Hi my NC 24 which is now fitted with an external fuel pump maxes out at 140 kmph and doesnt go above that and if I have an extra company it would top up at 120 can anyone tell me why would that be

plus there is no breather pipe connected

Not aware NC24 had a fuel pump?

Was this fuel pump a mod? I believe more information is needed, what happens after 140? does it just stop accelerating or does it bog down?

Bit more information needed.

Did you ride the bike without the fuel pump before it was fitted? How fast did it go?

Not sure why you would need an external fuel pump, the stock gravity feed is fine for the bike in almost all circumstances, some more information would be useful, the fuel pump could be the cause of the issue if it is not correct feed-rate / pressure etc.. since it is an aftermarket modification it would need to be installed properly.

These bikes were made before the mandatory 180km/h restriction so if it is all in stock trim it should be able to go off the the stock 180km/h clocks with a single rider... 210-215km/h is not unexpected on a flat road with a light rider and good condition bike.

Since they are all 30 years old it could however be due to many many things.. so first off as asked initially, did you ride it withotu the fuel pump? How fast did it go? Why did you add a fuel pump? Was it to "fix" other issues?

Whats the gearing like? Since these old 400s are not really about top speed anyway (buy a bigger bike if you want to go more than 120mph somewhere) the sprockets could be well off which would not only change the top speed but would also throw off the speedo.

Carb setup / valves / jetting / electrical system could all be part of it.

Any idea how the carbs are setup / intake airbox is setup / air filter etc etc?

thanks for the reply neosophist i couldnt ride the bike without the pump as fuel would not reach the carbs due to some odd reason and after 140 kmphr the bike doesnt go up the rpm stays at 10 and speed does not go above 140 or 120 if i am riding with a passanger

and no when i bought the bike the fuel had problem with the fuel delivery so ended up putting a fuel pump for it to deliver the fuel

Sounds like a fuel tap/flow problem that a fuel pump cannot solve. Fuel should floe freely from the tank whenever a vacuum is applied to the fuel tap. Fuel taps fail and vacuum hoses from the inlet ports can leak, both stop fuel flow.

i thought there could be a problem with the carbs or the jets because when i give the full throttle seems that the bike is not getting the full fuel

You need to start at the fuel tank and ensure that fuel flow freely and then progress through the system.