Hi,
Finally started to make some headway with my carb problems on my jap NC30. When bought, it would struggle to rev though 8 to 12 grand in higher gears, though revved cleanly up to there and after. Stripped carbs using cammos excellent guide, and found nothing untoward, all clean, other than some very odd needles. Jets are 118f 120r, emulsion tubes 4 hole f, 5 hole r. All jets were removed and cleaned, replaced needles with rick oliver ones and kept the single washer underneath them. Diaphragms in fine condition and all slides return evenly.
Test riding after this change now had made the full throttle higher rev problem go away, but now on partial throttle openings between roughly a quarter and half it would miss like mad, regardless of rpms, full throttle was fine.
On both instances, used fuel like mad, about 65 miles to reserve on a full tank then 75 with changed needles, and seriously sooty around the exhaust outlet. Exhaust btw is a horrible stubby viper can, originally had a db killer in, but I removed it in a simple bid to try lean it out, slightly better like this but still a nightmare to ride through corners trying to feather the throttle.
So stripped them again yesterday, all needles jets etc removed to check and clean. Checked float heights at 6.8mm or thereabouts, and idle mixture screws checked to make sure seals in place and no blockages. Set these back to 2 1/4 turns out. No different.
So today, have taken them off again (getting good at it now lol) have taken the washer out from under the needles, and turned the low end needles in an eighth in a bid to lean it out.
Now it's definitely made a difference for the better, nowhere near as bad on partial throttle as it was. Baffle is still out and would prefer to run with it in. It's not perfect, but useable now.
Question one, should I try maybe going to 118f 115r main jets?
Question two, anyone ever come across a similar problem?
Many thanks for any help or suggestions, wanna start restoring it over this winter, but not till the bugger is running to my liking!
Cheers!

How are you measuring the float heights?

65 miles to resreve sounds like leaking floats allowing fuel to pour into the engine, at high RPM wouldnt be so much of an isse.

How does it idle?

I had near enoguh same issue that was caused by leaking floats floodign the engine at slow speeds.

Good place to start I guess.

Easy way to check floats befoere oyu get it all apart is to take tank off and use a pop bottle or something as a "dummy" fuel tank, fill it wil fuel until you can see fuel in the bottle.. leave it for 10 mins or so with the bike off and see if it drops.. if it does your leaking fuel

Hiya,
Idle is absolutely spot on, pick up is good too unless holding it in the 1/4 to 1/2 throttle position. If one of floats was leaking the tickover would be allover with poor pickup from there surely. Mine is totally fine at the mo.
Floats checked from base of carb body to top of float just as the needle seats in position. Admittedly, I've not had the carbs connected up to a fuel supply off the bike to check, but have left it with fuel tap switched on for a good time with no flooding issues, though fuel tap diaphragm works well!
Next time they're off, will double check it though, good suggestion, cheers!

Yep, good to check.

Strange one.

Your pilot jets are all in place right?

Yep, most definitely, all in place, all clear, and all the correct size, 35's.
I've heard folk speak of worn jets, but myself, not sure if it's likely? Very tempted to try going down a size front and back, swop the 118's to the rear and buy some 115's for the front. and see how it runs like that.
Just starting to wonder if it might be wise to get it dyno'd to see exactly where it's rich or lean etc, and is a gas analyser for a car any use, work at a ford main dealer so have plenty of that type of gear i guess.

b1hrider wrote:Yep, most definitely, all in place, all clear, and all the correct size, 35's.
I've heard folk speak of worn jets, but myself, not sure if it's likely? Very tempted to try going down a size front and back, swop the 118's to the rear and buy some 115's for the front. and see how it runs like that.
Just starting to wonder if it might be wise to get it dyno'd to see exactly where it's rich or lean etc, and is a gas analyser for a car any use, work at a ford main dealer so have plenty of that type of gear i guess.

Gas analyzer should tell you.

But part throttle isnt main jets.

http://www.factorypro.com/tech/tech_tun ... gines.html

Have a read of this to get a good idea of what alters what on the carbs.

If its bogging really bad somewhere then something is really up

Yeah, the main jet thoughts were clutching at straws really, plus the high fuel consumption. Need to have a proper ride on it since last changes to see mileage and how it performs in real world riding, blasting up and down bypass doesn't really help and that's all i've done since. It seemed nowhere near as bad before, but it's still noticeable and would like it spot on if poss! Can't believe i've not found anything drastically wrong myself, but all seems to me as it should be?
Cheers for the link, gonna peruse it shortly.

Neosophist wrote:
Gas analyzer should tell you.

But part throttle isnt main jets.

http://www.factorypro.com/tech/tech_tun ... gines.html

Have a read of this to get a good idea of what alters what on the carbs.

If its bogging really bad somewhere then something is really up

Many thanks for that guide, just read through and it's exactly the thing i need to help me understand, cheers very much!

So after having a proper ride at the weekend;
Problem is still there, not as bad as before, but once fully warmed up not as improved as i thought.
Lot of gravel on some roads, and was mid afternoon so not giving it as much as normal, but got to 92 miles before reserve, so somethings improved.
Main conclusion i came to, just as bike is about to run out of juice, the throttling is perfect everywhere, part or full. Obviously noticed this just before hitting reserve, so switched back to on after carbs filled running on reserve for a short while. Sure enough, as the fuel in the carbs depleted, it once again throttled perfectly until it almost ran out. Repeated this a few times and same result every time.
Just as a recap,
89 jap nc30, jets 118f 120r, rick oliver needles, 35 pilot jets, pilot screws at 2 + 1/8th out, viper race can, no baffle, standard airbox and clean standard filter.

Need to double check float height, and raise their height a mm to lower the carb fuel level?
Any thoughts please?
Cheers!

Im having near enough the same problem as you, as you know from reading my post. I took my carbs off again tonight and i def have standard needles and emulsion tubes in the right carbs float heights 6.8mm too. I am starting to get suspicious about the hole in the carbs that the needle penetrates in the carb body where it lets fuel out of. Thinking that it is worn slightly around the bottom edge which is allowing too much fuel out past the needle which would normally be able to restrict it. I could be way off the mark, but that could be why a bigger needle or removing washers is improving it.