Gene, running a diesel up to temperature and at moderately higher RPMs periodically is better for a lot of things that are part of the engine's system.

You cloged up riser could be the function of too little to often -- as with your muffler.

However, it is possible (which I found out from several mechanic) to have a collapsed (or delaminated) exhaust hose. That can cause back pressure or reduced flow which can lead to calcifications in the raw water exhaust systems.

Make sure your vented loop, hoses all the way back -- plus the muffler are clear. It's hard to see collapse of the hose lining from the outside of the hose. It's just that inside lining "shirks" away from the hose.

(We didn't have this problem, it was something stupid that no one figured-out until way too long after they went searching -- like the first thing that they supposedly fixed. But it gave me a total system tour of our water cooling system. Cost me a lot of time and frustration on a "vacation".)