99 Yamaha 25 two stroke surging at 3/4 to full throttle

[Av8erdunn]

I have a 99 Yamaha 25 two stroke that's surging on top end. I have rebuilt the oil system, taken the carb apart numerous times while cleaning the Jets, float needle valve and flushing every port using alcohol. After all that it's still surging on top end. I Can start to feel it at a little more than half throttle and it gets worse the more throttle I give it. It surges every couple seconds and drops maybe 100-150 rpm if I had to guess. The hose is new and the fuel tank is new. Fuel filter is clean and spark plugs are new. What am I missing?

Thanks in advance.

[erniefish]

How about checking fuel pick up ? If it was my boat, that is what came to mind to check , even though it may be remote tank , it may have some " funk" on the pick up,.

[Av8erdunn]

Fuel pickup is fine. It's done the same thing on two new fuel tanks.

[Av8erdunn]

Changed the fuel pump diaphragm today with no luck on fixing it. Anyone?

[Seacraft12]

How are you cleaning the carbs? I had a similar problem and I fixed it by totally dismantling carbs and soaking them in carb cleaner over night, followed by using the spray carb cleaner in the little spots followed by compressed air.

[boscoe99]

Remove the air intake from the motor. Have someone mist some gasoline into the throat of the carburetor(s) when it starts to surge. If it picks up and runs for a brief period of time you have a fuel delivery issue.

In addition to completely disassembling the carburetors I find if they are not cleaned in a heated ultrasonic cleaner using a good cleaner such as Yamaha carburetor cleaner, you might be wasting your time. I don't think that alcohol alone can get the job done.

There are some incredibly small passageways within some carburetors that are difficult to clean. Using some soft copper wire to probe and rod out those passageways can help.