Whizzit that the joker valve is the first thing anyone suggests is the culprit for ANY sanitation system problem? I get emails from people whose toilet has decided not to bring IN any flush water, and the first thing they say they did was check the joker valve.
Most likely this problem isn't the joker valve or even in the toilet...the holding tank vent is blocked. All tank vents--water, fuel and waste--have two primary functions: to provide an escape for air in the tank displaced by incoming contents and to provide source of air to replace tank contents as they're drawn out. When a waste tank vent becomes blocked, air can't escape through the vent so the tank becomes pressurized, creating backpressure as the air seeks to escape wherever it can...the toilet discharge line is the line of least resistance. And that's the most likely reason your toilet is "burping." A blocked tank will also make it impossible to pump more than a gallon or two out before the pumpout or macerator pump pulls a vacuum. An especially strong pumpout pulling against a blocked vent can even implode a tank. And you weren't too far off when you wondered if your toilet was getting ready to explode, 'cuz if a tank becomes sufficiently pressurized, it can cause an eruption back through the toilet or even burst the tank.
Tank vent blockages are especially common in the spring 'cuz mud daubers just LOVE to build their nests in vent thru-hulls. So before you do anything else, get out a screwdriver or an ice pick or whatever works and scrape/dig whatever has set up housekeeping in your holding tank vent thru-hull. And you do that from OUTSIDE the boat. Don't use the toilet again or attempt to pump or dump the tank until you're sure the vent is clear. Chances are, if it's been at least several hours since the toilet was flushed, enough air will escape through the toilet to relieve the pressure, but I'd still be verrrrry careful when you open the cap on the decK pumpout fitting. And don't even THINK of removing the vent line from the tank until you're 100% certain the system isn't pressurized.
If your vent thru-hull is the usual cap with a slit or a few tiny holes in it, best thing you can do is replace it with an open bulkhead or "mushroom" thru-hull that you can stick a hose nozzle up against and backflush the vent line to keep it clear. As is true of most things, prevention is easier than cure.
Dave, you said, " I may have to rethink my system as I move to a freshwater flush system."
Why? As far as the holding tank is concerned, flush water is flush water...whether it's fresh or sea water.
Cardude...I'll be glad to help you sort out how your toilet--the whole system--works and how to maintain it. It's not rocket science, just a short learning curve that, once you understand it, will help you prevent problems instead of fixing 'em.
