Merc 20 HP won't run at med-high speed

[Olde Merc]

I have a 1976 Merc 200 20 hp outboard motor. I was out on the lake with it on the weekend, and it almost left me stranded. This was the first time I have run the engine this year and it was hard to start. Once started, it ran great. I ran it at WOT for at least 15 minutes touring around the lake.

Then the problems began. At WOT and the boat going full speed the engine suddenly just stopped running. I imediately thought that I forgot to open the tank vent, but it was open. After the boat stopped, the engine started up again, with relative ease (it always starts easy when warm). The engine then ran fine up until I turned the throttle past the "start" position (approx). The idle was normal, and up to start the engine ran normally. As soon as I turned the throttle past start it started to chug severely, lose speed, and would quickly stall unless I turned the throttle down. It always started easily after it stall (one easy pull).

On the way back into the boat launch, it ran fine unless I turned it past start (and it would stall). At one point it seemed to run normal again, and it ran for about 3 or 4 minutes at 3/4 to full throttle, until it shut down again. This time it would only run as before, and would stall if turned past start. This was the same regardless of being in forward or neutral.

Needless to say, the trip back to the dock was VERY long.

Any ideas where to start, what to check? The boat has had new plugs in the past year, and the CDI unit was changed four years ago (Merc part). It sees low usage during the summer months (maybe 10-20 hours per year).

Any advice greatly appreciated.

 

[arls14skiff]

Re: Merc 20 HP won't run at med-high speed

Same thing happened to mine...I have a 74 (200 model) 20 hp. I had some carb work and a new coil put on and that fixed that...check you voltages at your coil and make sure your carb. is clean..goodluck..

 

[Olde Merc]

Re: Merc 20 HP won't run at med-high speed

Do you know what was wrong with the carb? I will check the coil voltage as soon as I can. I just have to find the service manual for the engine.

Thanks for the help. Any other advice appreciated.

 

[Olde Merc]

Re: Merc 20 HP won't run at med-high speed

If anyone has any direction, I would really appreciate it. I'd like to try to tackle this repair myself, but I have no idea where to start.

I can't afford to bring the boat to the repair shop now, and I'd like to be out on the water.

Thanks!

 

[8075mercury]

Re: Merc 20 HP won't run at med-high speed

If it was mine, I would start by cleaning the carburator. Sitting up might be the cause. Also make sure your fuel diaphram is good (no pinholes). If you pump the bulb while it is stalling out at 1/2 throttle, this might tell you this is fuel pump issue.

After the carb is clean, I would start looking at possible ignition issues.

One other thing you might check too is see if the timing is advancing properly with proper trigger movement. I had the timing linkage come off on a 79 20 and it would idle fine but bog out at 1/2 to WOT.

Good luck, great motors !

 

[Olde Merc]

Re: Merc 20 HP won't run at med-high speed

Thanks for the info. I finally found my Merc service manual, and saw the carb tear down procedures. Looks pretty simple to overhaul, so I may start with that. I haven't down an outboard carb before, but I have done lots for cars, and they are certianly much more complicated than this one looks.

Anyone have any tips before I rip this carb apart? Any idea what rebuild kits cost?

Thanks.

 

[tundraguy]

Re: Merc 20 HP won't run at med-high speed

I was having the same problem with mine and after a combination of replacing one of the coils and a carb rebuild, it ran great. It is very easy once you get in there, just take your time.

Good luck.

 

[emckelvy]

Re: Merc 20 HP won't run at med-high speed

Pick up what Merc calls a "carb packing kit", it contains the gaskets etc that you'll need to disassemble/reassemble the carb. Typically you don't need all the stuff that comes with the (much) more expensive "rebuild kit".

If not carb, definitely check ign as it's quite common to see cracked coils, they'll arc to ground and kill the spark. Also possible it's a switchbox going bad.

You could use an inductive timing lite to check each plug wire for good fire while the motor's running. If there's an ign miss you'll see it right away in the timing lite.

HTH.........ed