them old C25's and 30's were pretty rough running at idle. NOTHING like the 395cc twin carb motors. Those idled pretty darn good in comparison. A sneeze at idle wasn't uncommon on the old 25/30 single carb motors, had to run them lean so they'd idle somewhat clean (not foul plugs), and until they were at operating temp they'd sneeze, fart, generally ran like poo. Replace the thermostat if you're restoring one. Makes a big difference in idle. If the thermostat won't open it'll never get warm enough to let a lean idle "work" like it should. Even then, on those motors, they were kinda noisy and rough at idle. Similar to Mercury/Mariner/Evinrude of the same or similar hp and year models. Also at full tilt they get kinda noisy too as some of the exhaust exits above the propeller and there ain't no muffler to quiet it down, just some water splashed around the tuner and exhaust to "muffle" it a little (very little).
yes you can make a 25 into a 30. Look at parts diagrams and compare part numbers. Carb, reed plate, and I want to say the tuner was different too but it's been so long I have forgotten all of the details. Make sure to check the timing too.
in the 1990's as the twin carb 25 yamaha came to be popular, the old 25 single carb--then called a C25--kinda fell off the map. I mean after all, it was heavier, a little slower, a LOT noisier, thirstier, rougher running at low speeds, and really smoky in comparison. They were the cheaper of the 4 25hp motors that the late 1990's were graced with (C25, 25 twin carb, F25, and 25 3 cylinder). Really the only folks that bought them were the ones who wanted the cheapest yamaha 25 they could get. Dealers--including the one I was working for at the time--had to give them away.
