High RPM's but boat doesn't move any faster.

[Fishing Michigan]

I have a 60 HP 1969 Johnson Hydroelectric Outboard 3 Stroke Motor. All carbs have been rebuilt, distributor is new, all spark plugs are new, fuel filter is clean, and is in relatively good working order except for this one issue.

I had a 13"x7" prop on and my top speed before the sputter was about 10 to 12 mph. I changed it to a 13"x9" prop and my speed went up to about 20 to 25 mph before it would start to sputter again. (both are alluminum props)

After I reach a certain speed, and still have plenty of throttle left, I try to go faster, and all the happens is the RPM's will go higher, the motor will sputter, and the boat will surge in unison with the sputtering, like it wants to keep up, but it just can't. It's almost like it's starving for fuel, but the fuel supply isn't restricted in any way.

I've been told it could be my prop, I need a deeper pitch. That it has something to do with the RPM's feeding back causing the sputtering, or some kind of module on the side of the motor that has a ton of wires coming out of it.

My buddy, who is a marine mechanic, just not on engines this old, did do a basic diagnostics of all electrical, out drive, etc. All to be in working order.

I just read about how to check a prop for splipping, so I will do that test as well, as I thought that was my first propblem, which is why I changed them.

Any advice or help on this matter would be grateful. I am willing to take it in to a reputable marine shop who works on these types of motors, ie: year etc. but am hoping to find a cheaper answer.

 

[Joe Reeves]

Re: High RPM's but boat doesn't move any faster.

The cavitation plate just above the propeller. Where is it in relation to the bottom of the boat... above/below... how far above or below?

What is the tachometer reading at that 25mph figure?

What do you mean by "sputtering"? .... A slow surge up and down increase of rpms, a very fast ignition type breakup? We could use a better description of the problem. The point setting of that model is critical and if set by any method other than the following can result in all kind of weird happenings.

(Point Setting Of Battery Capacitance Discharge)
( Ignition Models - 1968 thru 1972)
(Some have points - Some do not)
(Joe Reeves)

The points must be set to .010 but no wider than .010..... BUT in some instances due to a possible slight inaccurately machined crankshaft lobe or a slight offset of one set of points, a setting slightly less than .010 would be required as follows.

Whether the crankshaft has two or three lobes, when setting the points, check the setting of the points on each individual lobe by rotating the crankshaft by hand.

You may find that setting one set of points to .010 on one lobe, then turning the crankshaft to the next lobe, the gap measures .011 or .012 (too wide). This is where you would need to close that gap down to the required .010. A gap too wide can result in a ignition miss when throttle is applied.

Bottom line, pertaining to the point setting at the various lobe locations____ .010, .010, .009, is okay____ .010, .010, .011 is not!