Bad muffler

Will a bad (cracked) muffler on a 18hp kohler v twin horizontal engine make it run hot? Thanks in advance.

I suppose you could look at it this way, the cracked muffler might make the motor move more air, so it could be running lean which would cause hot running.
OR, did running hot cause the cracked muffler?

The only scenario that I can think of where a cracked muffler can cause an engine to overheat is if the crack lets enough cold air in immediately after shutoff that it would cause the exhaust valve to warp. A warped valve will not seal the other cylinder will carry most of the load and the engine will never be able to develop full power again, probably causing an overheat condition.
(I have personally never seen this happen on a Kohler engine, but it is why drag racers used to shove rags in their headers after a run.)

You are running the engine at full rpm, aren't you? The fan is what keeps it cool and the faster the fan turns the more air it can move.

There isn't a mouse nest under the fan cover is there? That will cause an overheat condition for sure as the nest keeps cooling air from reaching the cylinder fins where it is needed.

A lean carburetor will cause an overheat condition. Too much shrouding around the engine can cause an overheat condition.

The only scenario that I can think of where a cracked muffler can cause an engine to overheat is if the crack lets enough cold air in immediately after shutoff that it would cause the exhaust valve to warp. A warped valve will not seal the other cylinder will carry most of the load and the engine will never be able to develop full power again, probably causing an overheat condition.
(I have personally never seen this happen on a Kohler engine, but it is why drag racers used to shove rags in their headers after a run.)

You are running the engine at full rpm, aren't you? The fan is what keeps it cool and the faster the fan turns the more air it can move.

There isn't a mouse nest under the fan cover is there? That will cause an overheat condition for sure as the nest keeps cooling air from reaching the cylinder fins where it is needed.

The engine needs all its shrouds in place too. Its not a Harley going down the road at speed.

A lean carburetor will cause an overheat condition. Too much shrouding around the engine can cause an overheat condition.

A blown head gasket can probably cause an overheat condition if the engine is ran at close to rated load.

Like others, I doubt it. I would remove the shroud and covers and make sure the cooling fins are clear of debris.

Is the crack leaking back on the engine? If not, it should be fine

I think a leaky muffler can cause them to run lean, and overheat.