Dominator missfire

I have just spent the last 3 months rebuilding one
complete NX650 out of a box of two so I had no guide as to how thigs were
assembled.
Now its done and on the road and all is well apart from it will not
rev above about 2000 under load and about4000 with no load without very
erratic running. it
starts perfect and runs perfect at idle and up to about 2000RPM.
Above that it runs a little like a 2 stroke when its 4 stroking.and
there is a little black smoke which led me to strip and check the carb but
after checking all jets,float height,and choke return, the piston
diaphram, and blowing the complete thing out its still the same. I
have changed with a spare the coil and the CDI unit and it has a
new plug but still its was still the same. Have tried it with the air
filter removed
as well. The only thing I havent changed that I can think of is the
pulse generator and this looks like a lot of work so I am asking here
first although it checks on on the restiance according to the manual
One thing I am not sure of is the two pipes that go the the
carb,Are they both vent pipes, One on the bottom and one on the side. Oh
and I have replaced the rubber carb mounting tube as there was a small
crack.
Any ideas??

Thanks in advance
Ken in England

 

Your idle mixture system is probably all plugged up.

There are online diagrams of the carburetors for the 1988 and 1989
NX-650's at
www.partsfish.com and www.bikebandit.com also has diagrams. These are
US models, UK models may differ.

Item # 5 on the drawing is the idle mixture screws, with its little
spring and o-ring.

In order to find the original setting, screw it all the way in,
counting the full turns, and write down the number. Then remove the
screw and be sure you don't lose the o-ring.

Squirt an aerosol carb cleaner containing xylene, toulene and methyl
alcohol through
the hole that the idle mixture screw came out of. The carb cleaner
should squirt freely out of the hole directly in line with the screw
you removed.

If you put your finger over that hole and squirt more aerosol, it
should freely come out of a pattern of 3 holes near the throttle
butterfly.

OK, Cover all four holes with your finger and squirt more aerosol. It
should come out of the pilot air jet in the carburetor intake bell AND
the idle (pilot) jet in the float bowl.

If you can plug all of the outlet holes with your fingers and plug the
pilot air jet with another finger and keep spraying aerosol through the
idle mixture passages, you will eventually get the idle mixture system
clean and you should no longer have the problems described.

Item # 9 on the drawing is a deceleration air valve. It operates to
reduce backfiring during deceleration. It's possible that valve leaks,
but I would go for the idle mixture passage cleaning first.

The one coming off the float bowl is a drain. A hose coming off the
side is probably a float bowl vent to make sure there as as little
pressure differential as possible between the top of the gasoline in
the bowl and the outside air. There is very little pressure head of
gasoline from the gas tank to the float bowl, maybe 1/8th to 1/4 of a
pound per square inch, so, if the bowl isn't vented, the float bowl
won't fill up by gravity.

Some motorcycles have vacuum operated fuel petcocks, but the drawing I
looked at shows a mechanical petcock that wouldn't need a vacuum hose.

The tubing system on the right hand side of the drawing is probably a
California emissions control system.