Question about motor not running at same speed as before?

[Karla45]

I was in my fathers boat this past weekend. He has a 1994 Mecury force 120hp on a pontoon boat. When he purchased this boat it had a 90 hp on it and would run at about 25mile per hour. He starting having problems with it so the dealer put on the Mecury 120hp now the boat will not go past 16 miles per hour. It has been like this for years now and I am just wondering what might be the reason for this, it just does not make since to me.

 

[roscoe]

Re: Question about motor not running at same speed as before?

Only running on 3 cylinders?
Wrong prop?
Throttle not opening all the way?


What are the rpm's when running at full throttle?

 

[fucawi]

Re: Question about motor not running at same speed as before?

very often a prop for a pontoon boat is pitched low as they assume a heavy load..you should be looking at a 17 or 19 pitch for that size motor and speed

 

[jerryjerry05]

Re: Question about motor not running at same speed as before?

Before you go buy a prop you need to figure out the rpms at wot.Do a compression and spark test.
You will probably need a 13 or 15P as a pontoon needs much lower pitch.

 

[bentle]

Re: Question about motor not running at same speed as before?

By the book rpms need to be 5000-5500 at wot. Speed is great but a motor that is not running at the right rpms is not burning its fuel efficiently. If you try to put a car or truck in 5th gear and try to drive up a hill what happens? Same factors come into play if your motor is over propped. You may also need to give it a good tune up. Check for good spark on all cylinders and also good compression on all cylinders. Check for any bad fuel lines and a fuel pump diaphragm may be needed if its sat around awhile before purchasing motor.

 

[Karla45]

Re: Question about motor not running at same speed as before?

I can say that it burns gas more than any other boat I have ever been in. It does not even get near 1 mile per gallon. I would have checked the compression while I was there but did not have compression tester or any tools with me. I am not sure why the dealer did not take care him since it was this way from the start. This all took place in the early 90's and he bought the pontoon new. He just recently put a new diapram in fuel pum. It will be a while before I go back there. I will share this information with him. I was just wanting to know what you guys thought might be going on.

 

[bentle]

Re: Question about motor not running at same speed as before?

After you get back there and look things over then post what you find and someone will be glad to help. Motor could need any number of things done to it but start without good spark, good compression, fuel pump repairs if any, fuel filter, fuel lines in good shape, carbs may need cleaning and adjusting, timing could be out, short in ignition, and the size of the prop. These are all things that could need to be adjusted or fixed and could be one or more of them. Now ad for the prop I would put a 15 pitch prop on with that motor and you will notice a huge difference in performance. Good luck and let us know what you find out.

 

[Karla45]

Re: Question about motor not running at same speed as before?

okay will do. Thanks