1975 Johnson 70hp surging

[csj992]

New guy looking for help. I've got a 1975 Johnson 70hp that runs great up to 4k rpms. After that the motor feels like someone is on and off the throttle. Previous owner rebuilt carbs, new fuel pump and lines, still no fix. I have found no leaks in the fuel system. I was wondering if the timing could be advancing and retarding? Thanks, Charlie

 

[fireman57]

Re: 1975 Johnson 70hp surging

It probably isn't timing as that really can't change unless you have a sheared flywheel key and I'm sure that is not the case here. Had the same problem on a '78 70hp and I put clear fuel lines on it to watch for an air leak. I used lawnmower fuel line from O'Reilly's Auto as that is the only place that I could find them. They were a .32 smaller than what was called for but they worked beautifully. When I changed fuel lines originally I used the called for size in black and it ran the same way yours did. I don't know if the super tight fitting clear lines made the difference but I have to suspect that it did. If nothing else you will be able to see with the cover off if you are losing fuel to a cylinder at higher rpms.

 

[csj992]

Re: 1975 Johnson 70hp surging

Thanks Fireman. I will give that a try. The motor runs too smooth below 4k rpm so I figured it must be a fuel problem. If it was a small block chevy I could probably fix it easier..Thanks again, Charlie

 

[fireman57]

Re: 1975 Johnson 70hp surging

If it is not a fuel problem them try disconnecting the tan wire from your temp sensor and see if that takes care of it. Do you have a buzzer connected to it? I didn't and the overheat sensor would kick in because it was bad and short out the top cylinder. If it doesn't make a diff then hook it back up.