Cant get past 4500 RPM

[shannonlamb1]

Hey guys I have a new to me sun tracker PB 20 with a Mercury 90hp 4 stroke 2018. I can't get past 4500 RPM? Mercury website calls WOT 5000-6000 RPM. Is this normal sees like 4500 isn't really pushing it a ton to me but I feel like I'm missing something? Thanks.

[steve1313]

First thing I would check is the tachometer. Many of the analog ones have switches on the back need to be set correctly to match your motor (4/6/8 cylinder). Also, what size prop is on the boat now?

[shannonlamb1]

I think based on number it’s a black max mercury. Stamp says 78114a45 15P. Im guessing 15pitch but not sure diameter it’s not on the stamp.

9382A1B0-7E56-477D-B921-A7CC85EA2A1F.jpeg (86.3 KiB) Viewed 3136 times

[shannonlamb1]

In addition to above info this is also

F94D5362-1C7E-49AA-9E77-D41D8905A886.jpeg (71.08 KiB) Viewed 3132 times

the sticker for motor. Boat is 2018 guess motor is same. The “LCT” part of the one number leads me to think it’s command thrust long shaft. The mercury prop selector page says if current pitch 15 wot RPM of 4500 and max speed 21 drop to 12 pitch. I’m not seeing 21 mph more like 18-19 and it won’t let me input that.

[billb1981]

I put a new 115 motor on my Lowe SF214. They say it should be 5800-6000. I started with a 15, the holeshot was awesome but couldn't get them up to 5800 at WOT. I put a 13 on and I got 1 mph less speed and was at the 6000. So I went back to the 15 for now and running at 5400 WOT if that helps you any. I'm still tweaking and learning, but the 13 wasn't right for my situation. 21' pontoon, lifting strakes. Depending on your motor, you can buy a bluetooth adapter that will give you readings straight to your phone from the motor to confirm your tach is right. As of now I'm happy with faster, spending less gas.

I see you have the mercury, you could buy the bluetooth adapter. It's just money right?

[shannonlamb1]

Yeah I’m probably topping out 4500-4700 rpm and 19-20. That’s full boat 4 adult and 4 small kids. That’s almost the usual for us. Oddly today I had only 2 adults and 3 kids so I could get rpm up to right at 5000 and 22mph.
Should I even fix this? I mean 4500 rpm and almost 19-20 mph is okay for me but don’t want to create problems with motor. Doesn’t sound like if I lower pitch I’ll get more speed just more rpm?

[wwind3]

....not sure about 4 strokes but my 20ft Lowe with a 90 Merc Optimax does 27 at 5800 with a 16P. stainless prop. 17P got me 5400 and less speed.
Strangely my 16P aluminum spare gets me 28-29.. Still cant figure that one out. All GPS speeds That is just with me in the boat.

Are you able to trim the boat out? Check motor height too. Cavitation plate on motor should be even with bottom of mounting pod or close to it.

[shannonlamb1]

So out again today and have read a million things as well as talked to tracker dealer. They say supposed to be more like 14x11p. Again I’m at 15 1/4x15p black max 3 blade currently. Above is what guy said at tracker. Mercury prop selector based on speed/motor/etc.. says 16x12p which seems like not a big change. Again with 151/4x15p I’m getting 4800RPM max with trim up about 20% ish and max speed 21mph with 90hp command thrust 22ft pontoon.
Cavitation plate is basically level with bottom of “mounting pod” motor is on. Here are some pictures

[shannonlamb1]

Okay says file too large so no pictures. But above is still where I’m at.

[Marc K]

Just another reference point:
My 20 year old 115 HP Yamaha 4-stroke turns my stock 3 blade 13.5" dia. x 15 pitch at 5800 RPM at 27 MPH when running light.
My boat is 25' with two logs that are 23" diameter.

Running wide open, I blow out when trying to trim up any appreciable amount. This is my first pontoon boat, so I don't have a reference point, but the hole shot feels darned good to me for a heavy boat and not a lot of motor.

[shannonlamb1]

Well mercury recommended on the phone based on above information a spitfire 4 blade 13.8x13 pitch. Here goes nothing bc bass pro won’t take it back they said bc it’s not stock oem