I'm gonna play devils advocate here....
Thinking out loud, not gospel!
If bike is stock, it will run with stock jets if everything else is correct. Exception being that the pilot may be a tad on the small side to make emisisons compliance on 80's bikes.
If bike has a less restrictive intake, the pressure differential between the inlet and outlet of the carburetor is greater. This increases the mass flow rate which in turn increases the velocity in the venturi which LOWERS THE PRESSURE at the NEEDLE JET opening. Lower pressure in the jet opening and the same atmospheric pressure in the float bowl results in a greater pressure on the fuel and more flows.
If bike has a less restrictive exhaust the cylinder pressure will be lower when the intake valve opens. This further increases the pressure differential and speed through the venturi.
At first glance it would seem that the jetting need not change. After all the vacuum changes all the time anyway and we get more fuel flow with more vacuum. That's what the tapered jet needle is for!
The mass flow of air and fuel is pretty much proportional to their respective pressure differentials for a fixed opening size. BUT the pressure in the venturi is roughly proportional to inverse square of the carburetors inlet and outlet pressure differential(Bernoulli equation). So, the increase in pressure to push the fuel is not linear with the increase in pressure to push the air according to the physis as well. The fuel mass flow increases more slowly than the air mass flow!
This is WHY we need a bigger jet with pods and open exhaust.
Aside: A skinnier venturi needs a larger jet than a fat one to obtain the same mixture.
OK. so now we know what's happening in the carb. The math is way to complicated and there are too many other variables affecting the flow to calculate the required jet size.
So what to do?
1)You can't run a big bike at WOT for very long unless you're on a dyno. If on the dyno, you don't need to be reading plugs.
So for us it's iterative work.
Know what the stock jet sizes and jet needle position should be. If pipes and inlet air system is stock and nothing else is wrong, she'll run just fine. If she doesn't, fix what's broke before adding hot rod parts.
....Proposed scheme for tuning with hot rod parts.....
Open the throttle all the way. If she pulls good to redline in as many gears as you dare, it's OK the way it is. The stock jets were either too big or all your expensive mods didn't change anything. If pull is 'weak' or engine pings then try a larger main jet. When she pulls for all shes worth you have the right size main. If you keep going up in size you'll feel no difference and just burn more fuel.
2) Open the throttle half way. If she pulls good to redline then your needle is OK. If she's weak and you have a tapered needle, move it up so a skinnier part of the taper corresponds to the same throttle opening. If it's a straight needle except at the tip(and it may be for some carbs on some engines) try the next skinnier needle. If you run out of clip, try a skinnier needle but be mindful of the taper and set the clip accordingly.
3) Open throttle 1/4 way and do a chop here
.
Finer tuning is possible by messing with different taper rates on the needle and different needle jets. IMO at this point dyno time is well worth the $.
So, no plug chopping except at 1/4 throttle. At idle you adjust by the max RPM method. At other 1/3, 3/4, and WOT you adjust by needle clip position and needle taper. At WOT you adjust by main jet size. Remember, the main jet gets done first because it affects all the needle settings.
Does this make sense??? Did I miss something???
The little old CB350Four is running 90MPH at WOT in top gear. The CB400FK will redline in 5th gear and bang the rev limiter at 3/4 throttle. It will bang the limiter in 6th but that's too fast for me. I can't imagine being able to run the kz1000 at WOT for any length of time. Maybe cut off one coil and do two plugs at a time
-Duck