1978 Johnson 70 low power under load.

[Gibsonman2005]

My best advice is to settle down and stop buying motors
Did you ever take the propeller off the Johnson to see if fishing line was wrapped up in it?
Or get new primer valve and connections?

 

[David Young]

Seems strange to me, you buy another motor that now has the same problem that your other motor had? I would be looking at the fuel tank or even the gas station you are getting your gas from. Let us know what you come up with

 

[James R]

I had a puzzle similar to yours on a 70HP. Turned out that the lower unit was seizing when revs came up.

 

[Sashap]

Turns out new motor flywheel trigger magnet loosened and gave various timing while spinning. Any idea what the alignment should be?

 

[Sashap]

Had old motor all apart and no fishing lines were at issue.

 

[Sashap]

New fuel lines from tank to engine. All clean no bubbles either looking through clear hose section

 

[Gibsonman2005]

At this point cross your fingers and hope F_R chimes in...he's helped quite a few folks out over the years

 

[Sashap]

My guess is something with timing on old engine. Never checked it before since it started and idled ok. But it didn't increase rpm at wot. Timing screw was not touched but you never know. I am in process of putting it all back together. Just stripped and repainted power head.