1981 70hp evinrude outboard

I took her out for our first run together this season and this was happening.

Starts quickly and idles well.  When I shift into forward and start to throttle up I'd get up to 10mph or so but not accelerate any more, even at wide open.  It's not sputtering, dieing, gaining and losing power, etc.

I barge around for a bit before settling in to fish for a little while.  After I'm done, engine starts right up I shift into forward and idle around for a few minutes before trying accelerate.  This time I start to accelerate and the engine starts to rev up, sluggishly, and I top out around 25-28 mph.  Unfortunately my tach. isn't working, so when I hit top speed I'm quickly throttling down to 3/4 or less.  

I have a 15 pitch prop and brand new, the engine comes with a 17.  I was really expecting to have a higher top end than 28mph with a 70hp engine on a 16 foot aluminum boat (sea nymph, model # FM - 161).  Any ideas gang?

Sounds like a spun hub. Even though you say the prop is new it only takes one time to spin it. Paint a line across the hub and prop, try it several times, check the prop to see if the line is still straight or if part has moved. If it has moved the hub needs to be replaced.

Same symptoms were happening to my 25hp 4 stroke yamaha. Luckily for me all it was were the spark plugs. It started fine, It idled fine, On wide open throttle it would just not go. Changed the spark plugs and she was back to normal.

Yup I also was thinking plugs. Also what Mix Ratio are you using. Is it possiable your a little too rich on oil? That will foul plugs pretty badly, and quick.

Bill

Sounds like a spun hub. Even though you say the prop is new it only takes one time to spin it. Paint a line across the hub and prop, try it several times, check the prop to see if the line is still straight or if part has moved. If it has moved the hub needs to be replaced.

I will do the line test next time out.  Excuse me if I'm way off here, as I'm new to outboards, but wouldn't a spun hub mean I would have no power?  Or is that worst case scenario?

"Same symptoms were happening to my 25hp 4 stroke yamaha. Luckily for me all it was were the spark plugs. It started fine, It idled fine, On wide open throttle it would just not go. Changed the spark plugs and she was back to normal."

New plugs went in on sunday.  50:1 ratio.  Check the new plugs, some wetness on the threads, none on the electrodes.  Might be a little rich?  

I cleaned the fuel filter today, wasn't dirty, but what the heck.  

My 1984 Mercury 50 hp is in the shop right now with the same problem.  I'll let you know what the mechanic says when he calls.

Not necessarly. When the hub spins you will still have some power, the hub can slip and keep the boat from having enough power to put the boat on plane. The RPMs will go up, but it will lose the power at the prop as it tends to slip instead of turning the prop correctly.

My 1984 Mercury 50 hp is in the shop right now with the same problem.  I'll let you know what the mechanic says when he calls.

Thanks olebiker.

"Not necessarly. When the hub spins you will still have some power, the hub can slip and keep the boat from having enough power to put the boat on plane. The RPMs will go up, but it will lose the power at the prop as it tends to slip instead of turning the prop correctly."

The boat has gotten up on plane.  It feels more bogged down and not making power, than losing power at the prop.