kz900 A4 rough when warm - ignition coils?

Hi all Gurus!
Been assembling a kz900 A4 -76 with a kz1000 E2 (-79) engine. Done my first couple of short rides with it now. It starts nicelly and runs really well the first 5-10 minutes. Then it starts to be quite rough - specially in the lower revs. I know there's been quite a few posts on similar before so I have:
- Checked that there is no vacuum in the tank
- Checked the valve clearance (did also shim down a couple of exhaust vents that was on the "limit") - made no difference
- Tried to measure the ignition coils but just with a standard ohm-meter - at least the primary is reading ok
- Tried to feel if the ignition coils get warm - they get a bit warm - but not hot. Maybe the righthand coil is a little bit warmer.
- The resistor doesn't get warm at all (maybe it should?)
I'm suspecting something ignition related - maybe it's the coils... any hints would really be apreciated.

Cheers!

/Mikael

Coils don't typically cause the issue you mention. Who knows though with 40 year old motorcycle parts. A good place to start would be to remove the plug caps, check them for resistance, and if they check out okay snip off a 1/4" of the wire lead and thread the caps back in place. Changing the plug wires entirely would be a good idea too if you can manage the swap. I'm not familiar with those coils but on some of the old TEC coils the wires are glued to the coil proper, however you can still change the wire by digging it out of the coil. There is a spike at the bottom of the hole in the coil so be careful to not break this off or you will be buying new coils straight away after that.

Would double-check valve clearances.

Good Fortune!