1985 70 HP Johnson

[kmad]

My 1985 70 HP Johnson outboard motor does not run properly at high speed. It bucks and surges and will not operate at full power. It is so violent at times that it nearly knocked me down. It runs really good at slow speed. I have done some things already: I replaced the VRO fuel system with a conventional fuel pump; I replaced the in-line fuel filter and all the fuel hoses; I replaced the exterior fuel tank and fuel hose with primer bulb and i chaged out all the spark plugs. I got the motor to run smoother but it still does as I described plus sometimes when I put in at full speed it does not take off at all and when I back off and wait a few seconds and try again, it takes off but after a minute or so it starts bucking and surging again. Also, it does not seem to have the thrust that it once had and it rides heavy and will not skim the water and cut waves but rather wants to plow through. Any ideas as to what the problem is?

 

[jtexas]

Re: 1985 70 HP Johnson

This condition is known as "high-speed misfire", and from your description we can narrow it down to one of three possible causes: compression, spark or fuel.

Narrow it down further:
1. Compression test: compression should be about the same on all 3 cylinders (within 7% or so from lowest to highest). (follow the "compression testing" link from the "top secret file" at the top of this forum).
2. Spark test (not a "spark plug" test). get an inline spark tester (couple bucks at autozone), set the gap for 7/16", clip it to the engine block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it can't jump a half-inch gap in the open air without a spark plug, it's not firing in the hole under heat & compression.
3. Make sure the tank is free from water and debris, the fuel is fresh, the fuel line holds pressure, and the tank vent is open and clear.

How long since your last carb rebuild?

Let us know the results of your tests.