air box troubles?

[cb400]

I bought my 92 nc30 with the carbs off, and I'be been struggling to get the thing to run properly. Right now it idles ok, but riding it once I get to about 5.5K rpm it makes a sound like the slides are slamming about in the carbs. It'll pull through that range,but it's still surging and stuttering. I've tried to insure I've all the hoses connected correctly, and it has the 115 amd 118 jets with stock needles and the .5MM spacer installed. Anyone had similar troubles.

[duh_ave]

is the speedo derestrictor in place?
a friend had a similar thing happen as he had diconnected the speedo and not put a resistor in. once that was done, it ran like a top!

or, maybe you're not getting enough fuel?

[cb400]

I messed around some more with this, and found the front connection to the secondary airbox was loose. This made it better, but you can still hear the slides chattering.

I'm guessing I'll need to verify all these hose connections are sealing properly.

I thinking I'm not fighting a jetting problem here, more of a vac and airbox one.

Again if anyone can verify I'm headed in the right direction here, it'd be a great help.

--jeff

[Cammo]

Yo Jeff, a bit more information on your situation might help to fix your problem.

Is it running the standard airbox and filter, all parts intact?

Are you ssure that the carbs are seated correctly in the rubbers? If unsure, when it's idling, spray some crc around all the carb rubbers, if the revs rise then you have a leak.

I don't think it will be the carb slides chattering (happy to be proven wrong though!), I don't know how the hell you'd hear them over the engine noise! If you have doubts, remove the carbs and check that each slide diaphram is seated properly (and no holes) under each carb cover.

The hose connections to the sub air cleaner won't affect too much, so I'd suggest you keep looking for the cause.

[Danzio]

hey mate i had the same problem for agggggges!

turned out mine was due to the little plastic connector bit on the carbs themselves (the black t shaped pipes which when blown into open up the carb slides) where leaking ALOT of air as the rubber seals had perished and fallen off. try wrapping them again, in something like duck tape for a quick fix, i did until i could get some new rubber seals and they both worked. bike runs great now.

hope this helps, i mean its worth a shot.

regards, Dan

[cb400]

>Yo Jeff, a bit more information on your situation might help to fix your problem.

>Is it running the standard airbox and filter, all parts intact?



Almost. AS I'm here in the US, I've had to wait on an airfilter,so I used an auto pleated paper one cut to fit (for now)
Stock pipe/
Update.

I had a friend double check all the carb cleaning, jets, and settings, and we reseated the carbs properly.

It's using 35 pilots, 115 front jets, 118 rear jets two turns out on the fuel screws.

And it's off the scale rich at idle even without a filter.

Any ideas on that one?
According to all I've read (and the manual) this jetting should be it.


Thanks

[Cammo]

Those carb settings are correct for a late model nc30.

When you say it's rich at idle, how are you measuring it? It should be pretty lean below 4000 revs (>14:1 a/f ratio) so something is wrong.

Re: slides chattering, silly question but are you sure the springs are in each carb slide, and that they diaphrams are sealed properly? Aftermarket air filters can be notoriously trouble some for fuelling on these bikes. The standard jetting won't be adequate for your filter, but problems around this usually pop up at 8-9000 revs and above.

The carbs themselves are quite simple, your problems can't be too difficult to diagnose, I think you need to check the things above.

Are you sure it's running on all cylinders above 5000 revs? The nc30 electrical circuit has a built in limp mode where it cuts spark to 2 cylinders at 5000 revs if the speedo (or associated wiring) is cut or removed. There are easy ways around this if it is the case.

[cb400]

UPDATE to the below. It seems the previous owner drilled the jets. I'll start over with stock jets.... and see what happens/




I'm measuring it by the exhaust. It's not rich by a small amount, it's blowing black smoke (it's not oil, with that colour) and you can smell the richness.
So rich that it's interfering with middle and upper RPM usage.

Forget the "chattering slides" comment, at this point. All the diaphrams are in place, not holed or otherwise broken, with springs. The slides are working properly.

It's not in limp home, all cyls are running above 5K (there was a time when they weren't, so I know the difference now)

Valves are in spec, and carbs were synched.

It idles nicely, but you can see by watching the exhaust how crazy rich it is.


Now, I bought this bike with the carbs off, so I can't say with any certainty that, other than the externals (jets, pilots, diaphrams) anything about them.
I'm wondering if my next step would be to source another set.

Thanks for the help.

[cb400]

New jets and pilots have seemed to sort the mixture problem. I've yet to get it back on the road, as I'm awaiting new carb boots before I finally put it together for the last time.

I only wish I'd know he drilled the jets sooner. Would have saved a lot of time and energy.


Thanks all.


--jeff

[cb400]

That has indeed sorted it. I've now discovered the warped brake rotor.

Thanks.

--jeff