nc21 tickover problems

[128bhp]

Hi all, I have an nc21 and have had it a few months now. Its never run great from the start. When I bought it the bike would tick over at about 3k. if I revved it past 3k it would slowly go down to 1k sometimes and mostly cut out.
I needed restricters putting on so I thought the chap at the garage would sort the tickover out when installed.
He fitted them and gave me the bike back running a little better, with the tickover around 1.5k ish.
I rode it frequently over the next 3 weeks, and regularly it would die on me like it was running out of juice. then I would sit at side of road for 10 or 15 minutes and it would start up and i would get going. the distance I could get between breaking down got less and less over about 2 weeks until one day I had to call the aa. The AA chap didnt look much at all, but said there was too much petrol in the carbs. maybe as we could smell it. It sounds strange that there would be too much petrol in the carbs and to break down like running out of juice.
I have read loads on this site about the vacuum pipe and looked to find mine isn't connected to anything, apart from the fuel tap, and is really loosly fitted to that.
I will get a new piece of vacuum pipe and some clips in the week and I should have tried this before posting this, but with the revs not dropping fast enough in nuetral I thought there maybe another fault. Has anyone had this problem. Thanks in advance if anyone has any helpful advice.

[Neosophist]

If your bike runs at ALL without the pipe conencted then the fuel tap has been modified.

It sounds like your bike has been messed around with by somebody who hasnt a clue.

Good news is you can sort it out. I agree with the AA, it sounds like your having rich running issues. Follow these steps exactly and your bike should run fine.

Firstly, most people who don't know what they are doing fuck about with the carbs.

1. Get the tank off. Get a bowl ready but turn the fuel tap to On or Reserve, if fuel flows then the tap has been modified or is broken, best to strip the tap down and check to see what the deal is.

2. You already mentioned your vaccum hose is loose, unless the engine side has been sealed up this will be creating an air-leak which will add to your piss poor running.

3. With the tank off get the carbs off and do a full strip and clean and reset the float heights. Many people mix the bits up as they don't realise that the carbs have different jets and needles so you might find they are all in the wrong places.

4. If you want super smooth idling and peace of mind check and adjust valve cleareances while carbs are off.

5. Put the carbs back on after the strip and overhaul, if the rubbers are bad buy new ones, not worth the hassle.

6. Check the vaccum hose for the tap is connected or blanked off if your tap is modified, also check the 4 x vaccum screws on the engine and washers are in place, remove them and use a balancer to balance the carbs.

Bike should run like a dream.

Sounds like a lot of work but even for a novice with some paitence it will only take a day or so.

Seen it too many times to count, but your vaccum hose is leaking air which is causing the high idle, there might be other air-leaks too, the carbs are not setup right and overfuelling.

This mis-match is causing a lean / rich condition depending on rpm and causing the high idle / dropping idle.

Nothing that can't be sorted, go through everything above and it will idle like a swiss watch.

[128bhp]

Thanks for the reply, its just the help I need.
I tested the tap with tank off. In the on and res position, petrol only comes out when I suck the vac tube. no petrol comes out when in off position.
I found the vac nipple on #1 cylinder and fitted.
The bike started and ticked over better. but was moving up and down 200revs or so.
Also when turning throttle up to 4k it would still be very slow to return to 1k.
I then removed the vac tube from the tap. It ticked over for a while the same, then cut out. but I could keep it running if I turned the throttle. still slow return to 1k.
I'm very limited in knowledge so please assume i know nothing.
I take it that there are screws in each cylinder in the same place which should be blocked off. How do you balance carbs with these, or should I steer clear with minimal clue. I'm willing to try anything though.
Also is there a "how to" somewhere to show me how to check floats and which tubes and valves go where, or do I get a manual. And would the nc24 rubbers go on the nc21 as I read they don't make them now, Is that right?
With the vac tube not being clipped at the cylinder, would it mean there is air going in.
I'm going out NOW to find some clips and I'll post later. This is new, I'm actually trying to fix a bike.

[Neosophist]

Yes air will be being sucked in.

The cylinders are as follows

FRONT

[2]---[4]

[1]--[3]


The vaccum pipe connects to the rear left cylinder, theres a pipe that sticks out the side, its hidden by the frames engine mount but you will find it.

Have a look at the thread, cammo's carb guide is brillinant.

Dont change the jets or drill the slides but the float heights are right.

[128bhp]

I'm back and have fitted clips to vac tube at tap and #1 cylinder and there is no running away revs now.
But now the throttle is not responsive. If I twist quick, it wants to cut out. and the revs return really slowly.
Is it carbs off then? Thanks again for reply.

[speedy231278]

Sounds like it. Do what Neo suggested above. I'd also check to make sure the air filter isn't full of crap, mice etc. Yes, people really have found mice (or at least their nests) in there, although I'm not suggesting that I seriously think that's your problem!

[128bhp]

Dont change the jets or drill the slides but the float heights are right.

Do you mean check the float heights?
And do nc24 rubbers fit nc21?

Now you mention air box. Thers a pipe that comes off the rear bottom of it that seams flat/kinked, which goes to the top of the rear cylinder head. whats this? It doesnt look totally flat but looks restricted by the kink.

[speedy231278]

Yes, he means check the float heights. If the recovery bloke was right about too much petrol in the carbs, then the floats might be sticky, or at the wrong height.

That pipe is the crankcase breather. I doubt that if being kinked would affect the running of the engine as some people remove it and fit a filter over the end of the outlet.

[128bhp]

5. Put the carbs back on after the strip and overhaul, if the rubbers are bad buy new ones, not worth the hassle.

Do you know where to source these Neo, thanks.

[Cammo]

I think 21 & 24 carb rubbers are discontinued.

Nc30 carb rubbers will fit your carbs fine (same external carb dimensions). David Silvers or eBay.