'76 Merc 1500 Tower of Power Prop Question

[ClassyGlassy]

I got it to go 42mph at 4000 rpms WOT with a 21p 3blade Aluminum Prop. What's the rule of thumb for Propping down? How many RPMs gain as you drop pitch? I need to get around 5500-5800 RPMs at WOT. Does a 15p Prop sound too underpitched? What about a 17p?

 

[ClassyGlassy]

Re: '76 Merc 1500 Tower of Power Prop Question

I also went to a lake at 10,000 feet of elevation and couldn't get it over 2000RPMs........still with the 21p Prop.

Is this ONLY a prop issue or am I fighting something else as well?

 

[steelespike]

Re: '76 Merc 1500 Tower of Power Prop Question

Rule of thumb is "about" 200 rpm per inch. I think I would start by varifying the tach and speedo.A gps is best for speed.Be sure connections are clean and tight.Note the position of the selector on the back and operate it a few times to clean the contacts.For that little boat I don't think a 21 is excessive.
Check your setup The antivent plate(just above the prop) should be visable at speed.usually ends up about an inch above the bottom,measured with a straight edge along the keel.Carefully adjust trim,does the boat respond to trimming out.
(prop away from boat)Does the boat get up on the water or does it lay down like its glued to it.Prop in excellent shape?Throttle plates opening all the way?Motor timed correctly?
As far as elevation you technically need to rejet for the elevation.leaner, smaller jets.You could instead go with a lower pitch and after you have fine tuned your setup You will know better what you need for both conditions.Ideally a rejet and prop change is best.

 

[ClassyGlassy]

Re: '76 Merc 1500 Tower of Power Prop Question

I don't know what my final advance is for timing.......it does respond to trim though. I can pull the front up pretty good off the water by trimming out and it does pick up on speed. I know have the engine setup correct as far as the relation to the boat goes. The cavitation plate is about 1.5 inches above the keel and is visible when on plane and I don't have a problem with the prop sticking out when trimmed all the way out. The prop is brand new and not even a scratch on it so I know that that can't be the problem either. Speed was checked with my GPS but I have not other way to check RPM except for the Tach on the dash. It may be a timing issue I guess........

 

[steelespike]

Re: '76 Merc 1500 Tower of Power Prop Question

You may be able to borrow a shop tach a or get them to take a ride.
There are timing lights that have a tach feature.There are what I call motion
tachs that count revs of a dot placed on the flywheel.
Your speed seems pretty close for 4,000 so maybe it is just a motor issue.
You might confirm it is running on all cylinders.You wouldn't be the first to not realize a 2 stroke was missing a cylinder,including those with only 2 cylinders.

 

[ClassyGlassy]

Re: '76 Merc 1500 Tower of Power Prop Question

If I hook the timing light to each plug wire while its running and it flashes would that mean that I'm getting spark?

 

[steelespike]

Re: '76 Merc 1500 Tower of Power Prop Question

Yes but won't tell if the plug is firing.Pull the plugs,note their location and condition.
A lazy way to tell is to run a cold motor for about a minute turn it off and feel the plugs a dead plug will likely be cooler than the rest.