NC30 - Misfire at WOT ~7000rpm

[265chemic]

They are balanced with a drill bit which gives a decent idle and good low down response which is where balancing matters most. Even with very unbalanced carbs, the larger the throttle opening the less effect you should notice.

[magg]

What does "blow into the hoses of each pair" mean.

[265chemic]

If you blow air into the vent hoses (the 2 that are right next to each other attaching to the small box on the rear of the airbox) the sliders rise. One hose per pair, front and rear. Its not a bulletproof test as there tends to be air leaks around the swivel joins where the hoses meet the carbs but you can watch the reactions of each to see if they are close, which they are. I'm not sure how useful this method is for troubleshooting but you get to test their actuation without touching them.

[magg]

True but the vacuum on the throttle side of the slide is what lifts the slide. Have the slide vacuum port holes been modified or springs modified/changed.

[265chemic]

As in the holes in the plastic slide? They are 2mm. If you mean the port where vaccuum is present on to lift the slides then no I have'nt. Where are they? The springs all look standard.

[magg]

2mm slide hole is vacuum bleed to allow piston/slide to respond to changing level of engine vacuum. the top of the diaphragm is connected to another port located in the venturi to generate the vacuum to raise the piston/slide. Blowing air into vent holes does test integrity of diaphragm but does not test vacuum side of diaphragm. With the diaphragm cover removed, there should be port at the edge of the diaphragm that leads to venturi. Any blockage would affect the movement of the piston/slide.

Some thing might be worth checking.

[265chemic]

Okay I'm guessing that's the little offset lug that the relief in the slide cover lines up with. Hopefully I can get a piece of vac hose over the hole in the carb throat end and test them without having to pull the carbs again. So at this stage we are thinking there may be an issue aside from mains, even though the problem is in the last 1/4 of throttle and all midrage to 7000 is ok and at < 3/4 throttle opening all the way round the clock is okay? If really does sound like mains in light of doing some more reading - I found this article great;

http://www.motorcyclecruiser.com/motorc ... ion-basics

Just trying to consider all aspects so the carbs dont have to come off more than they need to!! Thanks again Magg and all for the responses so far

Here's a video on cv carb operation too in case it helps anyone in future https://youtu.be/wyspAHrMbb8

[Malc]

So at this stage we are thinking there may be an issue aside from mains, even though the problem is in the last 1/4 of throttle and all midrage to 7000 is ok and at < 3/4 throttle opening all the way round the clock is okay?

This contradicts what you said earlier, hence my thoughts about one slide not opening fully...

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.

This to me read as, you're starting at say 3k in top gear, and you wind the throttle full open and the revs start to rise nice and clean. When it gets to 7k it starts to play up and feels like it's running on three cylinders.

The only other that comes to mind would be check integrity of the ignition, specifically the HT leads. Perhaps you have one plug cap joint which is less than perfect and is breaking down as the revs get higher?

I mentioned carb balance earlier cos my 35 ran like a bag of shit from 8.5-9k onwards - had the carbs balanced with manometers and it ran clean all the way afterwards.

[265chemic]

Apologies if I wasn't clear, I was trying to articulate the same issue in a different way - ie Last 1/4 of throttle has the issue but up to 7000rpm is ok, plus less than 3/4 throttle opening all the way round the clock is okay,

the second quote was just saying the same thing but explaining it in a different way and explaining the noise.

I'm still not confident I don't have some kind of ignition issue, just that components have tested ok resistance wise. Another step could be to start it in the dark and spray a bit of water around to see if I can see any spark leakage.

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.

Nice call on this - it has actually partially cured the issue and my choke works properly for the first time ever. I would have assumed having the bung in would richen the mixture further, the opposite of leaning it out as freeing up the exhaust would normally do. Of course we aren't certain I was too rich or lean but logic would tell you rich - sticky black sooty tailpipe would also tell you so. In lower gears it revs out quite nicely now - I wouldn't say its perfect and you still have a bit of a dip in power around 7k but quite acceptable. In higher gears that hollow spot gets more pronounced but still keeps accelerating reasonably where it did not before.

[magg]

Good to hear you are having success. The 7000rpm dip may be fixed by adding the washers you removed, a shame you would have to remove carbies again.