After some remarks [ your machine smells like unburned gasoline ] and low mpg [ 1 liter at 13 km's or apprx 30 mpg ] we found that at low revs only the front cylinders are functioning. The rear exhaust pipes stay cold while the front are already hellish hot.
Only when the revs go up [ say 4krpms + ] the rears come in.
The exhaust fume however is dark, wet and smells after unburned gasoline.

Running 120 main jets, 35 pilot jets [ changed from 38s ], air screw 2 full turns out. Standard exhaust. New spark plugs [ nkg er9eh 9 ]. New air filter from HiFlo.

Already done: cleaned the carbs with Motul carb system cleaner. Took the carbs apart and had them cleaned ultrasonic. Checked the float valve needles: the spring action is verry verry poor so this might lead to problems?, checked the float levels [ 6.8 mm ].
Followed cam's tip to soak the diaphragm. Polished the carb slides and they are moving smoothly.

Regarding the electrical part: measured the ht coils primary and secondary resistance and they are within range.
The sparks all give a nice blueish spark when tested. Exchanged the coils and spark cables.
All with the same result: Front cyls ok, Rear cyls fail.

We have run out of options so any help is very much appreciated!

Regards,
Frank
[ my son bought the machine in april and it was equipped with the ultra loud Tyga race can. Therefor the 38s and 120 jets. Needles are non standard too. ]

Changed the plugs at all?

@bikemonkey: yes, changed them all.
Saw this same item only after I've posted my msg: viewtopic.php?f=69&t=38607&start=10 where 'Drunkn Munky' mentiones the 3 holes of the pilot circuit.
I can't find any reference as where these holes are exactly.. Only have this picture:



I forgot to mention 1 more important thing: oil in the crankcase smells like fuel.

Hi, the three holes are hidden by the carb butterfly.

Pull the carbs off and open the butterfly by hand fully, there are three very small holes in a triangle formation. Carb cleaner and a blast of compressed air on these and see if that solves it.

Also have you balenced the carbs, it could be that the rear two butterfly's are fully closed when at idle?

I balence them by eye using the three holes mentioned as a guide, line all the butterflies up so you can see the same amount of hole on each one, if that makes sense!

Try that and come back and let us know

EDIT: as my iphone autocorrect wants to mind its own business!!! piece of sh1t apple crap!!

If the oil smells like fuel, change it ASAP.

Sounds like you have a leaking float valve and maybe a faulty petrol tap.

Did you have the carbs cleaned professionally? Or at home?

Butterflies open exactly the same amount [ drill bit method ] so they must be unbalanced vacuum wise. Will sort that out after running ok on 4.
Regarding the float valve & needles: 1 of them was very poor. The internal spring was not reacting smoothly. So I ordered a non-genuine-but-applicable-for-nc30 float valve. [ ebay item 180682297963 ]
Quick tip: don't buy them , they are s..t! Vilton tip is larger than original resulting in not closing/opening as supposed to. After cleaning [ yes... home ultrasonic ] they function much better.
Will take the carbs of today again, attach external tank and see if they will leak. Petrol tap seems to be ok [ vacuum membrame functions normal and...the front cylinders are functioning normal as said. ]

I didn't know that microscopic eyes were nescessary. The 3 pilot holes on the rears were blocked. Cleaned them.
Now running on 4.
Muchos grazias 28hodge for the tip.

Will change oil right away and still perform a compression test.

Great news glad its gotten you sorted, had the exact same issue with mine as it had stood for years with fuel in it, even after home ultrasonic cleaning it wouldn't idle correctly until I blasted air through those holes.

Now I only wish I could solve my own problems as quickly!!!