1979 70hp Mariner unstable at low rpm (video + sound)

[johanw]

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

 

[DavidMoore]

Probably fouled up low speed jets, the point where you get a health engine is likely to be the point when the main jets take over.

Suggest a carb strip down and clean followed by a lync and synch is in order. It's not as intimidating as it might sound and a workshop manual will guide you through the entire process.

 

[johanw]

Thanks. This was actually my initial thought as well, but I was unsure if there is one carb for low speed and a second carb for high speed, or do both carbs have both a high and low speed jet? (I know there's two carburetors...) Can I do this procedure with the engine in a water tank or with a garden hose? Or do I need to be able to run it with the propeller in real load, sea-like conditions?

Another person suggested that there might be a problem with the switch box, trigger or ignition coils, or even the spark plugs, but your sugggestion seems more likely.

 

[sutor623]

Can you give us a serial number? These motors are hard to sort through the diagrams by year and hp alone.

The fact that it is worse, and not better when warm suggests ignition component failure. I am assuming this has been the issue since you bought it?

The first thing that you should do is get a timing light and see if the motor is misfiring at any cylinder. Especially after the motor is warm.

 

[schematic]

check spark consistency on each cylinder with a timing light. (especially at cold idle)
You must rule out ignition, otherwise you are spinning your wheels....

 

[johanw]

Thanks all,
Serial No is 5361999.
I would'nt say it's been semi-stalling as you see in the movie since I bought it, but the uneven sound has been there all the time. The seller even suggested cleaning the carbs to see if it could get smoother at low speed. The almost-stalling the way you see in the movie has just happened a couple of times lately. It might have been increasing over time though, it's hard to tell.
I am not actually sure it is worse when warm.