NC30 - Misfire at WOT ~7000rpm

[CMSMJ1]

I reckon Magg has it right.

I would comment and ask whether the airbox is standard and has the flaps and gubbins all in place? Anything other than standard can be a total bitch.

[265chemic]

Okay, last night I had a look and no you cant get to the front slides without removing the carbs - if you had allen screws instead of the little cheese headed JIS ones that strip just by looking at them funny you might just have enough room (far right screw is extremely close to the frame - cant even get a screwdriver bit in a shifter in there for eg). So I removed the carbs and removed the second washer, checking for diaphragm tears and anything else out of place. All looked good though the cheese headed screws are slightly more worse for wear. I haven't got the fairings and tank back on yet so can't tell you the result until tomorrow.

https://goo.gl/photos/8V5jVrbGPSobxbb87 - flap
https://goo.gl/photos/HcENwndHD6kaDfyN7 - flap and front slide covers close up
https://goo.gl/photos/8yhUS2D8RNgxSzCM8 - Carbs seem to be correct for a 1990 Jap import as below (E3DAEA is closest to the entry id expect in http://www.400greybike.com/files/models.htm but not exact);

NC30 - Grey Import 1990-1991 L NC13E-1300001 NC30-1050001 VDE3D A

Airbox / flap etc is all standard and correctly placed according to what I've read. With the airbox base on it seals off the front of the frame. I can see why people think it might be a good idea modify it to promote airflow cos it really does lock it off, but it probably does this to produce a good vaccum signal to get the carbs working as they should - makes me wonder if it was one of those engineering adventures that after many trials ended up the way it is.

Thankyou both for the help and advice, fingers crossed its done the trick. I have probably had the carbs on and off at least 10 times over the time I've had it, and while its a lot easier now than the first couple of times its not the most fun thing to do!

Yep, I've got the manual. This site is a great resource.

[magg]

265chemic, I might have to pull my air box and have a look. Can't remember the fine detail but if I am not mistaken I can see part of the radiator in your pictures. Thought that air inlet ducting would ensure that no hot radiator air could get anywhere near the air box inlet, but may be my poor recollection.

[265chemic]

Don't forget that pic is with the bottom of the airbox off - I was mainly showing the flap (the duct isnt perfectly centred in that one) and screw locations. Once the airbox base is on it seals it off and the air is drawn in through the top and sides of the frame

[265chemic]

No dice gents. I'm guessing I'm now looking at downsizing mains some more. It hasn't made a lot of difference removing one of the .5mm washers from each carb, but possibly a slight bit smoother. Previously when I'd whack the throttle open in top gear when Hot id get a small miss/hesitation but although I was able to reproduce that it didn't seem as consistent as before.. could have just been the temp today though.

What I don't get is I'm essentially stock carb wise except for the mains which others are having no problems with. In top if I whack the throttle wide open it will accelerate the through the rpm then just around 7k starts missing and sounds quite clearly like it's dropped a cylinder but keeps accelerating albeit more slowly.

As for sizing of the next set, should I be looking at 118r/115f or lower? Is there going to be enough difference dropping one step to solve this or is it a suck it and see situation?

So, I'm all stock apart from a loud can + mains and those carbs supposedly came with the aforementioned Jets. Safe bet?

[Malc]

How quick do the slides rise at wide open throttle? What I'm getting at is the slides are not directly connected to the twist grip, and as best as I know, they are lifted by vaccuum pressure, so at 7k they are not going to be fully open. This might mean the fuel is being regulated by other than the mains. If the hesitation occurs here onwards, perhaps a slide is sticking, not opening in sync with the other three?

Dunno, just theorising here...

[magg]

Ignition is shared between cylinder pairs, so an ignition misfire would drop two cylinders and I doubt the bike would do much accelerating. What brand of end can fitted, no db killer, must make you popular with the neighbours if my experience is any guide. Have you a db killer to fit, worth a try in case a fully open system has produced some weird flow issue that stuffs up the carburetion.

[265chemic]

I tested the primary and secondary coils on each pack ok - it seems a bit too predictable for ignition being the fault, as in you close the throttle a bit and it stops missing. everywhere else in the rev range is pretty crisp. Not sure of the brand of the can. Its fairly loud but not crazy obnoxiously. Yes I do have a silencer so I'll throw it in and see if there's any difference.

[265chemic]

Malc - slides seem good and even in their operation, both by manually retracting and letting them fall back and blowing into the hoses for each pair. The mid throttle positions seem pretty good where the needles have the most effect so it's sounding like mains are where work is needed. I would try a plug chop to see what the mains are doing but as we know they aren't the simplest machines to get at plugs, especially on the roadside.

[Malc]

Are the carbs perfectly in sync with each other, and not just from a butterflies opening together perspective? Might be worth having them checked with a manometer type carb balancing kit?