NC30 - Misfire at WOT ~7000rpm

[265chemic]

Hi All,

I've been chasing an issue with my NC30 for a while and thought I'd run it past the forum to see if anyone has any ideas;

At ~7000rpm (edit: and upwards) at WOT I get a fairly serious misfire and the bike barely accelerates but will rev out to redline, if I back off a touch it settles then cranking it again gives a surge of power but then misfiring starts again. If I accelerate under say 70% throttle it revs out okay in comparison.

My most recent change to the bike was the main jets - it had 128.5 and 125s which initially ran okay but I had issues with fouling plugs over time, especially when the air filter was getting on a bit. They are now 120/118s, which made the bike feel more punchy under 6000rpm (I know in theory this should be on pilots only but the mains seem to affect it). Needles are stock + raised via double washers, pilots and emulsion tubes are stock. Bike has an aftermarket can but stock exhaust.

I'm fairly certain there are no vac leaks (ive had the carbs on and off many times in the past) as the idle is decent, however was planning to spray some aerostart around the carb boots to rule vac leaks out.

Primary side coils test at 3.1 ohms. Secondary + leads for each is 25k. I've swapped plugs around with spares (these have all been cleaned up with a wire brush multiple times previously due to the aforementioned fouling) and don't see any change - all plugs pass resistance tests okay with a multimeter. They look okay when pulled and are all uniform in colour. Battery is 12.6v at rest, 12.1v with lights on. Running is > 13v @ 5000rpm. Airfilter has less than 200kms on it. Airbox is stock, shield/flap is present - I actually seated it the most 'properly' as I ever have (essentially sealing the area in front of the snorkel which looks wrong - but is apparently right, right?).

Valve clearances on the front bank are fine - have not checked the back yet but the front was the same as when I'd done it 30k previous so hadn't bothered with the back yet.

Might be worth noting this bike has never run well on the choke no matter what I've changed. Bringing the main jets down has helped but it still seems to be overly rich and seems to bog/misfire on choke. I just don't know what else to check - even thought about air bleeds but these are non adjustable as far as I'm aware.

Additionally, pulling the choke on / turning lights off makes no difference to the misfire.

Thanks all,
Pete

[magg]

6000 rpm is not pilot jet range. Given the original main jet sizes, I am surprised it managed to get much above 7000rpm.

My understanding is that a washer under each needle is supposed to fix a lean spot around 7000rpm. Two washers may be over-compensating. Why not try std no washer as a stating point.

Not sure what description regarding the shield/flap means but apparently this are can have dramatic effect on performance. There is a snorkel on the airbox inlet, a rubber flap/mat to shield the front cylinder head head from the air path and a funnel over the top of the radiator to keep radiator heat out of the inlet air flow.

Have you seen the carby setup document on this website.

[265chemic]

Hi Magg, thanks for the reply;

Ive been doing some further reading, it seems for an nc30 one .5mm washer is standard. A little more info, the misfiring starts at 7000 and goes all the way to redline, I can also get it up to 10k for eg on 1/2 throttle then crank it and it has the same behaviour. I should have said misfire from ~7000rpm upwards in the title.

I've looked at pictures of flap placement, mine is fitted as it should be plus the airbox bottom shelf is intact and the snorkel is unmodified.

I have been through the setup guide - everything is reflective of this so I'm trying to figure out what is missing. Carbs have been off and cleaned/floats set etc when Jets were changed back to recommended.

[magg]

Just to clarify, there is a duct/snorkel mounted on the air box lid, a rubber flap over the frame cross rail and the scoop over the radiator. My mistake regarding needle washers, should have said std one washer.

Although a side issue, what is the actual battery voltage at 5000rpm. Should be at least 14.2 volts, headlights on high-beam, to fully charge the battery.

[265chemic]

Yes that is correct - all you have mentioned is in place.

The battery voltage @ 5000 was 13.2v - it has been like this since I've owned the bike and hasn't gotten worse. I considered swapping in the battery from my other bike as a test but figured because it starts up strongly and the resting voltage of the battery is 12.6v consistently.

[magg]

Did the change to smaller main jets improve the wot performance. If so, then I suggest you reduce the main jet sizes further. The description you give is of an over rich condition and if you have two washers under the needles it would be worth removing one also to lean off the mid to top of the fuel range.

13.2 volts is not actually sufficient to fully charge the battery. Another test of the charging system is measure the battery voltage at idle with light on and then increase engine rpm slowly to 5000. Does the battery voltage progressively increase to at least 14.5 volts.

[265chemic]

That's the thing I can't get my head around - it ran okay on the bigger Jets up high rpm just not as well at lower/mid. I bought it with foam in place of the paper filter but replaced that with a new paper without a lot of change and it used to run rich (youd end up smelling like a fuel Bowser) and foul plugs. Even after playing with the pilot screws which seemingly had little effect I figured I'd go to a recommended Jet sizes to see if there was any effect - there was. Smaller mains also affected the choke - it worked better but still not how it should. I then wondered (seeing the choke manipulates air not fuel directly(?)) if the bike may have modified air bleeds. Ive yet to check this but believe it would be via the passage diameter on the carb mouth near the trumpets. Worth a look?

The bike had radiator hose sections for carb Boots (replaced) plus the foam filter when I got it so I'm wondering if someone was trying to solve a problem via jetting that originated elsewhere.

I'll aim to get the extra washer out next time I have some time to pull the carbs again and will post back regarding the voltage - do you think this could have any bearing on the misfiring?

[magg]

Choke is a fuel enrichment system. From your description the big main jets were making the mixture too rich on the mid to top end. This has been confirmed if smaller main jets have made the bike run better. The needle position will affect 1/2 to 3/4 throttle and main 2/3 to full throttle. Pilot jets and mixture screws are idle to 1/8 throttle usually set and forget. Some nc30 models imported into the uk have 110 main jets listed as std, so going smaller than than 118/120 is not out of the question and my own NC30 with 118/120 jets is probably jetted a little rich but I cannot be bothered to make changes, too much work.

Removing the extra a washer only requires removing the slides, a much easier task. I am not sure your running issue is actually an electrical misfire, if it was it would more likely occur throughout the rev range.

My questions regarding the charging system voltage are not related to the running issue more a case of potentially reduced long term battery life due to under charging.

[265chemic]

Thanks again, I'll remove the extra washers and see how we go. Can you actually get the covers off the front carbs without removing them? Id thought I'd considered trying it but deemed it not possible

[magg]

From what I can remember it is possible but it is some time since I have had the air box off. Have you downloaded the parts and service manual from this site, OEM components that should be fitted and their layout info can very helpful.