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Welcome to the forum. I hope you keep the bike all stock. These are very hard to find in this shape. I do see some one at some point changed the stock seat to what I think is a Saddlemen Double-Bucket Touring seat.

 

If it idles then no need to mess with the drilling. All those screws do is set idle mixture. If idling fine and as soon as you open throttle more there are issues, problems are likely elsewhere. Bike sitting that long would have me suspecting the diaphragms on the slides first.

 

Make sure you got the ones for 77-79 carbs as the ones for 80-82 are larger.

 

The xs400 used two different bs-34 carbs. The 77-79 used the mark 2 and the 80-82 used the mark 3. The 77-79 use a smaller slide than the 80-82. The outer part is the same 73mm but where the slide attaches it's not.

 

Make a manometer to sync them. I would make sure the carbs are clean and adjusted. Make sure the ignition points and advance unit are clean and moving as they should. Also do a compression test of the motor. You should get 150 psi or more. If it's under 125 your rings could be bad.

 

With ear close to exhaust exit the two sides should sound close to the same intensity of noise and speak at the same rate as far as impulses. As in pum....pum....pum.... does not compute with pum..pum..pum..pum.. or pum..........pum.......... You want them both the same.The idle mixture screws should be very close to each other in the number of turns open or something wrong there.Then you adjust the individual exhaust rate using the cable adjuster on that carb.

If you have good ears you can sync dead nuts using that alone on twins, I used to do it all the time, even got good on triples doing it.

If not synced, when you slightly open the throttle one side will speed up the impulse rate but the other won't, it will sound messed up as the side not opening on time will still be speaking slow. You don't need any tool to pick up on that unless you can't hear.

 

You can make a manometer for a few buck and it will save you a bunch of guess work. Mikuni put that sync screw between them for a reason. Use it. After that adjust you pilot mix screws for best idle. 3 turns out from a light seat is a good start. Be careful as the 77-79 carbs have a tendency to have the tips of them break off in the bodies. You may want to check that before anything just to be sure.

 

You need to pull off those caps

 

Yes. Make sure your adjusting and testing with the air boxes on the bike. They will effect your fuel/air mixture greatly.

 

Be careful removing the idle limiter caps, on some designs the idle screws underneath are made to break off easily doing it as you are messing with emissions settings. Open the screw up some as removing the caps tends to put side force on the screw, you don't want that anywhere near the seat when it happens.

I forget these use a positive linkage setup instead of individual cables, in that case any sync linkage or screw is the same thing as adjusting a cable. Usually one carb is chosen to be the 'base or master' and non-adjustable, the other(s) then set to match it.

If you guys are talking a single position manometer then you are setting to pure idle only and only part of the work. You are only doing a half sync like that, complete has you barely opening the throttle to check the drop off matching of both (or all) cylinders as you just crack the throttle and you think both are even at idle but they both can then act different at pullaway. The difference if there can say more than one thing and you can't tell that without two gauges on a twin and even then they need to be a matched set. Why I never use gauges on a 4, too hard to get 4 of them at the same setting, far easier to use the tube type that has all cylinders pull off the same medium to be perfectly matched so there is no question as to the action of each cylinder. If you pay attention you can even pick up on say one cylinder being slightly weaker than the other to then match them a bit cockeyed at the dropoff to then idle smoother and give a better pullaway too. When engines get older the cylinders have different demands on the carbs at idle and off-idle and you can fix some of that to not be as irregular as it normally would be. Simply getting the curb idle vacuum the same will not do any of that.

Of course, yours and do what you will.