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First of all 1983 Outrage 18, 115 Yamaha 4 stroke.
What's funny is I came on here during summer with a problem similar to this, and it turned out to be barnacles on the bottom. This however is not that.
All geared up for a stiper trip, cruising along at normal speed normal rpm (close to WOT), and bam speed cuts back but rpm's stay where they are. I slow down thinking I may have hit a crab pot or something, then go to speed back up and speed and rpm are normal until about 3500-4000 rpm, where the speed cuts back and rpm's surge to 6000.
I know what most of you will say, prop slippage, and I'm testing that today, but we started seeing an unusual amount of smoke coming from exhaust. Plus engine started sounding a bit rough. After much research from my last issue in June, and now this one, I can only think of two issues, prop or VST filter. I'm ruling out lower unit problem based on comments from last time.
Compression was checked in June and was good, and injectors were cleaned as well. Boats been on the lift for a couple months, and really only been used one time, about 2 weeks ago. Ran fine then.
I got nothing. Reading your title I would have been on the fuel supply problem bandwagon. but for you to actually slow down while the motor speeds up does not follow. Is there any chance your trim motor or switch is sticking? If the motor trims up without you knowing it the prop would slip. Of course that would have nothing to do with the motor running rough or smoaking more.
confirm by using a grease pencil across the hub and prop shaft when you get back, if the line is broken, you need a prop shop or a new hub insert, depending on what type you have.
My theory: It's not a spun hub and it has nothing to do with the fuel supply or any other part of how the powerhead is running.
What you describe is consistent with the gearcase picking up some debris, seaweed, twigs, a plastic bag or other piece of garbage and causing the propeller to ventilate which in turn will cause the sudden loss of forward motion and racing engine RPM.
If said debris covers the water intakes the motor will overheat and that can cause the cooling water in the exhaust passage to boil producing steam which will appear like smoke in the exhaust.
