Start it and get a can of brake clean and spray the heck out of it around the intake and carb base's. Do one area at at time and listen. It won't hurt anything and doesn't leave a residue like carb cleaner. Check the throttle cable and make sure it has some slack in it. If that checks out than on the carbs it could be the the idle screw on the slave carb keeping it idling too high if your running progressive. If it's 1 to 1 it still could be that when you lower the idle screw out all the way on the one carb the other carb idle screw is stopping the linkage from idling down.

I run this breather on one side and a pvc on the other.



Found this on my hemi using the brake clean method. It had us stumped because it didn't sound like anything we ever heard maybe because it was leaking at the top and front at the same time???