'86 Johnson 60 with odd performance issue

[Tin Yachtsman]

My project boat has made it to the point of having "sea trials", and I have had an outboard issue both times I launched. I want to see what the community here thinks. At launch, the boat idles pretty well, purrs like a kitten at WOT, has a nice top speed and planes the boat well. There is a good stream of water, so I believe the outboard is running cool enough. But about 20-25 minutes into the run, the outboard seems to starve for fuel and will only run at, or just above, idle no matter what I do with the throttle. The first trip I immediately returned to the dock at idle, but yesterday I anchored for a couple hours and fished, thinking that the outboard might come back after cooling down. No luck, and I had to idle back in again. After the first trip, I cleaned out the fuel filter, changed gas, and added Seafoam to the gas to clean out the fuel system. Other posts have talked about gas tank venting, but I am not sure how to check that out on my system. Any other suggestions are appreciated. The motor is an '86 Johnson 60HP. It had a VRO, but a previous owner disabled that.

 

[Pappy]

If you think it is fuel related then when the issue occurs pump the primer bulb by hand and see if the engine picks back up and runs. This will tell you if there is a fuel pump issue. If you are still running a VRO with the oiling system and tank then make sure you have oil in the gasoline when you do this.
If the power loss is sudden it would be more like ignition break down. If it is more gradual then fuel would be more suspect.

 

[Buddychrist]

If you are thinking it might be the vent just take the gas cap off of your fuel tank and give it a run. If it runs good with the gas cap removed then you either need to replace the tank or fix the vent.

 

[Tin Yachtsman]

If the power loss is sudden it would be more like ignition break down. If it is more gradual then fuel would be more suspect.

Can you suggest any specific tests I can perform on dry land? I'm not going to have an opportunity to launch again until next Monday. I'm just confused as to why the outboard would perform so well at first and then bog down, whether it is a fuel or an ignition problem. Thanks for the input.