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The Curse of the Yamaha

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Old 07-31-2005, 11:55 AM
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Default The Curse of the Yamaha

After many problems, questions and all around disgust I think the curse of the Yamaha may finally have been broken.

The cause of the intermittent on/off RPM losses seems to have been due to the resistor spark plug caps (Thanks Dave for the diagnosis, as I would never have though a spark plug cap could be a problem). I found 3 with 100-1000 times the resistance of new caps, and 3 with about 10 times the resistance of new ones.

With the on/off RPM jumps gone I was quite dismayed to find an idle problem with the port motor had returned. Still havn't beaten it completely but it is getting better and I think it's down to a clogged carb passage somewhere.

Dave had also suggested checking the fuel system for high suction and air bubbles. I setup a vacuum gauge just ahead of the engine mounted seperators and above 3800 RPM under load on the lift I'd start to see more bubbling and the suction was at 5", which Dave said would be enough to be a problem. I bypassed the fuel routing to eliminate the selector valves that the prior owner had installed to access a bow tank (but is empty and I never use). That dropped the suction at full throttle under load to about 3", and greatly reduced the bubling seen in the seperator.

While I had it running with the cowls and silencers off I noticed that at full throttle only 4 of 6 carbs on the port motor and 5 of 6 on the stbd had visible fuel coming from the main jet wells. I back-flushed the wells to get all carbs flowing on both motors and was rewarded with about 600 more RPM at full throttle. I pulled the bowls on the carbs and found a sliver of brass (photo coming) in one of them which was small enough to fit in the main jet and not quite fully close it off. The other two bowls (port motor) had small fiber clumps in them that looked like possibly sawdust??

I never fully eliminated the port idle surge/quit (it is back-firing, as I can feel the "pop" in the motor fuel pumps and also see the prop momentarilly stop) but wanted to see what to top end looked like so took it out for another test run.

I kept the idle out to 1000-1100 RPM which smoothed out the port motor, and it never did quit on the way out or back. In fact, after one good sputter on the idle back in it seemed to be idling a little better by the time I got back to the dock. I let it run for quite a while and it never did quit.

The real improvement was in the top end. Both motors were now running pretty well in throttle sync (both levers the same) at anything about 4500 RPM. They develope some mis-match below that and will run down to about 3200 RPM. The benefit was the motors would pull at normal trim to 6000 RPM, and run along quite well at 5600 RPM (sucking down 56 gallons an hour according to the fuel sensor).

Now, on to the new problems identified and still needing solution:

While running at the dock I noticed the port motor pumps a steady stream of water out of the lower unit through the hole right next to the forward/port side mounting bolt. I'm guessing the water pump is filling the cavity so has a crack/leak/missing o-ring or somthing. Need to get the boat pulled so I can get someone to overhaul both pumps.

I also noticed that when turning to port the port RPM drops, which I expect as it is putting the prop deeper in the water. What I think is still a problem is that turning to stbd the port RPM really drops off, much more than the stbd motor. I'm guessing something is getting pulled/pinched inside the rigging tube between the transom and motor.

Finally, the mechanic thought before that the motors needed to be raised as the cav plates were too low in the water when on plane. If I go ahead and raise the motors I'm going to need new props, since I can pull the motors now up to 6000 RPM at the current height. That's the kind of problem I could learn to live with for a change.

-Greg
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Old 07-31-2005, 02:15 PM
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Default Re: The Curse of the Yamaha

''I also noticed that when turning to port the port RPM drops, which I expect as it is putting the prop deeper in the water. What I think is still a problem is that turning to stbd the port RPM really drops off, much more than the stbd motor.''

Greg;

Could this have something to do with the direction of prop rotation?

Your friend
Roger A. Holmberg
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Old 07-31-2005, 04:53 PM
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Default Re: The Curse of the Yamaha

Havn't a clue. They are both rotating out so I would "think" any cross-flow coming over the hull would be the same for the right prop when turning left and the left prop when turning right and vis-versa. On left turns the stbd motor doesn't seem to have any RPM drop (or rise) at all.

-Greg
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