Just got a 1985 2700...I have the same issue, but mine are all messed up...let me know how yours goes. I'll post some photos next week as it is getting the bottom repainted right now. Once I get it back, I'll take photos of mine and we should compare notes.
I have a 1995 3100 Open and all 4 are leaking. I think it is the black rubber seal. The 2 turn locks are tight and press the window firm but, when water hits them for a bit I get lots of water running down the inside. Change the seal or the entire window? Thanks
Have a 84 3100 Convertible. Does any one know where to get the rubber seal for the porthole windows? It appears to be 1/2" square with rounded corners. Haven't been able to find a source.
Make sure that you see how to adjust the pressure on the levers. It's not the metal screw that the lever pivots on, but a black screw recessed in the lever arm about half way out the lever.
I "lube" the black gasket with vaseline every year. I never open the portholes so the grease in there seems to help seal them better. The other thing is check the seam on the outside between the inner and outer halves. The caulking tends to peel away from the stainless after time. If you see this, pull all the caulk you can and re-silicone.
If there is black rubber on your portholes because the heat has melted the gasket on to the porthole window you need to clean the rubber off the window. If you see that the rubber gasket has micro cracks and look as if it is deteriorating you can put some black silicone over the gasket, smooth it on and let dry for 24 hours before closing. Ultimately a new porthole is the best repair for a gasket that has seen better days.