Think I'm Over Propped??

[greatoutdoors]

Hi,

I have a 16' Sea Nymph bass boat with a 1972 Johnson 50hp. The motor seems to be in great running condition. It fires right up, has 140+ compression, new carbs etc. etc. I cant get it to run over 15 mph or get it on plane. You guys could estimate the weight of the boat, regular 16 ft'er with decks livewells and rod lockers. Two 200 lb guys on it. If I had to estimate the rpms the motor is getting at WOT now I'd say 3000 at the highest. The prop size is a 13 3/4 X 15. The blades are literally 1/4 inch from the cavitation plate. It just seems too big. I had a 40 hp Merc on it last year and with three guys on the boat it ran 28 mph..up river. Can anyone suggest a prop size for me? Its seems a 50 horse should be pushing this thing over 35 mph. Thanks for the help.

 

[BonairII]

Re: Think I'm Over Propped??

Its seems a 50 horse should be pushing this thing over 35 mph. Thanks for the help.

Doubtful that a 50 horse will get you to 35mph. You're motor is pre-1986, so you're probably only getting 40hp at the prop.

You need to install a tach and find out your true rpm. You also need to look at your transom plate and find out what your motor's operation range is.

You're probably missing on a cylinder. Start the motor and disconnect each plug wire one at a time. If motor doesn't stumble when a plug wire is pulled...that cylinder isn't firing.

Even if you are overpropped, dropping to a 13 or 12" prop will only gain about 400-600rpm.

 

[greatoutdoors]

Re: Think I'm Over Propped??

Doubtful that a 50 horse will get you to 35mph. You're motor is pre-1986, so you're probably only getting 40hp at the prop.

You need to install a tach and find out your true rpm. You also need to look at your transom plate and find out what your motor's operation range is.

You're probably missing on a cylinder. Start the motor and disconnect each plug wire one at a time. If motor doesn't stumble when a plug wire is pulled...that cylinder isn't firing.

Even if you are overpropped, dropping to a 13 or 12" prop will only gain about 400-600rpm.

In theory, 400-600 rpms, right? If a smaller prop would allow the boat to get on plane, that should take all that load off the motor and allow it to reach a much higher rpm, correct?

 

[BonairII]

Re: Think I'm Over Propped??

Don't bother with a different prop until you find out what's wrong with your motor. Your current prop has nothing to do with why you're only getting to 15mph.

Your most likely only running on a single cylinder.

 

[greatoutdoors]

Re: Think I'm Over Propped??

Don't bother with a different prop until you find out what's wrong with your motor. Your current prop has nothing to do with why you're only getting to 15mph.

Your most likely only running on a single cylinder.

Would a good way to test that out of the water be to crank the motor with a single plug pulled at a time, and hold it against the block to check the spark? Or would there be another reason for it to run in one cylinder? Wouldn't the cylinder that isn't firing fill up with gas while I'm running it on the water? Thanks for the help.

 

[BonairII]

Re: Think I'm Over Propped??

Start the motor THEN pull one plug at a time to find the bad cylinder.

The only reliable way to check for spark(esp finding out the QUALITY of the spark) is to use an adjustable spark tester.

The cylinder not firing won't fill up with fuel, the unspent fuel will just go out the exhaust into the water.

 

[greatoutdoors]

Re: Think I'm Over Propped??

The cylinder not firing won't fill up with fuel, the unspent fuel will just go out the exhaust into the water.

That's what I thought. No fuel comes out the exhaust. Could that mean a fuel problem?

 

[BonairII]

Re: Think I'm Over Propped??

Start with the basics. Check compression and check spark with an adjustable spark tester. Report back with the results.

 

[steelespike]

Re: Think I'm Over Propped??

Yes you wouldn't be the first to not realize you were one cylinder down except for the lack of power.

 

[dazk14]

Re: Think I'm Over Propped??

Yes you wouldn't be the first to not realize you were one cylinder down except for the lack of power.

+2

A 15" pitch prop is the norm for that motor/boat size. 13 3/4" diameter isn't your problem.

Has this motor ever run properly for you?

 

[pole position]

Re: Think I'm Over Propped??

+2

A 15" pitch prop is the norm for that motor/boat size. 13 3/4" diameter isn't your problem.

Has this motor ever run properly for you?

I agree,that is probably the correct prop.I have a 1973 50 hp Evinrude with a 13 3/4x15 prop on a 15 foot Atlantic Craft (bowrider) and it will run 30 mph with 2 people and crab/fishing gear no problem. Check the motor to see if it's hitting on both cylinders.

 

[steelespike]

Re: Think I'm Over Propped??

Could be the link and sync maybe the throttle isn't picking up the carb as it should.