I say plugged risers, because here in the north we winterize and our motors sit, depending on how one winterized their boat each year, our blocks and exhaust components are highly susceptible to building rust and causing cooling and or exhaust issues due to the rust breaking free in larger pieces and plugging the small orrifices. Risers in my category should fall on a basic PM weather fresh water use or non just to avoid the what ifs.. but like ya said pap, cracked manifolds can do the same. Look what another member of the forum went through recently... cracked heads...
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