needle and jet

This is a very general question, and I have little experience working on carbs:
If pod air filters air used to replace the stock air filter and box, then should the jet or the needle or both be replace? Also, are the jets/needles adjustable? perhaps it's model dependent?

There are three 'jets' coming out of my float bowl and into the carb venturi tube. One of them is covered with a rubber stopper, why?

Thanks, and btw I am working on a 79 kz400.

Usually the jet needles are adjustable, but some models are not adjustable (wiredgeorge and some others can verify this). Usually all that is needed is a main jet swap to correct the lean condition that is created when pods are installed. I encountered this when I installed pods on my 82 KZ550, switching to a larger mainjet cured it.

In short, if you install pod filters and/or 4 into 1 pipes on a KZ400, you need to rejet, else the bike will run lean; especially with the pod filters.

You have three jets on your KZ400: primary main jet, secondary main jet and pilot jet. You don't need to adjust your jet needle/needle jet.

The pilot jet controls the amount of gas induced to the air/fuel mixture at idle. It is the one that has a plug over it because it doesn't pull gas in through the bowl. There is a little opening from the main jet tube where gas is sucked over into the pilot circuit. If you leave the plug off, it will cause the idle circuit to be VERY rich and you won't get the bike to idle properly as it will be drowning in gas. This jet SHOULD be changed if you swap to pod filters but finding replacement pilot jets is difficult. They may call this a "slow jet" in the parts diagrams.

The "Main Jet" controls gas flow off idle to about 3/4 throttle or so... It is the round and barrel shaped jet set off from the center of the carburetor. Replacements main jets are available. You should go up from whatever size you are now by at least one increment in jet size if you need to rejet.

The secondary main jet controls gas flow at wide open throttle and this jet is readily available and should be bumped in size at least two increments from OEM jetting if you go to pods. Thre increments would also work fine I think.

Since the slow jet is fed via the main jet through a passage located above the main jet, the rubber plug must be there to avoid fuel bypassing the main jet and directly to the needle jet. If the plug is missing, the bike will run rich and there is no controle of the fuel delivery to the main system.

I would recommend that you buy the three next sizes of the secondary main jets, and then start with the largest. And the only way to find out if the mixture is correct, will be by riding the bike for 1/2 mile at full trottle on 3. gear, then pull in the clutch and stop the engine with the engine kill switch at the same time, stop the bike, and then check the spark plug colour. If it runs rich, you can go down on the jet size till you get the correct colour on the spark plugs. When the mixture is ok at full trottle, it's time to deal with any problem you might have in the mid range, or at idle.
It is also important that you adjust the fuel level in the float bowls to the correct level before you start tuning the carbs.

And you should also be aware that it is difficult to find parts for those carbs. Secondary main jets can be bought, but the primary main jets and the needle jets are hard to find. So in many ways, you will be better off by using the stock air box. And with the stock air box, and an aftermarket 2-2 or 2-1 exhaust system, most 400 owners have experianced that they don't have to rejet. But again, you can only find that out by testing.