First make sure your timing is set to where it will run well and not to much total advance. The 15 initial seems to be in the bag so now set the total. Just hook up the light and slowly rev the engine and watch the mark. Or have someone do the revs while you check it (easier). For starters you want to fall between 35 and 38 deg total all in by about 2500 RPM. Also no vacuum leaks or other mechanical problems. You can't tune carbs until every cylinder is spot on and each contributing equally to the engine output. I can't stress this enough. Once that is dialed, and the engine is at operating temp then proceed to the carbs.
Is your intake a dual plane type or a divided single plane? I'm sure it is as that is the most common. Are both carbs hooked so they work together as 1:1? Not progressive like a hemi? I'll talk more about this later but for now you need to get a good basic adjustment.
Start by disconnecting all the linkage except the throttle closing springs. Back off both primary idle stop screws until the throttle blades have seated in the bores. Then run the screws in until they just make the throttle arm move and go one full turn. Both carbs should be set pretty close at this point. You can start the engine and see where the idle is and if it's low or high make equal adjustments on both carbs. For mixture follow a similar procedure. Lightly seat all screws then back off 1.5 turns and see where you are. To rough in the mixture make adjustments in 1/4 turn increments equally on all 4 screws, moving either lean or rich. It helps to keep track of where you are so pay attention! There will be some dancing back and forth while setting ideal idle speed then following up with a mixture tweak, and when I have things pretty close I follow up with a lean drop method on each mixture screw.
That final lean drop mixture tweak is where I zone out everything around me and listen to the rhythm of the engine while making small adjustments and pausing for the result. Lean drop is finding that fine line where the engine RPM just starts to drop then go another 50 RPM lower. There is often a small time delay between making the mixture adjustment and feeling the result. Most hot cammed engines might like to be on the plus side of the lean drop so find that fine line and go maybe 1/8 turn richer. Proceed with each mixture screw, finding that fine line on each one and make the appropriate adjustment.
Once the carbs are set loosen all the linkage lock nuts and make necessary adjustments as you hook it up while not disturbing the carb settings. The linkage should not tell the carbs what to do at idle. Once you have the linkage connected but adjusters still loose, apply force where the cable connects to open the carbs slightly and observe that both primaries are opening EXACTLY AT THE SAME TIME (in a 1:1 application). The slightest movement from the throttle MUST result in EQUAL movement of the primaries, while taking into account all the linkage slop, etc... This will take a few tries.
This is the best I can explain without actually standing over your fender doing it. I have tuned tunnel ram setups, cross rams, six BBL's, Tri powers, individual runner Porsches and VW's, mechanical fuel injected Porsches (air balance only), Honda CBX motorcycles and probably a few others I forgot all using this method. It's not magic; just paying attention and having a good ear for a lean misfire.
