yamaha bogging down at half throttle

[mikkydee]

Hello to all you motorheads out there. I am seeking some help on my 1999 Yamaha 100 HP 4 stroke. Just acquired the boat with motor and it ran well on the test drive before I purchased it. Drove it home a few hour drive and put it in the water a few days later. Ran ok at half throttle but when I kicked it up a little boat started cutting out badly and bogging down. The longer I ran it the worse it got requiring me to continually lower the throttle to keep it from cutting out. I took it home and the next day I emptied the fuel tank to get all water out and flushed the fuel lines and carb bowls to remove any water in the system. I also replaced the water separator. The previous owner recently had carbs rebuilt and synced and spark plugs were replaced at the same time (he gave me the receipt for the work done). I replaced some fuel lines, replaced the water separator with the see-through glass bowl type, and added some Heet to the fuel tank. I really thought this would cure the problem as I assumed it was due to water in the tank that came from a missing O'ring on the cap (I discovered when troubleshooting why there was water in the tank). Took the boat out and it ran a little better but still will not go over about 2400 RPM without bogging down badly, and periodically cutting out. I am thinking it can potentially be a fuel pump delivery problem not supplying enough fuel to the motor at full throttle but that is just a guess. The fuel pumps on these things are about $139 a piece X two. I hate to just start throwing parts at it hoping the fix the problem especially since they are quite expensive. Is there a way to test the fuel pumps to see if they are able to produce enough fuel for full throttle or if 1 or both of them may be on their last leg?
I also would love to hear any other thoughts on what the problem could be that I haven't tried yet.

 

[mike_i]

What's the condition of the primer bulb? What's the condition of it when the motor starts to die? Is the fuel vent/line clear? Have you seen any crap/water in the fuel since installing the new see thru filter? Do you have any idea how much water was in the gas? Do you have access to another fuel tank to temporally run the motor? Did you buy the boat from a dealer, if so any warranty? Are the fuel line clamps tight are any of the fuel lines collapsing when the motor is bogging down?

 

[mikkydee]

Hi Mike, Fuel seems clear now as I have not seen any water in the new separator since I drained and refilled the tank. I replaced a bunch of the fuel lines and clamps thinking it could be pulling air into the fuel line somewhere so all that seems good now. primer bulb looks to be ok and isn't sucked in when the motor is bogging down. When the motor was bogging down I tried pumping up the bulb again to see if I could get more fuel to the motor. No change. I drained about 25 + gallons of fuel out of the tank and there looked to be maybe 1 quart to half a gallon of water in it. The boat was purchased from an individual in another state so no warranty. The motor has not even been broke in yet after a recent rebuild. It has like 3 hours of run time on it. I do have a 6-gallon tank for my smaller boat that is full of 2 stroke fuel mix right now. Most of my experience on boat motors has been with the 2 strokes I have owned previously so this 4 stroke is new territory for me. The fuel tank is vented through the gas cap/fuel fill it looks like so I am not sure how to check that. I can see a separate smaller hose from the tank to the filler neck which must be the vent line I am assuming.
I will see about draning the portable fuel tank of 2 stroke fuel and using that tank. I assume this would tell me if the problem is originating from the tank to primer bulb side of the fuel delivery or on the engine side itself.

 

[mikkydee]

Ok an update. I synced the carbs and removed the pickup tube from the fuel tank. No obstructions and ball check valve seemed to be working okay in the pickup tube. No issues with the tank vent. I hooked up a portable tank to the motor with some fresh fuel and duplicated the same issues. Motor bogs down at high throttle after a few seconds. Pulled apart the fuel pumps to check them and the diaphragms looked good. Just seems like somehow the engine is getting starved of fuel at high throttle. Primer bulb is fine on both the onboard tank and the portable one I used. I guess I will order a leak down tester next and give that a go. Got some new plugs also. Desperate for some ideas here.