NC30 fuelling/carbs (again)

[Wilson]

OT: I don't like the way this forum handles timeout

Apologies that the questions I'm asking should be answerable by me simply by checking my carb jetting, but the circumstances mean I don't have the opportunity to do so for a while.

I picked up a 'race ready' NC a few weeks back, which was in the process of being tuned. It has a free flowing 3/4 system (locally made), an airbox with the snorkel cut off, no air filter, and no fuel filter. As no one dyno tunes carbied bikes here anymore, the owner said that the guy who did the tuning work was going to accompany them to the track and progressively mod the airbox to get the bike running right.

The bike had been sitting for about a year and a half before I bought it. I swapped all fluids, but there was a little standing fuel in the bike, so I don't know if the carbs were affected/gummed up by that.

I didn't have time to play with the fuelling before going racing, as I spent all the time I had on it swapping in the better suspension from the road NC I had been tracking. Knowing that airbox mods are generally recommended against, and figuring it was safer to run rich than lean (as the bike hadn't been properly tuned yet), I installed my stock airbox and air filter before taking it racing.

The RGVs I was racing were walking away from me on the straights (they were well tuned, though). Hell, there was even an FZR250 I couldn't bully on the straights (I was faster, but not as much as I'd have expected). The bike revved out smoothly and didn't really seem to have any flatspots, but it just didn't pull as hard as I expected, and somewhere between the top of 3rd and the top of 4th, it seemed to brick wall a little. The only other symptom of note was that when revved at idle, the revs would drop very slowly below 3k or so.

Does this sound like the bike running excessively rich? I really don't have much experience with fuelling and how the bike responds to too much/little fuel. If it is too rich, am I likely to risk damage from running too lean if I run the cut airbox (to see how it pulls in comparison)? Is there any way to tell when I'm running lean enough to risk damage?

I don't think I'm going to get an opportunity to do anything with the carbs before the next trackday. If you guys have some diagnosis advice, I can perhaps get started ordering some jets and working according to Cam's guide on the carbs on my old NC, then do a carb swap when I know I have the time to get the carbs in and out of the race bike. Carbs are a new and scary world

[Drunkn Munky]

The brick wall your hitting in 3rd and 4th is likely due to turbulant air caused by the lack of the rubber flap in the induction system. You really should get the carbs off, what's stopping you? Its only an hours job at most.

If it were me id resort to the settings in cammos guide and run a stock induction system until such time you can get to a dyno and dial in getting it setup without a air filter.

[Wilson]

The rubber flap just below the airbox snorkel? I'm pretty sure that's in place. It's running stock induction at the moment (I put in the stock airbox and stock air filter off my old NC).

Working on the bike in the very short term is hard because the bike doesn't live at my house (due to space and lack of tools issues), I don't finish work till well after dark, and I'm not going to be home for a lot of the coming weekends. While I might be able to get the carbs out in a couple of hours when I do get some weekend time (I'm really that green with carbs), I'm afraid of running out time to get it all reinstalled for the next trackday.

[CRM]

OT: I don't like the way this forum handles timeout
:

WTF does that mean ? timeout ? can you explain or maybe even put it in the correct section if you want to try and resolve an issue you are having


has anyone been butchering the clocks / restriction circuit ? if it sits at around 5k and wont rev in the higher gears that would be that causing it.
if it revs out in all gears but just doesnt feel strong, then you need to look at the jetting / setup

[Cammo]

I picked up a 'race ready' NC a few weeks back, which was in the process of being tuned. It has a free flowing 3/4 system (locally made), an airbox with the snorkel cut off, no air filter, and no fuel filter.

Knowing that airbox mods are generally recommended against, and figuring it was safer to run rich than lean (as the bike hadn't been properly tuned yet), I installed my stock airbox and air filter before taking it racing.

Airbox mods work great for the track, not so good on a road bike. The problem is that too many people stuff around with the airbox without adjusting fuel requirements properly, leading to poorly running bikes.

I think you have 2 options:

1. If you want to run a filter and standard airbox you'll need to rejet the carbs to suit it. I'm guessing the main jets you have in it now will be way too big for this setup. Most people will stick around 160 size mains in there if they don't use the velocity stacks. Compare this to the size you need (around 120) and you'll see why it's running so rich. You need to get inside the carbs and take a look at what's inside for yourself - while there make sure you have standard needles and pilot jets (size 35). This is all covered in the carb jetting guide sticky in the v4 workshop section.

2. Run it with no airbox or filter (e.g. nc30 hrc carb setup style). This will net much better power gains than above, but you need all the bits to make it work properly - main jets (around 145 size), needles, velocity stacks, emulsion tubes, bigger pilot jets (I used 40 size) and fuel tap mod. Rick Oliver sells replicas of all of these parts.

[Wilson]

Thanks for the info, Cam. I had no idea it was such a big difference between open and standard airbox jetting. No wonder my bike was running like a dog (assuming they've put halfway right jetting in there for the cut airbox).

I'll have a more serious dig around the forum for airbox mod advice then, and get in and have a look at what jets I actually have as soon as possible. Lucky for me, I have 2 sets of carbs and a stock airbox, so I might just jet my old set up for the stock airbox, and switch between the two while trying to get the tuning right on the open one.

Don't like the idea of running no filter though... a mate rebuilt his RGV with an open airbox mod, but decided to keep the filter. Good decision, as the bike had a play in the sand on the very next trackday.

OT: I don't like the way this forum handles timeout
:

WTF does that mean ? timeout ? can you explain or maybe even put it in the correct section if you want to try and resolve an issue you are having


has anyone been butchering the clocks / restriction circuit ? if it sits at around 5k and wont rev in the higher gears that would be that causing it.
if it revs out in all gears but just doesnt feel strong, then you need to look at the jetting / setup

It was just a throwaway whinge at retyping my post because I got logged out while typing. It wasn't a serious complaint.

The bike pulled through, but just weakly. Going off what Cam said, it's probably just waaaay too rich.

[Neosophist]

Yep the fuelling on the NC30 is like wizardary!

They require radically different setups.

Either go for one (standard airbox filter setup) - This does work quite well for most cases and is the easiest to sort thanks to cammo's excellent carb guide.

Or buy the whole kit from Rick O (he makes a great reporduction setup) different needles, stacks, jets etc and run an open system without a box.

Jetting and setups are well documented for both of these but if you try and do a mishmash you'll be forever tinkereing with it.

[Wilson]

Cheers for the advice. I think that's going to be my plan of attack... set one set up like Cam's road jetting guide and use that as a safe option while I try and get the other setup working with the HRC bits.

I managed to get hold of the guy who'd done the work previously. He couldn't remember what he had put in there, but the plan of attack was to tape the airbox opening up to about the same as stock, then progressively open it up and see if it got better. From what I've read of the NCs, I don't think that solution's going to be idea. I'll root around for this HRC guide then...

[Wilson]

Bump. Sorry to pester, but carbs and jetting are a totally new world to me, and the state of my bike is making the learning curve steep.

I have finally had the chance to take the carbs off the bike and check the jetting. It's running 115 all around. They were planning on running this with a cut airbox .

I have run the bike on stock airbox and filter, as well as trying it with a slightly cut airbox (but still with filter). I didn't log the difference, but seat of the pants dyno could only tell me both were slow.

I expected the bike to be rich, based on what I was told of the airbox/jetting plan, and on how it ran (no guts, did not pull through redline, started to brick wall in the higher gears), but it turns out it's lean.

Would those with experience say the 115 is so far out with a 3/4 system as to cause massive power loss? For reference, we were at the long track last weekend, and the RGVs were hitting just over 190 at the bottom of the straight. I was struggling to get 170. My lack of power is really noticeable.

I've got new jets ordered, but now. I'm trying to figure out if I'm going to have to take the bike to someone to get compression tested and/or more.

[Wilson]

Oh, a couple of the emulsion tubes are in the wrong place as well.