Question about WOT RPM on old Johnson motor

I'm new to the forum but I have read all the great information people give here to other posters, I have a problem of my own. I have a 1974 Johnson V4 85 hp motor, the motor is on an 18 Ft Cobia sportster open bow, Hull is FBGLSS. The motor starts every time I turn the key and idles good, the problem is at wot i'm only getting 3000 rpm, I took the boat out last evening and when my Buddy went to the front the rpm went to 3800. The prop on the motor is 13/25/17 Aluminum. The numbers are as follows 85ESL74B ser # J4082983 4500 / 5000 RPM 85 HP 5000 RPM Johnson 1974 V4 85 HP like I was saying i'm only getting 3000 RPM and when my buddy sat in front the RPM went to 3800 However the Motor ran smooth no stumbling or power surges just steady. Took off air breather while running and held my hand over the set of carbs, the motor had noticeable change in idle, took spark plug wires off one at a time and the motor had noticeable change each time. Do I need another prop, and if so what pitch. Thank you

Re: Question about WOT RPM on old Johnson motor

I figure your boat dry is about 1500 lbs Your motor should operate in the 5,500 rpm range.You may be slightly over propped A 15 may do better but its not going to solve your rpm problem.Will only add about 400 rpm. Something else is going on.Do you trim out for the best speed? How is the overall performance?Strong hole shot,
push a load well?What is the speed at 3,000 Some boats won't even plane at 3,000.From your description it sounds like it isn't planing until your buddy goes to the bow. The switch on the back of the tach may be set wrong.What is rpm at idle?Is the throttle opening the carbs all the way?Is the motor in excellent tune?Is the prop in excellent condition?Boat bottom clean,paint in good condition?Check the compression should be at least 100 psi and even cylinder to cylinder.

Re: Question about WOT RPM on old Johnson motor

Thank you for helping me with this frustrating problem

This is what I am sure of.... The tack is set to 6, the boat is in great condition, new paint job, the idel RPM is 800, the throttle is opening the curbs all the way,compression was at least 100 psi and even cylinder to cylinder, leak test was less than 5%, the prop is in excellent condition brand new, hole shot is strong and yes it isn't plaining until my buddy goes to the bow.

What i'm not sure of is... What the speed at 3,000 is, motor in excellent tune.

I also wanted to note the prop is new, the prop that was on the motor did the same thing, I was looking for numbers on the original prop but we could not find a thing even after taking it off, my buddy thought it might be a 19 pitch over propped, So I got the 13-25-17, i'm thinking I got the same size as was on the motor in the first place I don't think the original was a 19 as my buddy suggested as nothing changed with the new prop still the same.

Re: Question about WOT RPM on old Johnson motor

If the boat isn't planing at 3,000 your probably about 12 or 15 mph.
74 brochure says 17" is standard prop.Of course final prop is selected based on the application.My feeling is your boat would require a 15" pitch.
But as I said the 15 isn't going to solve your problem.
Have you checked the timing?Full advance is 28 degrees BTDC.
Checked at full speed in gear.Timing is adjusted with the motor off.one full clockwise turn of the screw retards 1 degree.Check the plugs look for evidence of lean burning. Plug listed for prior to 86 85 hp is L77J4
This is from a 89 aftermarket manual.May now be same plug different number.
Check for water intrusion.Sometimes indicated by an exceptionaly clean plug.