1981 Johnson 70hp loses power.

[tmhailey]

I have a Johnson 70hp Seahorse mounted on a 16? fiberglass Uniflite. I get great performance for about 20-30minutes. Then the motor becomes increasingly sluggish until it will produce only about 1500-2000 rpm. As the performance deteriorates there are glimpses of hope as the motor surges and it seems like its clearing itself out.

I have a main tank and a portable tank, each with its own line. Switching from one to the other, sometimes resolves the problem, regardless of which one I switch from and to. Yesterday this problem occurred again as I ran out (about 3 miles) to set a skate. The way out was fine (on my portable, 6 gal. tank), 4,000-5,000 rpm for about 20-25 mins. I ran slightly above idle speed for about 5 minutes while setting the skate. About half way back, performance began to fall off, but I was still planed out.

I pulled the boat out for about 3 hours, and then ran back out to pull the skate. Performance continued decreasing. I pulled the skate. By the time I was half way back, it would not plane at all. The portable tank eventually ran out. I switched to the main tank, and it ran great the rest of the way in. But in the past the problem has started while on the main tank and been corrected by connecting the portable. I've also had the problem occur and not be corrected by switching lines.

Thanks,

Todd

 

[jtexas]

Re: 1981 Johnson 70hp loses power.

both tank vents being blocked seems an unlikely coincidence but I guess stranger things have happened.

You're changing fuel lines at the motor? Have you replaced the quick-disconnect fitting on the motor?

Have you compression tested the motor recently?

 

[tmhailey]

Re: 1981 Johnson 70hp loses power.

I am changing the lines at the motor. I have the quick disconnect fittings to replace on the lines and plan on doing so tomorrow. I hadn't even thought of replacing the one on the motor. Thanks. I'm almost positive that's a part I will not find locally (I live on an island in Alaska), so are there any other suggestions before I start having parts shipped one at a time?

While the performance was fading, I removed the vent on the portable tank completely and it had no efecct. I have no recent compression check on the cylinders, but it doesn't seem like a low compression issue would explain the sudden good performance when switching tanks.

Thanks for the help,

Todd

 

[tashasdaddy]

Re: 1981 Johnson 70hp loses power.

i think this is one of those things that needs to do the trouble shooting when it is happening, try pumping the fuel line primer bulb when it happens, also check spark and see if you are loosing spark on a cylinder. it does not sound like a compression problem. check fuel for water, is a thought.

 

[cmchesse]

Re: 1981 Johnson 70hp loses power.

Just corrected a similar problem on my `85 70hp. Seems it was changed form VRO to premix in the past.
Anyway, It would improve if some pumped the fuel bulb non stop. So I put a new fuel pump on and it and the problem went away.