A trend that we probably would have never recognized as a problem, were it not for the sheer volume of our service calls, relates to the pesky business of clogged exhaust systems.
In the latest manifestation of this problem, a customer was reporting on 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” 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 remove 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.
The blockage in this photo is also in the immediate area where engine cooling water is introduced into the hot section, which leads us to conclude that minerals and other crud coming in with raw water have a tendency to build up serious encrustation in this area. Notice that only the area directly above the piece of pipe used as a hose barb is open for exhaust to pass through. The fact that the water nipple comes so deep into the pipe doesn’t help this particular situation.
A valuable diagnostic technique grew out of this latest episode. Whenever any one of the four spark plugs was removed, the engine ran and even accelerated somewhat better. Clearly, by the time a restriction grows to this extent, the exhaust system is only capable of handling a three cylinder engine. As soon as the forth cylinder is added, the engine bogs down due to being unable to discharge its exhaust.
Blockages of this magnitude don’t develop overnight. There is clearly a time period of several years of buildup, before the acute symptoms described above will begin to manifest. During this time, the engine will show early signs of the blockage, such as losing power, fouling plugs, and/or experiencing sticking valves.
Unfortunately, these symptoms can also be caused by more routine things, like low compression, poor ignition, rich fuel mixture, etc., so I am at some risk of initiating a bunch of exhaust system work, fixing things that aren’t really broke. However, in cases where compression is good (85 psi or better), and an engine is otherwise being well maintained, it could well be suffering the early stages of exhaust system blockage if it is still manifesting one or more of the above symptoms.
In those cases, removing the hot section of your exhaust system for inspection and replacement gets my personal vote for a most important winter project, particularly if it has not been removed for 30 or more years and is showing signs of age.