Race bike: nasty jerky running 6-8k

[StrayAlien]

Hi all,

I'd like to see if I can improve on a bad spot between 6-8k on my 93 NC30 race bike. Bike has no airbox, HRC tray and associated gubbins equiped. Short rubber stack etc. It had it when I got it and even after a recent replacement of carb bodies and general clean up it is still there, so it ain't the carbs fault.

The spot is really pretty nasty - I am not calling it a 'flat' spot, it feels more than that - it simply feels like the bike has no power - it splutters and jerks and carburettes badly and then when at about 8k it fires into life.

The odd thing is, and it could be a red herring, it doesn't seem to happen (and I can never recall it happening) when upright .. like when accelerating down the slip road onto the track at Phillip Island or wherever. I can pass through that rev range and all is okay. But banked hard left or right at that rev range it seems to suffer.

Like I said, the 'at lean' thing could be a red herring for sure, but it kinda seems like it.

Some things to note:

HRC vacuum tap mod
I do not run the tank near full (nor near empty)
145 Mains (but did the same thing when smaller)
40 Pilots (but did the same thing when smaller)
Fuel screw 1.5 turns out
8ZG needles front and rear with 0.5mm spacer.
Carbs run NC21 half moon sliders with drilled hole.

I know the basic answer if "well, it is a race bike mate, dont let it get into that rev range" ... but every now and then I do like when you get baulked or slow mid corner when someone does something unexpected .. and when it happens, I reckon I can kiss goodbye to a second or two on the lap.

All help appreciated.

[CMSMJ1]

nowt to do with fuel level in the tank and it getting a gulp of air?

[dobbslc]

You've got so many variables in the bike that it will be a nightmare just fiddling with it. I would take it to a decent dyno centre for some set up work.
You will save yourself so much pissing about it will be worth it in the long run.
Make sure you go to a good old skool dyno operator though and not just some geek that is only interested in pushing buttons on a laptop rather than getting covered in fuel and getting you a good setup.

[Cammo]

Fuel tank level.

Nexg time on track try running a full tank. If it helps the issue you might want to think about an alloy fuel cell.

[VFRkieran]

Sounds just like fuel level, or you have drilled the NC24 slides out by too much, they shouldnt need drilling at all because they have 2 slide holes that equate to the same amount as drilling out the NC30 single hole.

[RoninZX-10R]

Hi Greg,
If you do start to suspect fuel levels it would be cheaper to install the foam baffles (basically blocks of foam that don't break down and only take up about 1% of space) that are used to stop the fuel sloshing about, ie when braking hard and cornering, instead of building an ally fuel cell. I've looked into buying the foam blocks, they're not cheap, but would certainly keep the fuel where it needs to be.
Cheers,
Josh

[Cammo]

I wouldn't be surprised if fuel level is a factor here. Similar issues on track bikes have come up on the forum over the years and the culprit was fuel level, it's a known issue on nc30's. Running the open carb hrc setup seems to worsen the issue as the big main jets can empty the float bowls pretty quick.

I used to keep my racebike tank at about 3/4 full to avoid any issues (regs only allowed stock fuel tanks to be used), these days there are some cool options for fuel cells and tank covers, the foam mentioned above sounds interesting too.

p.s. the previous owner of that bike never drilled the carb slides, they were standard nc21 round-bottom slides (2 holes) if my memory is correct.

[mo haggs]

race bike? - just keep it above 8k rpm = problem solved. lol.


being mid range with most likely a partially open throttle - id say try a different stetting of needles.
however, the fact that it happens at lean - it could be fuel, but in the float bowls rather than your tank.
so while you're in there check your float heights are correct.

[CRM]

was just going to say that.
set the float heights and try again, then set them 1mm less and try again

[StrayAlien]

Thanks all,

Sorry for the late reply and going all quiet after posting. I got side swiped this week.

Cammo: NC24 not drilled. Cool. I do recall the previous owner said he had troubles with the bike at lean sometimes, but his solution was *not* to have a full tank. Bit I could be wrong on that.

I would have thought that despite a bit of fuel sloshing about in the tank the carbs get their fuel from the bowl. It is really possible that a 145 main jet will empty the bowls that quickly .. in one corner?

.. come to think of it, you *do* use a hell of a lot of fuel in 5 laps of Phillip Island ...

CRM/haggs, the float levels are spot on - I did them recently and agonised over getting them perfect. Would lowering them 1mm cause issues elsewhere in the range with fueling?

Ronin: thanks, foam sounds interesting ... because carrying a full tank on a race bike seems a little unnecessary to me., but I'll give it a go next time at the island.