not getting enough rpm's

[atk82]

i have a 94 50hp evinrude with a 13 pitch prop and i can only get 4700 rpm at wot i rebuilt carbs,replaced spark plugs and wires any suggestions

 

[SparkieBoat]

Re: not getting enough rpm's

what boat is it on?? did you get more RPMs before, or have you just put this motor on this boat?? A little history could help.

 

[atk82]

Re: not getting enough rpm's

its on a 17' lowe aluminum bass boat and the motor came on the boat

 

[tashasdaddy]

Re: not getting enough rpm's

start at the beginning, check compression and spark with testers on both cylinders.

brp site recommends:
0386841 PROPELLER, 11 3/4 x 17 - Aluminum (40). Refer to OMC SysteMatched Parts & Accessories Propeller Chart

0386786 PROPELLER, 12 1/4 x 15 - Aluminum (25). Refer to OMC SysteMatched Parts & Accessories Propeller Chart

sounds like too much boat for power. what HP is the boat rated for.

 

[SparkieBoat]

Re: not getting enough rpm's

I agree with TD, check compression and spark and fuel lines/carbs. Spark must be checked with a spark gap tester at 7/16 gap. tester sold at auto parts stores for around $10.

 

[SparkieBoat]

Re: not getting enough rpm's

and to TDs other statement about possible under power, I assume you do not know if this motor ever turned more RPMs than this on this boat, so it may need a 70 HP. you do not volunteer much info so, if the boat has rod lockers, live wells, console, trolling motor, multiple batteries along with other heavy type stuff you may have a heavy boat, but then again I am just guessing. also check throttle linkage and cable. do link and sync.

 

[Tim Frank]

Re: not getting enough rpm's

A few doors down on the left you'll find the "prop question room".
I think you need to ask this question in there; they'll have some advice.

 

[atk82]

Re: not getting enough rpm's

the boat is rated for a 75hp and i dont know if it ever got more rpm's also the compression is good and i got good spark the boat has a rod box, 2 livewells that hardly ever have water in them,2 batteries,and a 12 gallon fuel tank im having a link&sync done tomorrow hope it helps

 

[tashasdaddy]

Re: not getting enough rpm's

you are under powered by 33.3 %, you cannot ask anymore of that motor. nothing is going to help but more hp. that motor is doing all it can do with that load.

 

[SparkieBoat]

Re: not getting enough rpm's

If your motor is producing the full HP and you plan on keeping that motor on that boat, I would suggest an 10 or 11 pitch prop, this should get your RPMs into the mid 5Ks which will give you better performance and longer engine life.

 

[atk82]

Re: not getting enough rpm's

thanks for all the help everyone i guess im going try a new prop but this gives me a reason for a new motor

 

[trendsetter240]

Re: not getting enough rpm's

thanks for all the help everyone i guess im going try a new prop but this gives me a reason for a new motor

Check your compression at a minimum first. No sense buying a low pitch prop to make that boat plane if the motor is on it's last legs anyway.

I had a 50hp merc/mariner push a 1600lb 17ft fiberglass runabout with a 13p prop. It would plane easily with a light load and wind out to 5000 rpm.

You will not be happy with that setup with an 11p prop. You will get the rpms but your motor will be screaming while you chug along at 18mph.

 

[kenmyfam]

Re: not getting enough rpm's

Make sure that the motor is running 100% then move on to your motor height, trim, loading and then to the prop. What you get at that point is the best you will get with that motor. Anything more will mean moving to the max hp.
My 2 cents.

 

[SparkieBoat]

Re: not getting enough rpm's

it has been my experience that changing prop pitch by 2-3 inches does not usually change top end speed very much, because the increased RPMs will mostly make up for the lower pitch. for instance with a 13 pitch prop at 4700 RPMs the theoretical speed is 29 MPH. with a 11 pitch prop theoretical speed is 27 MPH. All of this is just theoretical because every boat is different and each boat will have what I call a sweet spot prop that seems to give you the best over all performance.
It is also my understanding that over loading an outboard with too large of a prop so that it does not reach the proper RPMs can lead to shorter motor life and excessive fuel usage.
But I do agree that the motor should be checked and eliminate any motor malfunctions as prop sizing is not a substitute for a needed motor repair.
Ken also mentioned checking motor height. possibly jacking the motor up would reduce drag but if it starts cavitating you have gone too high.
There are a number of things that can be done to help, but if after finely tuning the motor your RPMs are still that low I would change prop to get over 5K at least.
That boat probably would do much better with a 70, if your 50 runs good sell it and find a good 70, I just sold a 1995 rude 70 in great condition for $1200. I got a 225 for sale if you want to really make that boat go...just joking

 

[trendsetter240]

Re: not getting enough rpm's

it has been my experience that changing prop pitch by 2-3 inches does not usually change top end speed very much, because the increased RPMs will mostly make up for the lower pitch. for instance with a 13 pitch prop at 4700 RPMs the theoretical speed is 29 MPH. with a 11 pitch prop theoretical speed is 27 MPH. All of this is just theoretical because every boat is different and each boat will have what I call a sweet spot prop that seems to give you the best over all performance.
It is also my understanding that over loading an outboard with too large of a prop so that it does not reach the proper RPMs can lead to shorter motor life and excessive fuel usage.
But I do agree that the motor should be checked and eliminate any motor malfunctions as prop sizing is not a substitute for a needed motor repair.
Ken also mentioned checking motor height. possibly jacking the motor up would reduce drag but if it starts cavitating you have gone too high.
There are a number of things that can be done to help, but if after finely tuning the motor your RPMs are still that low I would change prop to get over 5K at least.
That boat probably would do much better with a 70, if your 50 runs good sell it and find a good 70, I just sold a 1995 rude 70 in great condition for $1200. I got a 225 for sale if you want to really make that boat go...just joking

Hey sparkie I agree with everything you said there but you are a bit off on the numbers.

That motor has a 2.41:1 gear ratio so with a 13p at 4700 RPM with 10% slip the theoretical speed is 21.6 mph. With an 11p it's 18.3mph. A 10% slip ratio is also optimistic.

Also why would you sell that 1995 70? Those are the keepers

 

[bassfearmee]

Re: not getting enough rpm's

FYI I have a 92 Tracker 17' bass boat very similar to your boat, it's rated for 70hp. I have an 89 50hp Evinrude on it with a 19" pitch cupped prop and it screams, I hit 30+mph no problems. No problem getting it up to 5500rpm fully loaded with three batteries, trolling motor, etc. I will use a 17" prop when I have passengers.

It's possible that the thottle stop on your motor needs to be adjusted. I had to back mine down to keep the rpms from going over 5500. 50hp should be plenty to run that boat. The boat can't weigh more than 800lbs.

Bob

 

[atk82]

Re: not getting enough rpm's

i have no problem getting my boat on plane and im running 26 mph by gps up the missouri river im going to check the motor height when i get it back from the the shop how do you adjust the throttle stop

 

[trendsetter240]

Re: not getting enough rpm's

i have no problem getting my boat on plane and im running 26 mph by gps up the missouri river im going to check the motor height when i get it back from the the shop how do you adjust the throttle stop

hmm..are you sure that tach is accurate? 26mph should be around 5600RPM for that motor and prop setup.

Unless I am mistaken in the gear ratio but I believe the 93-95 evinrude 50, 60 and 70hp all have 2.41:1.

 

[bassfearmee]

Re: not getting enough rpm's

On mine it's in the linkage. I'd recommend you get a factory service manual for that motor. FACTORY SERVICE MANUAL, not an aftermarket. There is a very precise procedure outlined in it to "Link and Syn" the motor. I got one for mine and it's 100% better than an aftermarket manual.

I agree about the tach, it may not be reading correctly, it may be set for a different motor. Either way, 26mph sounds about right with the motor all the way down. When you get up to speed, raise the motor slightly with your switch, you'll jump up over 30mph.

 

[SparkieBoat]

Re: not getting enough rpm's

my numbers were with no slippage at 2.0 ratio...they were meant only for reference..I was not sure the correct ratio and was too lazy to look it up.
I have also seen a lot of older tachs give false readings, try borrowing another known good tach and check it then, also make sure your tach is set to the correct pulse setting.