Perhaps small creatures built a home in the cowling somewhere.-----Also look for wires that might be touching the cowling and killing spark to a cylinder.
Interesting comment. Here I have a "dirt dobber" problem. I get them by the thousands durning the summer and you won't believe where I find the nests. I have been concerned about them getting under my cowl and in my intake manifold to do their dirty work, thinking what their decaying nest's dirt will do for my engine.
I would dismantle anything in front of the carbs and inspect for such. Also look up under the cowling, usually near the top of the engine where air is allowed in to feed into the cowling for the carb inputs. A big nasty nest could be blocking that off.
They are just amazing...they live to build nests and lay eggs seems to me; very determined. The other thing amazing about them is their navigation system. They look around for a dark place to build a nest and seccure a route to that nest they decided to build.
Sometimes I deliberately block their path with something. They test different directions in an attempt to get around whatever it is to see if they can get back on the "beam" (for lack of a better word) back to the nest.
I close my big 12'x 12' sliding doors sometimes to aggravate them. Some go along the doors testing for an entrance till they find a path to get around the doors in the cracks along the side of the door to building interface. Others go around the building and come in a side door. Once they find that route they use it on return flights.
They love electric motors and have ruined several around the shop. I have to build screen wire cages around things like my drill presses, band saws and hand operated electric tools.
A real PIA I assure you.
