Hi Rich,
Your burning the propshaft seals is pretty rare but it does happen occasionally in boats that use jackplates, in some tunnel hulls, and often when operated in water with plant life or debris. Aerated cooling water getting ingested into the water inlets can contribute to the problem. It is not a common problem for normal recreational boating.
Your dealer wanting to pull the powerhead and drilling out the cooling water passage in the inner exhaust housing accomplishes 2 things. It allows more water into the exhaust stream to cool the hot gasses plus the larger hole has less of a chance of being restricted or plugged up when running in sandy or debris laden waters.
Your dealer can test for aerated cooling water by running the boat using a piece of clear hose temporarily attached to the EMM to VST water hose and watching for bubbles at various speeds. In addition, if you have a recent computer printout of your engine, you can examine the EMM temperature profile. If there is a noticable percentage of time within the 120-160°F range or even higher, then that is a dead givaway that the cooling system is ingesting aerated water from the boat or from the motor being mounted too high and sucking air in. Aerated water does not absorb heat like liquid water does. It is similar to a "head of beer" that does not quench a thirst like a mug of liquid beer does. It could also mean that the cooling system is restricted in some way, often from mud, sand, or debris.
You may want to have the above tests and inspections done before going through the time and expense of pulling the powerhead and exhaust system apart.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
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The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.
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