We learned quite some years ago from Universal tech service that the Atomic 4 likes a very low exhaust back-pressure; between 1 ½ and 2 psi. Back pressure in the 3 to 4 psi range will result in sooty plugs and carbon build-up in the combustion chambers. If back-pressure gets much above 4 psi caramelized goo (as seen below) builds up on the stems of intake valves until they are prone to stick.
Basically, exhaust back pressure is the resistance your engine has to overcome to push its exhaust gases (and in our case engine cooling water) out the back of the boat. Any blockage in this system will increase your exhaust system back pressure.
NOTE: To aid in measuring exhaust back-pressure, Moyer Marine sells an exhaust back pressure measuring kit which utilizes the 1/8” threaded hole in our exhaust manifold flange. This kit is designed to periodically measure the total exhaust back pressure down stream of the exhaust manifold flange (every couple years unless symptoms of excessive back pressure develop sooner). The kit is normally removed between tests. If you don’t have one of our manifold flanges, you can drill and tap your existing flange with this kit if you have sufficient access.
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.
The blockage in this photo is 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. The recommended fix here was the immediate replacement of the inline water entry fitting, in addition to the replacement of any clogged exhaust piping.
A valuable diagnostic technique was discovered during this episode. Whenever any one of the four spark plug wires was removed, this particular 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.
Another common cause of elevated exhaust back pressure is a collapsed inner layer of the rubber exhaust hose in the first couple feet as it leaves the water lift muffler. One of our more recent customers aptly referred to this condition as an “aneurism” in the rubber exhaust hose.
In more rare cases, sticky intake valves can also be caused by restricted induction air available to the carburetor. Some years ago, one of our customers was suffering sticky intake valves and the problem turned out to be his brief case which he inadvertently sat in front of the big 6” air duct between his engine compartment and the lazarette. This air duct provided the only available air to the engine and his briefcase blocked the air flow available to the engine for proper combustion.
Bottom line: If an engine with good compression, and a well-maintained ignition and fuel system is manifesting symptoms of sooty plugs, and sticky intake valves; there is a good chance that it’s suffering from an elevated exhaust back pressure, or restricted induction air.