my mc18 runs great , but when at tickover and then rev to 5-6k the tickover then sits at around 3k then slowly comes back down to 1k, the same happens when i,m riding then come to a stop
the carb slides snap fully shut when throttle is closed , the carbs have been cleaned and new jets installed,
i,m thinking it could be an air leak somewhere, any ideas anyone

Weak on the pilot, air screw or float hight, air leak from the inlet manifold or crank case seal.

Start easy and work your way down. Try half a turn on the air screw.
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If I have to take the carbs off once more...

just tried the air screws (richer and weaker) did,nt help
gonna check floats and check inlet gasket
can the crank case seal be replaced without splitting the engine

Crank out to do the seals.

Can be done with the motor in place, but it'll be tight on an MC18.
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Andy.
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More than likely cracks on your inlet manifold.The same happened to me on my NC30.
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got the same problem with my mc18. Can anyone shed some light on the problem? Just built my motor up inc rebuilt crank, all new everything inc a hi flow kit. Im getting the same problem but only when the tick over has been adjusted accordingly. When revved up the bike hovers around the 4000rpm mark then eventually settles but if i slacken the tick over screw right back off then it revs perfectly but obviously wont idle.

Carb balance.
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If I have to take the carbs off once more...

Churs Steve but im still a bit new to the nsr. How do i go about balancing the carbs or checking them?

There are 3 ways.

1, You can remove the two screws just infront of the inlet manifold on the carb bodies (on the outer side of each left and right opposite of the oil feeds) and attach a vacume guage as you would on a normal bike.

2, If you have a late MC21 you can remove the air box and looking down the throat of the carbs you will see two drill marks (one on each of the slides). Open the cabs slowly untill the top of one of the drill marks touches teh top of the carb throat and adjust the other so its the same.

3, Get two drill bits the same size maybe 1mm dia. Open the carbs and slide the drill bits under the slides. release the throttle and just the balance screw untill both drill bits and being held. If one is being held looser than the other twiddle untill they are under equal pressure.
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If I have to take the carbs off once more...

Churs Steve i will set about getting a vacuum gauge. I have checked visually when twisting the throttle open and both slides look as though they are synchronised but i know this way not be the case.

think i should check this , was about to do the crank seals