I don't have any experience w/a 2 carb Stovebolt 6 but I do have experience w/multi-carb setups on various other engines so I'll throw some basic checks out in hopes it'll help.
1st easy check would be to remove both aircleaners and let the engine idle.
Slowly cup your hand over the throat of the front carb and listen/watch to see if you hit a point where the engine rpm picks up. If it does your front carb is running lean, troubleshoot accordingly. IF it stalls you probably don't have a vacuum leak on it. Key is to do it slowly. If at any point it picks up you probably have a vacuum leak. Keep in mid that in any case you will stall the engine if you totally cover the opening.
If it doesn't alter the idle do the same for the rear carb, if it does troubleshoot accordingly.
Troubleshooting accordingly: use Jerrys suggestion on using propane or Mapp gas to check for the vacuum leak. I've successfully used carb cleaner but using gas may be quicker.
To add to Jerry's suggestions also spray around the throttle shaft where it pivots in the carb base as worn shafts and/or bosses are known vacuum leak sources.
You could also have a stuck float in one of the carbs causing a lean condition (stuck closed)or stuck open which causes an over rich (flooding) condition. Are you seeing black smoke when it starts missing?
I've used a "snail" carb flow meter to setup multi carbs on other engines but I haven't tried the Stovebolt 6 as of yet. They are very helpful in getting both carbs setup...
Stumbling under load can also be bad points, the low volt coil wire grounding out on something (in dist, engine block) cracked dist cap, wet dist cap etc.
Good luck & Semper Fi
Dave
