If the hot section of your exhaust has developed a leak, you are much better off to replace it than to attempt a repair.
Removing the 9/16″ hex-headed bolts of the exhaust flange can be a real problem if they have had water damage over the years. Your best chance is with a six-pointed box-end wrench and rubber mallet (or socket if space permits). If the bolts fail or break off, the easiest approach is usually to remove the manifold and take it to a shop to have the bolts ground off, drilled out, etc. If the hot section is not that large, you may be able to simply remove it from the inlet to the muffler, or by cutting it off as close to the manifold as possible with a hacksaw).
One of the benefits of removing the manifold is that you can have a local machine shop hot tank it to clean the water jacket. – Updated: March 24, 2004
The piping between the back of the manifold and the inlet to the water-lift muffler is normally called the “hot section”, or “dry section”, due to the fact that this section of the exhaust system is not cooled by engine cooling water. The only engine part of this system is the flange which bolts to the back of the manifold. The center of this flange is threaded to accommodate inch and a quarter pipe threads.
The most common approach is to construct the hot section of the exhaust out of regular inch and a quarter pipe (black iron, stainless, brass, etc.). This pipe is available at most local plumbing supply shops. The hot section should basically extend as high as possible (no doubt the reason that it is sometimes called a “riser”, and then down to the inlet of the water-lift muffler which should be mounted as low as possible. This vertical dimension affords protection against water backing up into the exhaust manifold.
If you’re replacing an existing system, it is very convenient to take the entire hot section to the local plumbing shop and simply have them match up the same nipples and elbows that were used in the original system.
Engine cooling water should be introduced just before the water-lift muffler. You can drill and tap a 1/4″ pipe-threaded hole directly into the piping of the hot section, or install a “T” fitting, with the stem of the “T” bushed down to receive a 1/4” male pipe thread x 1/2″ brass hose barb.
NOTE: We now list two different types of water entry fittings to facilitate the introduction of cooling water into the hot section (product numbers CSOT_01.1_324 and CSOT_01.2_334). Search on “324” and “334” (without the quotes) to find them.
Manufacturers of water-lift mufflers like to see 8 inches (or so) between the water entry point and the muffler, but many boats do not have the luxury of that kind of vertical space. The Catalina 30 fleet, for example, has barely a foot to work with, due to their center cabin-mounted engines.
West Marine stocks 4 or 5 different brands of plastic or fiberglass water-lift mufflers. I’m sure any of these units will work fine. In case you might want a recommendation, we believe we hear a slight consensus from among our customers that the Vetus brand may have an edge in terms of reliability.
In operation, while the engine is at idle, it’s common for cooling water to “batch” out from the back of the boat, as water in the water-lift muffler sequentially builds up a bit of head while the exhaust builds up enough pressure to blow it out through the exhaust. At RPM above idle, there is enough exhaust pressure and volume to keep the water moving through the system, so no head of solid water ever builds up.
If you have a copy of our service and overhaul manual, you will find more details of exhaust system replacement in Chapter 7. – Updated: March 24, 2004
A trend that we probably would have never recognized as a problem, were it not for the shear 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. The blockage was also noted 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. Only the area directly above the piece of pipe used as a hose barb was 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. – Updated: November 4, 2003