1971 25hp Johnson, stumbling/surging on accelleration...

84toyota

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Mar 15, 2011
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48
Hi everybody,
I'm got a 1971 Johnson, 25hp outboard that I have been going through and fixing up. I have rebuilt the carb, being very careful to clean out all passages, made sure the float was set correctly, and made sure the low-speed needle and packing was in good shape. I've completely replaced the points, condensors, coils, plug wires, and plugs. I've put in a new thermostat. The motor starts just fine, and idles well. It will run at top speed very well, with no stumbling. My problem arises when I try to accellerate rapidly from low speed to high speed, as well as when I drop from high speed to low speed. The motor will start stumbling and in many cases "bog-down" and die. It will start back up just fine. The problem doesn't seem to go away over time (once the motor is warmed up and the thermostat opens, it still does this). If I accellerate/decellerate very gently, it seems to do ok. I have tried pumping the primer bulb when this is happening, and it seems to kill the engine quicker. The only thing I haven't looked at on this motor is the fuel pump. Do these symptoms sound like fuel pump symptoms, or does this point back to the carb? I was very meticulous when tearing down the carb...

Thanks a bunch,
Ken
 

kfa4303

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Sep 17, 2010
Messages
6,094
Re: 1971 25hp Johnson, stumbling/surging on accelleration...

HI 84toyota. The symptoms you're describing sound more like spark/ignition issues to me, rather than a fuel deliver issue (assuming the carb is good to go). These old motors can start, run and idle surprisingly well on only one cylinder. I'm thinking you may want to give the magneto another look to see if anything is out of place, or grounding out. Be sure that each cylinder is producing a nice strong, snapping, blue spark that can jump a 1/4" gap. You can get a gap tester from most auto parts stores for about $5. Also, make sure the spark plug wires are firmly connected to the coil posts. The wires need to be twisted on to the posts, rather than simply jammed in place. You may nevern need to snip a 1/2" of either end to get to some fresh copper. You may also want/need to perform a "link-n-synch", which just coordinates the spark and opening of the butterfly valve on the carb. Do a search here in the forums for the complete procedure. Give these guys a try and let us know what you find. Here are some other handy links too.

http://www.outboard-boat-motor-repa...on 3 HP 1952-1967 Ignition System Tune-up.htm

http://forums.iboats.com/showthread.php?t=167352

http://www.duckworksmagazine.com/07/columns/max/index1.htm
 

84toyota

Seaman Apprentice
Joined
Mar 15, 2011
Messages
48
Re: 1971 25hp Johnson, stumbling/surging on accelleration...

Thanks for the reply! I forgot to mention in my original post, that I did test the spark and had a nice blue spark jumping the 1/4" gap. I used FR's timing fixture tool to adjust the points exactly (not just a ballpark .020 feeler gauge). I also did a link-n-sync. So I pretty much covered all of your suggestions. I did burn through about 2 gallons of gas on my test run yesterday (with some Sea Foam in it to help clean some of the internals). I'll probably make another run or two to really dial in where my problems occur. I might take a look at my fuel pump, as I have not looked at it yet (it seems to pump just fine, but maybe it needs cleaning?) Thanks for your suggestions - I'll keep you posted.

Ken
 
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