im stumped

[m.qualls89]

Ok so ive posted a few threads and im still working on this, so i hope yall will will cut a new boater some slack lol. My 1978 mercury black max 150 hp is not running full rpms. I figured out it was the prop im running a 13.75" diameter x 21 pitch prop (not 24 pitch like i had previously posted) im only getting 4000 rpms and lugging the motor so im not using that prop now. The boat came with a smaller prop that has no numbers except MICHIGAN SPM 319 PATENT PENDING and its blue with a sticker that says michigan super prop. It lets the motor run over 6000 rpms at WOT so i have to hold the gas back. The diameter is much smaller than the 21 pitch. The guy at the marine store said to try a 19 pitch. Does the diameter make a big difference? Any suggestions will be greatly appreciated.

 

[123roger]

Re: im stumped

Rule of thumb: you gain 200/250 RPM's for each # pitch you decrease.

 

[steelespike]

Re: im stumped

Your rpm doesn't mean much without the gps speed.
also what size/brand boat is it? Unless the boat is a monster your numbers don't add up.
To get to about 5400 from a 4000 would take about a 7" pitch drop putting you at about 14"
thats close to what you would find on a pontoon.
So you may have a motor problem or a tach problem.
Some cells have a gps app.

 

[m.qualls89]

Re: im stumped

How much does the diameter matter? I get over 6000 rpms with the small prop that isnt marked my speed on my speedo app with the small prop is 29mph and with the 21 pitch its 35 mph... The boat is a 1978 glasstex sea star tri hull 18 foot open bow total capacity it 1650 pounds.

 

[steelespike]

Re: im stumped

Things aren't adding up Do you know if it has a V6 gear case?Do you know the gear ratio?

 

[m.qualls89]

Re: im stumped

No i dont... It looks to be all original but i cant say for sure. Here's the serial # 5072466

 

[steelespike]

Re: im stumped

Is this an inline 6?
Based on the numbers I have to think either your setup is wrong or the motor is weak.
Does the boat respond to up trim? Is there a wale tale or foil on the motor?
Is the bottom clean and smooth?Does it have a strong hole shot?

 

[m.qualls89]

Re: im stumped

No it is a v6, yes it has a whale tale, the hole shot sucks with the large prop but with the small prop its out of the water on plane in about 5 seconds... Not sure what bottom you are asking about unless you mean the bottom of the boat if so yes it it minus one little nick where the first day i got it i was loading it and my trailer came off the ball somehow... I check compression in all cylinders even though i did it cold the highest cylinder compression was 120 psi and the lowest was 110 psi.

 

[m.qualls89]

Re: im stumped

Also yes it responds to up trim i usually leave it low until i get going tgen trim up to get my max speed