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Mariner 642 3.5 hp power problem.

C

chadj123

New member
I wonder if anyone can help me with a strange problem. Firstly I received this outboard because it was old and leaking badly. Not only from the carburetor, but also from the tank which was rusted through in a few places. After repairing the tank, I was able to use it for short runs but it would flood the carburetor if I left the tap open when it was not running. At the beginning of the year the carburetor bowl broke and I was unable to replace it as the motor is obsolete, so I turned a new one on a lathe and redirected the overflow out of the side. I also turned a new seat jet which now stops the carburetor from overflowing. I am now able to leave the tap open, and after 24 hours am not getting any overflow. The motor runs well with no problems while idling in or out of gear.

The problem is that when I leave the moorings I can go at full throttle for about 100 meters and then I loose power. After that I can keep going as long as I keep the revs low. I have managed to run for 1 nautical mile if i stay under three knots. First thought was that there was not enough fuel getting into the carburetor and it was starving for fuel. Today I turned a new bowl out of white HDPE and made side walls thin enough so I could see the fuel level (1 mm). Once again I managed to get about 100 meters before it looses power and then must stay at low revs to keep moving. The level in the carburetor has not dropped below normal and the flow of fuel stays constant.

Does anyone have any suggestions?
 
Thanks for your comments.

As this motor is almost 40 years old and obsolete I have been reluctant to start stripping it further than I need to in case I can not get any spares for it , but it looks like I might need to take the plunge. Because I have spent so much time on the fuel system I did not think about the exhaust system. The cover on the drive shaft housing is missing (Part no 15) could it be possible that this is increasing back pressure when trying to move at higher revs? I am starting to think that this might be the problem, as it has no problem revving when I am not moving. I am going to try and cover it up this afternoon and see what happens.

http://www.marineengine.com/parts/mariner-outboard-parts/35/64223740-and-below-m/drivshaft-housing
 
Well it looks like your diagnosis was correct, I closed up the hole with some duct tape and took the tender around the marina this morning a couple of times. I did not push it too much because there was a 20-25 Knot wind blowing and I did not feel like paddling back, but I went at least twice the distance it normally took before it lost power. Well done and thanks for the advice. I have ordered a new cover which should be here in a week or two. Based on the weather forecast I won't be able to use the boat before then anyway.
 
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