As an example of possible causes of exhaust back-pressure, a customer was reporting a serious power loss, even in neutral. The engine had become more and more sluggish over a two year period, and by the time the customer called us, the engine would not accelerate past 1500 RPM, even in neutral. Whenever the throttle was advanced much past idle, a “gasping” or “gagging” sound came out of the mouth of the carburetor, along with small droplets of fuel.
After much “let’s try this” troubleshooting and repeated cleaning of the carburetor (all to no avail), we finally convinced the customer to separate the hot section of the exhaust from the back of the exhaust manifold. It only took several seconds of running to determine that the engine ran perfectly OK with the exhaust removed.
Further investigation revealed that the one and a quarter inch exhaust pipe had become at least 80% restricted with rusty scale in the immediate area of the water entry point, just ahead of the water lift muffler. We never got a photo of this particular exhaust blockage, but here is a photo of another similar blockage which we discovered in a section of exhaust pipe that was left attached to an engine we received for rebuild.