1975 Johnson 70hp

[lehkyc]

hey everyone. iv got a 1975 johnson 70hp i bought and im having some issues. i water tested it for the first time the other day and this is what happens; it starts up perfectly, always within a second or two of cranking. goes into gear fine. the factory spec for idle speed is too low for the engine to keep running. it runs pretty good from 1-3k RPM but will not exceed 3500 rpm for any reason. very sluggish on take off. im wondering if perhaps its fouled up carbs. but if the motor is in neutral, it will rev up just fine, all the way to 5500 rpm. im runnin a 13 1/4 x 17inch prop on it. its on a 16 foot boat. the motor is not underpowered for the weight its pushing. all 3 cylinders check out at 130psi. i put new plugs in the other day as well. any clue to what this could be? any help would be extremely appreciated. im dying to get out on the water.

 

[Barnacle_Bill]

Re: 1975 Johnson 70hp

Have you checked for a strong spark at all plugs? Don't be reving up that motor in neutral more than 2000 RPMs. You are inviting a major disaster.

 

[Daviet]

Re: 1975 Johnson 70hp

As bhile stated, check the spark on all cylinders, should jump a 7/16" air gap and be bright blue. Check compression, hopefully it will be in the 125-150 range, but more importantly be within 10% of each other. If all that checks out you are going to have to start on the fuel system, fuel pump, lines, filter and carbs.

 

[lehkyc]

Re: 1975 Johnson 70hp

i will check for spark tonight or tomorrow. compression was at 130 on all cylinders. if im not getting spark on 1 or more cylinders what am i looking at doing next? can this be a major problem?

 

[Daviet]

Re: 1975 Johnson 70hp

If you don't have spark on a cylinder you can try swapping coils and see if the no spark condition moves with the coil, if so you have a bad coil.

 

[lehkyc]

Re: 1975 Johnson 70hp

ok, cleaned the carbs, checked for spark and it was good on all 3 cylinders. took it to the lake and nothing changed. still doesnt want to rev above 3500 rpm or so. fuel pump? i checked and cleaned the fuel filter.

 

[fishfeatures]

Re: 1975 Johnson 70hp

Have you carried out a lync and sync as per owners manual ?

 

[eavega]

Re: 1975 Johnson 70hp

Do a cylinder drop test to see which one is not running. I am going to bet that you have one cylinder that is not. If you can, get a timing light to verify the spark while the motor is running.
If it is fuel, look again to the carbs. Did you check the lines that run fuel to the top and bottom carburetor? I had a leak in one of the lines on my '76 70 HP. The fuel system would check out fine from the tank through the fuel pump and into the middle carburetor. It wasn't until I isolated the problem to the top cylinder that I was able to find the air leak that was causing the top cylinder to run badly.

Good luck