flat spot / jetting

[lee.pb]

Hi all,
had my new race bike out on the track on friday, only to end up doing 1 lap
bike starts fine and passed noise testing but when trying to open her up out of the pit lane, she would only rev to 8000rpm and then was flat. changed up to second only to find it still flat. must of had the worlds best siting lap in 1st gear.

tryied lowing the needles but no change, also raised the needles a washer at a time, which still had a flat spot at 7-8000rpm.

the carbs are fitted with 115 jets and j51b needles front and j51a needles rear, it is running a hrc airbox too.

repalced carbs with a second set off another bike (standard set up 110 jets) and runs ok still slight flat spot but once through she goes well to the red zone.

have a new set of carbs on there way but what would be the best jets / needles for the above mods also running half system and scopion can.

i'll leave the card behind the bar for peps who would like a drink as it is my first post

[CMSMJ1]

Welcome to the site.


You say HRC airbox? What exactly is this? NC30 run no airbox at all for the HRC setup - huge jets and no filter etc.

Can you describe your setup in a bit more detail?

Where are you based? UK? It sounds like it is too rich but with only 115 jets I would have thought it was lean!

What air filter have you got? Is it clean? Did the bike ever work or has this just started?

[lee.pb]

on further investigation it looks like the nc35 hrc air box top, the air filter has been removed.
the slides seemed to be alittle worn and a bit rough when slid up by hand.
the bike seemed to run fine apart from a small splutter at 7-8000 rpm with a standard set of carbs on but with the carbs that it cam fitted with would not rev at all past 7000 rpm in first and was worse in second gear.

bike spec with fault:
nc35 hrc airbox
no filter
115 jets
j51b needles front carbs
j51a needles rear carbs
half system
scorpion can
quietner gears removed.

[CMSMJ1]

The lack of filter is your problem. Stick one in or be prepared for ballache trying to get it jetted.

The sizes for use without filter are of the order of 135 to 150

[lee.pb]

when the bike ran ok with the standard carb set up i used the standard airbox and filter, beleive there may have been a couple of more mods but with out a strip down who knows. bike pulls well after small splutter at 8000 rpm then seems to get a second set of wind at around 10-11000 rpm. standard bike just seems to pull with a quite linear curve.

may have a word with rick oliver and get the hrc carb kit and jets to suit the no air filter situation.

[lee.pb]

cheers CMSMJ1 for the help, will have to try the new carbs when they arrive and keep you informed.

[CMSMJ1]

no worries Lee..

I have the same issues on my race bike actually..

Fitted some open carbs, running 147 jets and it is too rich and messy. bogging down and making nasty noises.

As my first test is Weds and race on Sunday I am fitting std carbs, 118/120 with a normal box and filter. It won't be fast, but it will work and for now, that is good enough!

[lee.pb]

one of my mates has the full hrc carb setup for open carbs / nc35 hrc airbox said he would send it to us next week when hes home will pm you what he sends if any help. bike been on dyno to find best settings.

wil also post on here.

[Cammo]

may have a word with rick oliver and get the hrc carb kit and jets to suit the no air filter situation.

The hrc carb kit will help, but you really need to rstrict the carb air jets or it will never get rid of that midrange hole! The nc30 hrc velocity stacks do this.

Jetting depends hugely on this air jet restriction. No restriction (as it currently is) and you'll need about 160 size jets. If you restrict them you'll need anywhere from 130-145 size.

There's not much point running an airbox on nc30 if you mod the carbs to full hrc spec. NC35 hrc carbs are intended to run with an airbox.

It would be easier to put a filter and airbox back on. My racebike has full hrc carbs though and it rocks in the fun stakes!

[lee.pb]

is it possible to get the hrc velocity stacks from the uk?
i would add a picture of the airbox but unsure how too.