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duece88

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i have a 78 mercury black max 1750 outboard. the problem i am having is i cant get my rpm up. if i use a 3 blade aluminum 15 pitch prop i can get about 3800 rpm and get around 45 mph with 2 people in the boat. I have tried a 2 blade brass prop that is a 21 pitch and get about 3300 rpm and 46 mph. The factory specs says this motor should turn around 5500 rpm at max throttle. Any idea how to get the rpm up?
 
Finding the right pitch is another.Basically,the higher the pitch the faster you go.Depends on your rpms at WOT.Then,depends on what type,size of your boat as well as gear ratio.
The lower the pitch the higher the rpms.The higher the pitch the lower the rpms.
The lower the rpms the better.Less work on the engine.You can't find your correct prop pitch by changing from 2-3 blade props.That's just crazy.If your gonna run a 3 blade then run a 3 blade.
(Your saying that you are getting 3800 rpm at 45 mph on a 3 blade aluminum 15 pitch prop.)First,thing I would do is to throw away that aluminum prop and install a 3 blade quicksilver stainless steel prop.
(Any idea how to get the rpm up? )A lower pitch will higher the rpms but,in turn lower your speed.
(The factory specs says this motor should turn around 5500 rpm at max throttle)Sure red lined.What's your factory specs say about the propper pitch at 5500 rpms?
Well a 3 blade 12 pitch should get your rpms up where you want to be from using a 15 pitch prop.But,will greatly lower your speed for sure.The main question is what do want?Speed or Rpms?Can't have the cake and eat it too.
I'm very surprized you got 45 mph out of a 15 pitch at 3800 rpms.Sounds to me your rpms should of been much higher.I'm beginning to think it's not your prop after all it's your engine not at par.The more people you have on board will lower your speed.Go to your local shop and ask if they have any test props for you to try?Most do have them for sale reasons.
 
"The lower the rpms the better.Less work on the engine."
I disagree. These engines are designed to run WOT @ 5500.
If you are running WOT @3800RPMs,the engine is laboring and will overheat,destroy powerhead,lots of $s.
You don't say what kind o' rig you have or it's weight.So I suggest you listen to the sound of your engine.WOT it should be screaming.Does it sound as if it's labouring? Yes-change prop,or,increase HP for the rig. No-check your tach. Is the tach set on the right pole position?
 
agree on the need to turn max rpm....when i ran a 2 cycle part of every trip was at max rpm even if it was only a mile or two......depending on the rig of course but 45 mph sounds high for 3800 rpm...as haventaclue states the difference in 3800 and 5500 sound is easy to hear for someone that has run outboards....if your experience level is low find a friend that has some experience to take a ride with you...or better yet take it to a shop that has a tank and buy an hours time and let them run the motor in the tank...if you continue to run as is and the rpm is low then at a minimum you are going to have a carboned up engine...
 
Still think there is something wrong with the motor.----Find a shop that has a dynomometer and have the engine checked.-----------If the motor is confirmed to be making the proper horsepower , then you can find a prop that allows 5500 RPM.-----Running that motor full throttle at 3800 RPM is a mistake and a big mistake at that !
 
I get the feeling the stator may be bad. Or your dash gauge is wrong. 3800 is not even close. No amount of repropping wil fix that.
 
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