Carb, timing, or both?

I have a ‘86 Johnson 20 on a 14 foot boat. It starts hard. I can chug around all day at idle speed when I’m trolling. Basically idling in gear gives me 2.5-3.0gps speed. Pop it out of gear to land fish, back into gear to troll again. No problems. Runs for hours.
Trouble is, if I run it down the lake at speed, it will not start again. It stalls as soon as I throttle back. It will go 26.0gps opened up, with 2 adults onboard. It runs smoothly at WOT. But no way, no how will it start again, like until tomorrow. Anybody ever experience this? I thinking a carb rebuild is in order, but wondering if there’s something else I should be looking at also. Thanks, Bill

had a 25 do this exact thing. Ended up being a sheared flywheel key.

Pappy, who posts on here is THE GURU, but it could be the idle adjustment. Usually if an OB stalls when backing off from crusing speed it is sign of improper idle speed adjustment, typically too lean. See photo, steps 6 and 7, but follow all.

BTW, you can buy these online for $20 or less in PDF form. Plus there is a post somewhere for access to free OB manuals already posted on this site, like a library.

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next time you go out, loosen the gas cap on the tank. not talking about the vent, but the cap itself. worked on a neighbors yamaha 25 last year. he had same problem. the chinese vent (rubber duckbill) in tank would not vent the pressure OUT! only in.

Thanks for the suggestions.Pappy you are correct, no air box on this model. I did as you said and duplicated the problem. 1: I am getting a spark. 2: I pumped the bulb and activated the electric primer; it doesn’t make any difference. Also, the slow speed-knob on the carb is adjusted correctly per the literature. So what’s the next step?

Thanks Pappy. It’s gonna take a week or two before I can get the boat back in the water. My tow vehicle has electrical problems that need to be straightened out first. I will do as you suggest and post the results in a couple of weeks. Thanks again! - Bill