NC24 air filter problem

[bbqnm56]

Hello- my first post.
Recently acquired an old NC24

It was running ok when I first got it but it was not revving too well. So decided to strip the carbs.
Followed the good advice on here- float height 6.8mm, 2 turns out on the pilot screw, checked the jet sizes and emulsion tubes- all good. Ultrasonically cleaned all the jets etc. All put back on the bike and fired up first time and revved crispy and sounded great. At this point I must point out that the air filter was not in place. So feeling all happy and jolly I proceeded to fit the air filter and tank etc. Went to start it again and would not start up. Tried for aged but would not fire up. I removed the air filter and it started first time!!. Put filter back on and no go. I did notice that that when running that the revs would shoot up to 4K and then back down to idle- air leak possibly?

Anyway I wonder if anyone has any tips to what could be the problem?

[richyrd5]

Check the carb rubbers in case there perished.also would replace/at least clean air filter.and check no pipes/wires are getting snagged when putting air box on.

[bbqnm56]

ok- whipped the carbs off again- rechecked everything. One set of floats was sitting at about 8mm so put that back to 6.8 ish. Made sure all pipes were routed properly and carbs seated right down. Fitted the air filter. Started her up- fired up first time- sounds great. So at least it now works with the air filter fitted!. However it sits steady revs for a few minutes at 1300 rpm then climbs to about 3000 rpm then drops down again to about 1000/900 rpm and I have to give it some throttle to stop it cutting out. I am sure it is not an air leak as I sprayed wd40 all over and it did not seem to affect anything. Did some more thinking and I had previously noticed that the fuel tap does not do what it is supposed to do- i.e. when the engine is turned off and the fuel tap is on or on reserve- fuel pisses out. I thought these NC24s are supposed to have a special diaphragm that required a vacuum to open up the flow of fuel. Anyway it seems my tap is not working correctly. I have read about the "HRC" mod and suspect that this has been carried out or the diaphragm is broken. Anyway do you think blanking of the vacuum nozzle or connecting a short piece of tube from the vacuum nozzle on the tap to the air bleed nozzle will sort out the uneven idle? Also I assume the nozzle that points down vertically in the Vacuum nozzle. I will have to remember to turn the tap to off every time the bike is parked up as I do not want to find the engine full of petrol in case the floats suddenly stop working and let all the petrol out!

[richyrd5]

On the nc30 the nozzle that points down is NOT the vaccum take of point.but dont know about 24!

[Neosophist]

that uneven idle sounds like you still have an airleak somehwere but cutting out might be too rich or flooding.

i suspect airleak and the idle is set too low so when the leak seals it cuts out.

are they old rubbers?

blank the vaccum pipe from the cylinder head off temporarily and make a dummy tank out of a pop bottle or funnel in the pipe etc and see how it runs to eliminate vaccum tap..

[bbqnm56]

Thanks!
Over the weekend I blanked off the vacuum pipe and connected a short tube between the two small diameter connections on the tank to effectively cancel each other out. The bike runs much more evenly now- 1300rpm+/- 50ish. So I am happy now!. Still suspect that my carb rubbers may not be the best so will get another set. I say this because when I rev the engine the revs dont come down as quick as I think they should. I also think my pilot screws are probably not on the best setting. currently 2 turns out but the bike has a non standard can so maybe the screws need to be out a bit more? anyway I will play around with that.

[Neosophist]

2.5 shoudl be ok on the pilot screws, and yes the idle should drop back nice and quick... the extra 1/2 turn on the screws should help that but never trust old rubbers (of any kind!)