of the rubber flaps being moved and the turbulent air at speed not wanting to play...long shot, but worth 20p

[lostinflyz]

ho hum you may be onto something there. good thinking. will try that out

[superlite]

Yep, as above my money's on airbox/induction problems.

Check to see whether the airbox is standard (chopping the front off seems fashionable?!), and that the airbox base front lip and rubber flap is in place. What sort of filter is it running?

[lostinflyz]

its got a custom filter. airbox was modded and i took the front flap thingy out, the right angle piece. Looking at my good race bike thats still in there so ill stuff that back in and give it a whirl. makes alot since on the airbox induction side.

You'd think with a massive fluid exam tomorrow i would have been thinking turbulence. what an idiot.

[Neosophist]

Haha yes you really should, Kolmogorov's (sp) theory and all that...

Good luck with the exam.. try and use your NC30 as an example in an answer!!

And if it all does go pear-shaped you may as well be creative!

[lostinflyz]

haha well the exam went well but the bike went like shit.

couldn't even pull outta 3rd. bastard.

have played with everything (even chopped my good race bike to bits)

i think im just gonna blow the POS up

[Sligeach]

i just had the EXACT same prob on my bike.
it was totally and utterly to do with the airbox and turbulence.

[zimm]

fuel starvation due to a blocked filter/split vacuum pipe ??

this would explain why it was ok on the dyno, and in the lower gears

[superlite]

It will be running lean.

What sized jets are you running? Foam filters will typically need 128-136 mains (depending on the type of foam: pipercross, hrc?). First thing you NEED to do is to find out what your carbs are running.

I would bet 1 aussie dollar (about 50 quid it seems...) that it's running way too lean up top. Dynos can't always replicate the airflow that the carbs will see at speed (higher gears).

Putting bigger jets in is easy, but then it typically highlights problems with transition from the needles to mains when you have airbox mods. Best way around it is to use a standard filter and airbox setup. You won't lose much hp but gain a whole lot of linear response.

Modifying the airbox/induction commonly leads to fuelling problems unless you have the whole hrc kit.