Spark Plug Gap size 1978 35 HP Johnson Engine

[Moose1am]

I am trying to find the correct spark plug gap for my 1978 Johnson 35 HP Engine.

One dealer's marine mechanic said to gap it at 0.30" but the manual said to gap the plugs at 0.40".

I am running the motor at slow speeds for long periods of time and want to keep the spark plugs clean and unfouled.

Should I gap the plugs to make the engine run hotter at slow speeds.

I am currently fishing "Idle speed only IDNR F&W lakes in Southern IN making it impossible to open it up and clean out the carbon. They give tickets if you go too fast on these small strip pit lakes.

I am going slow to use a humminbird 898c SI sonar also. Can't go over 4 to 6 mph or the SI won't work right.

 

[JB]

Re: Spark Plug Gap size 1978 35 HP Johnson Engine

The current spec is .030", Moose, but in the past it was .040".

You can probably get good performance anywhere in between as well.

I recommend .030" and Champion J4C plugs for all-round and high speed operation, but .035" or .040" and Champion J6C plugs for extended low speed operation.

Good luck, and welcome to iboats.

 

[torbjorn]

Re: Spark Plug Gap size 1978 35 HP Johnson Engine

I am trying to find the correct spark plug gap for my 1978 Johnson 35 HP Engine.

One dealer's marine mechanic said to gap it at 0.30" but the manual said to gap the plugs at 0.40".

I am running the motor at slow speeds for long periods of time and want to keep the spark plugs clean and unfouled.

Should I gap the plugs to make the engine run hotter at slow speeds.

I am currently fishing "Idle speed only IDNR F&W lakes in Southern IN making it impossible to open it up and clean out the carbon. They give tickets if you go too fast on these small strip pit lakes.

I am going slow to use a humminbird 898c SI sonar also. Can't go over 4 to 6 mph or the SI won't work right.

The correct gap is 1mm/.040", I have five of that model. I don't know why they later wrote '.030"'.

 

[Vic.S]

Re: Spark Plug Gap size 1978 35 HP Johnson Engine

The correct gap is 1mm/.040", I have five of that model. I don't know why they later wrote '.030"'.

The plug gaps for practically all OMC engines are now recommended to be 0.030". I forget the reason for the change.

I make the currently recommended plug QL77JC4

The original plugs would have been L78V surface gapped plugs I think you will find

 

[Moose1am]

Re: Spark Plug Gap size 1978 35 HP Johnson Engine

Thank you guys. That's exactly the information I was looking for. I'll try to pick up some of those low speed spark plugs for my boat.
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I have the current new plugs gapped at 0.30".