Tohatsu 9.8 four stroke

[kpalfreeman]

At the beginning of the season, being very short of time to get ready for the first race, I only had time to change the plugs, check the oil level, then go. Engine Started well enough, but on the way home after the race, with now a total of about 30 mins running, it became very rough, and cut out just before we got to our berth. It would start but not run for long.
Since I owed it an oil change I did that, thinking I would then have to get involved in cleaning the carb, checking the electrics, etc.
After the oil change I fired up the engine and amazingly it ticked over sweetly and has run all season. About 30 outings, about 40 mins motoring each time, with a bit extra here and there, say 25 hours.
I have had to adjust the idle screw, but I think that is because with the passage of time and use, the linkages have all loosened up and become a bit worn.
Last week the ragged running started again; changed the plugs, as that is most common problem, no improvement. Today changed the oil, running sweetly, ticking over on idle like a clock.
The engine was new in 2009 and has had four seasons use, and altogether probably 150 hours.
The old oil is always a nice black colour.
The engine seems to eat plugs, they usually need to be changed 2/3 through a season. Old plugs are dry, a bit brown, scraping them clean and resetting the gap does not seem to work.
Accepting that my maintenance leaves a bit to be desired, what I do not understand is why changing the oil makes such a different to the actual running ? It is the difference between an engine that runs so badly it produces about 1/4 power and will not idle and one that idles nicely and produces full power.
Can any Tohatsu experts give an opinion ?
Palfreeman

 

[Lakesailor]

I can't see there could be any connection.

 

[VicS]

Like Lakesailor I cant see the connection.

I'm a bit concerned by "The old oil is always a nice black colour." Why should that be so...??

Running very rich or carb flooding ??? except that the plug colour does not support this idea ... correct plugs presumably ??

"The engine seems to eat plugs" perhaps ought to tell us something. Why should it eat plugs ??? What's it doing to them ??

What you using for fuel ??

 

[kpalfreeman]

Fuel is regular unleaded - same as for the car.

 

[catalac08]

I am puzzled by the black oil-my Tohatsu 's oil (20HP) is still a light golden brown after about 40 hours use this season. "Eating plugs" is also strange. I am also interested in what this strange problem could be!
My best guess is that the choke is being applied and by taking off the cover this is releasing the choke problem until a time when it is activated again. How does the choke operate on the engine

 

[kpalfreeman]

UPDATE

Since changing the oil we have done about 2 hours running, mostly at 80% to 100% throttle and the engine has run faultlessly. After the initial start for the day (three pulls) it starts on first moderate pull.
I am happy the engine is working so well, but still trying to understand how this problem has happened twice - so that I can avoid it in future.

Ref the comment that oil should still be golden after 150 hours, whereas on our engine the oil turns black: The engine is in a well in the transom, so there is always a lot of spray about and the air going into the engine must have a fair amount of water in it. Maybe this is working its way into the oil.
Possibly someone with a copy of the full service manual could see if there is reference to this.

Ref: Choke comment: the design is quite simple and all the parts seem to move freely. After four years of use I have had to adjust the idling screw, probably because there is bit of play now in the linkage and the adjustment is very sensitive - a half turn of the screw is the difference between a gentle tick over and stalling.

Without knowing the cause of the problem I will change the oil and plugs twice per season since reliability is the end objective.

In the meantime if there is a Tohatsu expert (professional ?) who has met this problem and dealt with it, it would be good to hear from them.