Johnson SeaHorse 70 getting slower

[Crimson Beard]

Bought a '93 21ft pontoon with a 1981 Johnson 70hp outboard. It ran pretty quick when I bought it this spring, but over the weeks it has gotten much slower. Motor cranks right up, runs and sounds great. No sputtering or smoking. Replaced spark plugs. Prop is in fair shape. Pontoons are drained. No indications of any problem yet I'm losing speed. Not draging anything.<br /> Haven't tested compression yet.<br /> Could a weakening battery cause this or the 87 octane fuel?<br /> Is there a way to troubleshoot the power pack(s) without just buying new ones? That's an expensive test.<br /> I'm baffled bu this loss of speed/power.

 

[JB]

Re: Johnson SeaHorse 70 getting slower

Howdy Redbeard.<br /><br />Low battery voltage will not cause a loss of power. 87 octane is the recommended gas.<br /><br />Since you haven't put numbers on your impression of loss of speed it is hard to guess what it might be.<br /><br />If you keep your 'toon in the water it has probably grown a bit of a foliage beard, which will greatly increase drag.<br /><br />You can check out your powerpack by removing each plug wire, one at a time, while the engine is idling, and see if it will prodice at least 1/2" of hot spark to the block.<br /><br />You can check your fuel pump by pumping the primer bulb while running wide open. If you gain speed you have a weak pump.<br /><br />If none of the above identify a problem, borrow a tach and tell us rpm at wide open throttle (WOT).<br /><br />Let us know what you find.