Major NC21 running issues.

[death46]

Hi all, i own/ride a 1986 NC21, and this is going to take a bit of explaining. My bike is restricted to 33bhp, and rode fine up until i hit about 80/85mph, where it would bog down severely and my top speed would be reduced to 40/50mph (today it backfired as well), it also won't tick over when this happens. The bike feels like it is lurching backwards and forwards beneath me, if i park up, let it stand for 15 minutes and get back on everything returns back to normal. Earlier this evening i removed the 4 washers from the carbs to see if the bike behaved the same when at full power, and it does to some extent. Obviously it is WAY faster, but there is hardly any power in the lower revs and now it will only tick over at 3k revs or not at all... The bogging down also occurs (i twist the throttle and there is hardly any response) quite regularly. I will probably install the washers again as it overall it runs better with them in.

Anyway, sorry for the long post but i know you guys will know what you're talking about, cheers in advance.

[death46]

Just thought i'd add that the garage who fitted the restrictors checked over and cleaned the carbs/plugs etc...

[Neosophist]

If it's idling high then it suggests you have an air-leak when you re-fitted the carbs.

Spray carb cleaner around the carb rubbers when it's running.. if your revs change then it's leaking.

Did the company dyno the bike who fitted the washers? Have a look at the fuel-air graph.

Theres quite a lot of things that can cause this though.

Sorry this is going to be a long list but check them all.

Electrical system.

Check battery voltage at idle and at 5000rpm with multimeter set to DC volts.. 20 if it has a range.

Gearing.

Check sprokets are standard 15/45

Valve Clearnances.

Do you know when they were last setup?

Vaccum Pipe.

The fuel tank should be vaccum operated.. this is worked by a small pipe.. if it's old and broken.. at HIGH rpm the pipe can leak air or collapse.. this shuts the fuel off and the bike dies.

Check / replace the vaccumn hose (replace it if its old anyhow.. they are cheap)

Carbs.

Providing the electrics test out ok then this is going to be where the issue lies.

Is the air-box all standard and complete?
What jets are in it? Main / pilot?
Are the emulsion tubes in the right carbs?
Do you have air-leaks? (spray the carb cleaner on the rubbers)
Is the float height correct?
Are the carbs clean?
Needles in the right carbs?
Balance them to improve the idle smoothness (only after check/set of valve clearnaces and making 100% sure there are no leaks)


My initial diagnosis providing electricals are ok is that you have a bad vaccum pipe and poorly setup carbs.

Often the rubbers are 20+ years old and perished / split so makes sealing the carbs properly difficult.. plus they can be hard to install for some people.

The carbs also have different bits in each carb and people do get them mixed up

[death46]

Nice one for the response. It has never idled properly, even when it is idling properly (at about 1k revs) and i twist the throttle it gets stuck at 4k rpm. From what you have said i'm tilting more towards a vacuum hose leak, as the bike will at least move forwards if i keep the revs below 5000rpm, anything above that and it feels like it is being starved of something.

[magg]

If you have a problem getting over 5000rpm you might want to check the speedo sensor. No sensor or a faulty one and the ignition to the rear cylinders is cut above 5000rpm.

[Neosophist]

If you have a problem getting over 5000rpm you might want to check the speedo sensor. No sensor or a faulty one and the ignition to the rear cylinders is cut above 5000rpm.

Not on an NC21

[death46]

After examination the fuel flow out of the tank seems to be a problem... on either 'on' or 'reserve' no petrol comes out unless i shake the tank... I don't know if suction created by the vacuum pulls the petrol through or not but on every bike i have had petrol freely runs from the tank regardless if switched 'on' or 'reserve', what do you think?

[Neosophist]

After examination the fuel flow out of the tank seems to be a problem... on either 'on' or 'reserve' no petrol comes out unless i shake the tank... I don't know if suction created by the vacuum pulls the petrol through or not but on every bike i have had petrol freely runs from the tank regardless if switched 'on' or 'reserve', what do you think?

Yes petrol shouldn't flow unless you suck the small vaccum pipe.

It shuts off the fuel even if the bike is ON or RES.

If you can still make some come out without sucking the vaccumn pipe then it sounds like that you may have a problem with the fuel tap (broken diaphram is common)

[death46]

Oh ok. Towards the left side of the rear left cylinder there is a small nozzle protruding which blows when the engine is running, should anything be connected to this? There is a small pipe coming from the petrol tank which is of perfect reach and size.

[Neosophist]

Oh ok. Towards the left side of the rear left cylinder there is a small nozzle protruding which blows when the engine is running, should anything be connected to this? There is a small pipe coming from the petrol tank which is of perfect reach and size.

Yes!! Thats the vaccum pipe for the fuel tank.. if that isn't connected then your engine won't idle or run properly.. you also shouldnt' get any fuel coming from the tank!

Vaccumn pipe sounds old and broken.. easiest is to replace it with a new one... less hassle