Quote:
. . .But after about 5 to 10 seconds at 5000 rpms the slight to moderate surging starts again.
Could this be that I'm not getting enough fuel to the engine / carbs. IE old weak fuel pump??
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What you've described is how long it takes for fuel in one or more float bowls to get low enough to reduce fuel flow thru the main jet, so one or more carbs is starting to lean out. I'd bet on the top one. Be careful if you're running premix, because you're starving that cylinder for oil as well as gas, so it's a quick way to burn a piston or score a cylinder. Don't run it long in the surge condition!
You can tell which carb is running lean. Take off motor cover and pull the air baffle/inlet cover off the carbs. Have someone else run the boat, and shine a flashlight down the carb throats with the engine running wide open. Fuel will be pouring out of the venturi on the good carbs and it'll be obviously less on the one being starved for fuel.
Could be a weak fuel pump OR some restriction between pump and carb. Did the dealer replace all the individual hoses between the pump and carbs? Old fuel lines can build up a film of crud on the walls if you let fuel sit in them long enough. This stuff will eventually liberate and can PARTIALLY plug the carb inlet needle and seat; it flows ok at lower speed but won't allow enough fuel to pass at higher speed. Ethanol fuel will make it happen quicker. I had this happen once on my old 115.
Also make sure the fittings on your portable tank/fuel line are compatible with a big V-6. I'm not familiar with Yamaha's but OMC used to use 5/16" fuel lines on V-4's and 3/8" on the V-6's. Not sure if the connectors were any different. My old dealer told me of a problem they found on a boat one time that had been repowered from a V-4 to V-6 . . . the inside passage in the 90 degree fitting in the fuel tank wasn't drilled out to correct diameter! It was OK for the V-4 but caused a blown powerhead on the V-6!
