Hi,
got a 70hp Mariner, 2-stroke, 3-cyl from 1979 or 80, has worked fine this summer, except that it runs unstable below a certain rpm threshold. It starts fine at the first or second attempt, but runs with an unstable/uneven sound. When I slowly increase the throttle, I reach a pretty evident threshold where the unstable engine sound transform into a smooth, spinning, "healty engine" sound. My tachometer is disconnected, but I'd say its around 15-25% throttle, which gives 10-12 knots of speed. After a while's running, returning to dock, and turning the throttle down below the rpm threshold, the engine usually start sounding even more unstable than in the beginning, when cold. This phenomenon has been more prominent the last 5-8 times we've been using the boat. At one point did i get a complete stall at almost idle rpm, when throwing the anchor at slow speed...
https://clyp.it/vc2wztgs - Here is a sound recording during acceleration with the transition between unstable - stable engine audible. Not the best recording but if you listen carefully you can hear the engine sound shift in character around 00:07-09, and 00:54-56.
https://vimeo.com/141407456 - Here is a video that shows the engine and the sound just after returning to dock after a 45-min run. You can see the whole engine shake at the point of almost stalling
got a 70hp Mariner, 2-stroke, 3-cyl from 1979 or 80, has worked fine this summer, except that it runs unstable below a certain rpm threshold. It starts fine at the first or second attempt, but runs with an unstable/uneven sound. When I slowly increase the throttle, I reach a pretty evident threshold where the unstable engine sound transform into a smooth, spinning, "healty engine" sound. My tachometer is disconnected, but I'd say its around 15-25% throttle, which gives 10-12 knots of speed. After a while's running, returning to dock, and turning the throttle down below the rpm threshold, the engine usually start sounding even more unstable than in the beginning, when cold. This phenomenon has been more prominent the last 5-8 times we've been using the boat. At one point did i get a complete stall at almost idle rpm, when throwing the anchor at slow speed...
https://clyp.it/vc2wztgs - Here is a sound recording during acceleration with the transition between unstable - stable engine audible. Not the best recording but if you listen carefully you can hear the engine sound shift in character around 00:07-09, and 00:54-56.
https://vimeo.com/141407456 - Here is a video that shows the engine and the sound just after returning to dock after a 45-min run. You can see the whole engine shake at the point of almost stalling