hi all, my nc21 is having some running issues. basically what's happening is a massive lack of power/bogging down. the bike was running fine last week, i have recently had the petrol tank thoroughly cleaned, so it's free from rust/debris. i have modified the fuel tap by removing its diaphragm and eliminating the need for a vacuum, the spouts on the tap and the engine have both been blanked off.

it runs fine for maybe 2 miles, then suddenly it loses virtually all of it's power (twisting the throttle has no effect). it will continue to lose its power until it completely stops and won't start for another hour or so, if it does start again, twisting the throttle hardly increases the revs as it is extremely bogged down. it will also sometimes backfire and pop. i am on a 33bhp restriction, so i thought that maybe the restrictors were the cause of this problem, removing them however has had no effect.

fuel flow seems to be fine, the carburettors have recently been stripped and cleaned, and the tank cleansed of all it's contaminants. any advice is greatly appreciated, i'd rather this be a quick fix than a garage overhaul .

fuel tank breather blocked ? try riding with the fuel cap open slightly

will try that now and get back to you.

CRM wrote:fuel tank breather blocked ? try riding with the fuel cap open slightly

you can also unscrew the breather on the top right and try that.. if you remove it try blowing into the tank through the hole.. it could be bloced with crap from the clean

Neosophist wrote:

CRM wrote:fuel tank breather blocked ? try riding with the fuel cap open slightly

you can also unscrew the breather on the top right and try that.. if you remove it try blowing into the tank through the hole.. it could be bloced with crap from the clean

That can also be the problem if you put Petseal etc in when you cleaned your tank

tried riding with the breather unscrewed to no avail.

It seems like it's being starved of fuel but i just don't see how it can be.

I'd say your problem is definitely fuel starvation, so it's either caused by a blocked breather creating a vacuum in the tank as the fuel is sucked out by the carbs, or a blockage in your modified tap assembly.

Have you checked that the breather hole itself isn't blocked? The breather you unscrewed is nothing more than a one-way valve... take the valve off altogether and use a torch to see if there's any fuel in the tank.

with the cap open see if you can blow into the hole you remove the breather valve from... and then blow through the breather valve.

is the tap still vaccum operated? the diaphram in the tap could be playing up, or the vaccum hose is very common.. have you checked it has no splits etc/