Check to see if you have good water flow at the generator exhaust. I had similar issue. Like JB Marsh I also found bits of impeller in the heat exchanger and narrowing of the water ways due to build up. I disconnected the hose coming from the exit side of the raw water pump, removed pencil zinc...
Mark.60 and Tenacious, I pulled a chip from the bilge area and had a local store color match for me. They added the following; to one Qt of the White Marine Top Side. 10/10 perfect match for my bilge.
Do you happen to still have the formula, also did they use a tint base or the white? I looked at Home Depot and can only find solid colors. They do have one called "oyster" but on line it doesn't look like a perfect match.
Do you remember what the difference is? Aside from $100 more for the Ultra II's. I need 4 in total so the price difference adds up. I have one bad one in the stateroom and one over the helm but if I have to remove the hatch to replace the rivets I might as well do them all while they are out.
Google Onan key 132-0374, seems readily available with bunch of sites to order from.
https://shop.cummins.com/CSSNAStore/s/product/a2w4N000003sJUiQAM/onan-key-1320374
I was taught to top off the bowl with diesel after replacing the filter to keep air out of the system and always have done it that way without any problems. I only drain the bowl when I see deposits in there, I have been using the Frank's treat every ounce of fuel method found here on CSR for...
Love Mustangs! This 72 belonged to my Dad, he bought it brand new, kept it up pretty well and occasionally still drove it until he died at age 84. It took me several years to restore it and I hope one of my kids will want it some day when I am gone.