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Max RPM vs. Cruising RPM and Overall Performance Suggestions

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DavidNC50

New member
Hi Everyone I have a question about engine RPM's and what is safe/normal. I'd also like some general feedback on performance... looking to tap into the knowledge that you guys have gained from years of experience.

I have a 135 Mercury Black Max V6 and I was looking for feedback on what RPM the engine should be turning at full throttle and what's the Max I should turn.

Currently it seems to bog on take off but will spin at 5000rpm's when on top end. Is this to high for this motor? It sound sooooo sweet!

135 Mercury Black Max
23 pitch Laser SS prop
water block off plates
jack plate lifted maybe 3 or 4 "
water pressure is about 4 or 5 at idle... 10-15 full speed
speedometer will max out at 60 with a little trim
chine walk starts around 60+ and forces me to back out:brickwall:
I do have hydraulic steering



The boat does not jump out the hole very fast but seems to love top end. Please let me know what you guys think about changing the pitch of the prop. I don't really want to lose top end. Some guys have suggested a 21 pitch would be better... what do you think?
 
That prop has vent plugs right? remove them and see how it is out of the hole.

Or try something with vent plugs.

Addition:

Where is the prop shaft in relation to the bottom of the boat when you have the motor jacked up?

You could safely spin that motor 5800-6000 RPM.
 
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Yes it has vent plug holes but they are empty. They are about as big as a dime with no insert. These let air surround the prop and help let the motor rev out of the hole correct?

I will measure the distance on the jack plate and see if I can snap a couple pics.
 
You may want to drop down to a 21 pitch, you should be able to maintain or even increase your top end if you can get it up to its max rpm range(5800).
Calculations for a 2.0l 135hp with a 2.0-1 lower. .08 propslip (estamated of course)
max rpm @ 5000/23p you should "theoreticly" only be seeing about 47-48 mph
max rpm @ 5800/21p you should "theoreticly see about 51 mph.
Check your speed with a GPS, Water pressure speedos are notorioulsly inaccurate, especialy over 40 mph.
You should have a much better hole shot and more top end with a nice 21p, I think your trying to swing to big of a wheel to get the best performance out of your set up.
Good luck with it!!
 
I've got a 78 Predictor with a 135 Blackmax, and a hydraulic jackplate. I"m running a 23 pitch spinelli, and the boat tops out at 60 gps. RPM's are around 5300.
 
Adambomb said:
I've got a 78 Predictor with a 135 Blackmax, and a hydraulic jackplate. I"m running a 23 pitch spinelli, and the boat tops out at 60 gps. RPM's are around 5300.
Click to expand...


23 pitch is too much for that motor. I'd run a 19 pitch at 5500-5700 or so or a 21 pitch 4 blade at 5500 ish. You may or may not lose top end speed but you'll get a lot more low end grunt. Even a 17 pitch 4 blade will put you into the 50's and will be a lot more fun.
 
I don't believe the Mercury PVS holes are meant to be run empty. I tried it on my starliner and it would not even get on plane. They sell plugs with different sized hold plugs, or solid plugs.

But it sounds like that's not your problem though - 5000rpm max is way too low. You're losing holeshot and top end. As suggested, drop back a pitch or two and you'll be happier.
 
21p was my 1st thought, the 135 is a great little mota,They ran circles around my fresh 150 in-liine. If loaded it up with to much prop for your load you can easily damage the mota. let it spool up and run!

my 21p lazor II is 20 yrs old and the vent holes are square with no plugs available so I don't believe you absolutely need plugs in them?
 
Plugs/no plugs/hole sizes depend on the hull and motor. Ram's boat runs best with no plugs in his Tempest, I tried it like that on my Liner and it would struggle to get on plane even with just me in it because it slipped so bad.
 
atc250r said:
Plugs/no plugs/hole sizes depend on the hull and motor. Ram's boat runs best with no plugs in his Tempest, I tried it like that on my Liner and it would struggle to get on plane even with just me in it because it slipped so bad.
Click to expand...


Yup! I run no plugs in the vent holes and my engine has the same block as the OP. Merc 2.0
 
Thanks for all the replies!! I need to get those pics taken so you can see the holes in the prop.

I had the speedo quit on me... gotta get it back right and compare to GPS also. any ideas on fixin...

I have a 23 pitch SS & a 15 pitch aluminum.... such a huge gap. I will get those pics and maybe a video to show how it performs. :thumb:
 
You should have a black tube running up from the front of the lower unit, that may have come diconnected or if you look at the front of the lower just above the tip of the nose there should be a small hole see if its blocked. These speedos work on water pressure into that hole thats why they are so inaccurate once you get flying, But you can even use the GPS from your car for a comparison any type of portable GPS will work. I use one of those small garmen etrex. its water proof and it will hold my max speed for that trip till I clear it.
I watched a guy use his car gps record 105 mph pass in an allison GS, he just kept it in his pocket.
 
Plenty of free apps for apple and android to check gps speed. My boat has a pickup attached to the transom and a tube attached to that. Sometimes when running it hard the pickup gets folded back and the speedo stops working.
 
DavidNC50,

My Faria speedo works on venturi as well and has a small adjustment screw on the rear to let the user tweek it in.
 
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