I guess the bike in question is a twin since you mention 79... In any case, does the bike have the stock airbox on it when you try and start it? Mikuni BS series carbs can be fussy to start if you lean them (not using an airbox/filter) and they have the stock jetting. If you DO have the airbox/filter in place and you can't get the bike to run, then you need to check some things...
First, does this bike have a vacuum or gravity petcock? If it is a vacuum petcock, put it in PRI (prime). If gravity, put it in ON position. Take the hose off the carbs and see if gas comes out. If not, then crud inside the tank is obstructing the flow of gas. You will need to remove the petcock and clean its screen that is inside the tank. Use an inline gas filter...
If gas flows out the hose, then check the vent lines are not pinched or blocked or obstructed. See the "BS carb connections" page available for viewing from the main page of my web site (
www.wgcarbs.com).
OK, if the vents are clear, get a clear and flexible plastic hose. Put it on the drain nipple of #1 carb as you sit next to the left side of the bike while the bike is on its centerstand. Open the drain screw a few turns while holding the hose STILL and against the side of the carb so the end comes up OVER the line where the bowl gasket is. The gas will fill the hose... tap out ANY air bubbles as these give very false readings on the gas level... Anyway, gas should come to about 1/8" below the gasket mating line of that carb. If it doesn't, you will need to remove your float bowl and carfully bend the tang (the bit of metal on the float that contacts the needle) up or down to adjust the level. Bending the tang UP puts less gas into the bowl and down puts more. Anyway, refill the bowl and recheck the level and repeat till you get the correct amount of gas in the bowl.
I would be pretty surprised if one of these things, done in the order I suggest, doesn't fix your problem.