I am new to this and hope some one can help,

I have a lovely NC30 1992, and it has developed a missfire, it will some times be fine but as it warms/ runs it starts to run very badly, missfiring and and back firing up the inlet tract, any ideas?, opening the choke whilst running has no effect, it uses no coolant or oil either.

It has aftermarket exhaust can, which it is jetted to suit, and used to run fine with

I have swapped -
coils,
CDI

I have removed and cleaned carbs, air filter

I have removed the fuel tap diaphragm

I have checked the spark and plugs

I have checked the valve clearences

I am lost.........

Welcome..

We'll sort it...

So - start at the start. When did it start? What exactly does it do? Is this riding it, on the stand?

do you have a standard airbox setup?
What jets are in the carbs?
Put the fuel tap back together..no need to remove the diahpragm

Get us some more info..what has changed on the bike- they are simple beasts

plugs fucked and breaking down when warm and under load. replace them

It happens worst under load, it is not so bad at part throttle, the missfire takes place from 6k upwards

It just happened, when i went to ride it after it being sat for a few weeks, ran fine before being stopped, nothing changed from it running fine, to when the miss fire started

It is 2 or 3 jet sizes up on both front and rear cylinders, it has been like this for 4 years, so i don't believe this is responsible for the miss fire

It runs a standard air box and air filter

I removed the diaphragm to eliminate fuel flow issues, however i have now deduced this is not the case.


As for the plugs, is this possible? i have never heard of this before, how ever this is not to say it doesn't happened, have you had it happen on a set of your plugs?

Plugs are super fragile on these bikes, look at them the wrong way and they shit their selfs.

Saying that tho you say its above 6k rpm, on a NC35 that would suggest the bike is in limp home mode although im not 100% sure the same applies to the NC30. If it is then you need to look at things like blown fuses, broken wires and your speedo, is it connected?

^ this.

PLugs are prone to die if flooded or there is poor running.

There is a limp mode at 5.5k revs - you got a standard speedo?

CMSMJ1 wrote:^ this.

PLugs are prone to die if flooded or there is poor running.

There is a limp mode at 5.5k revs - you got a standard speedo?

Plugs

Cheers for the responses, it is a great help, especially as the weather is nice and i want to be on the thing scrapping the pegs around every corner i can find!

Yeh i have got a standard speedo, with a mph converter on the back, and it is working, didn't know about the limp home mode, does it then missfire above this rpm if it defaults to that?

i think i will investigate the plugs next then, i had them out and checked the gap, but i have some old ones than used to run fine in another bike, that i can drop in.

When they are bolloxed is there any visable sign on the ceramic or electrode, or is it an internal failure?

Another thought,

presumably they have a crank position sensor, for timing, is this known to be tempermental? and would cause the bike to enter limp home mode?

It's not 'limp home mode' this is what many cars do when they have an engine fault of some description.

It goes into 'limited mode' and this is by cutting the rear spark plugs above 5500rpm.

The bike has pickup coils. Most likely to be plugs, it always is and they are very easy to replace.