Johnson 150HP revs up

[mochosla]

Happy New Year everybody!<br /><br />I have a 1991 SunBird 21' with a Johnson 150HP of the same year. When I open full throttle from a stopped position or when I'm going slow, the motor revs up and the boat does not take off as it should. If I slowly gas it, the boat takes off fine and it will go as fast as 55mph and with a lot of power (by the way, it does this regardless of the trimmed possition). I'm wondering if I need to bring down the motor down to the last whole on the motor base on the transom. I have room to lower the motor about one to two inches more. The prop has a couple of small nicks but I don't think they affect it. Also, the motor has a planning board (I guess that's what they are called) that is in good shape. <br /><br />Thanks for your help.

 

[jimmbo]

Re: Johnson 150HP revs up

sounds like your motor is too high on the transom for the prop you are using. Withe the motor down and the anti-ventivention plate parallel to the bottom of the boat, is the anti-ventilation higher or lower than the bottem of the boat?

 

[G DANE]

Re: Johnson 150HP revs up

Hi mochosla<br /><br />Which prop is on it ? Lowering your motor would shurely give you the bite you need for hole shot, but lower your wot speed and mileage too. A prop with more cupped blades and more bite would probably do the job better. Jimboo's question is very relevant too, can you provide both informations ?

 

[G DANE]

Re: Johnson 150HP revs up

Hi mochosla<br /><br />Which prop is on it ? Will it also ventilate in turns at speed ? Lowering your motor would shurely give you the bite you need for hole shot, but lower your wot speed and mileage too. A prop with more cupped blades and more bite would probably do the job better. Jimboo's question is very relevant too, can you provide both informations ?

 

[angus63]

Re: Johnson 150HP revs up

It is also possible you have a spun hub on your prop that is beginning to slip under load.

 

[Dhadley]

Re: Johnson 150HP revs up

Concentrate on the prop. Lowering the motor at this stage is not what you want to do.<br /><br />Did the boat ever accelerate properly?<br /><br />The hub may be OK but the blade tips may be worn. If the blades are worn and the cup is disapearring it will do as you describe.

 

[mochosla]

Re: Johnson 150HP revs up

Wow!!! thank you very much for all your quick responses. This is a great site. I will look into the prop and ventilation plate. I thought about the prop hub but the problem is too regular and predictable so it sounds like the prop is ventilating. if changing the prop would cut my top speed but resolve my problem, I can live with it as I don't care to be driving at 60mph.<br /><br />I'll update you tonight. <br /><br />Thank you again.

 

[mochosla]

Re: Johnson 150HP revs up

One more question, what's miles per galon I should expect from this motor? The boat is a 20' center console.

 

[Dhadley]

Re: Johnson 150HP revs up

If it requires a prop change or rebuild you probably wont lose top speed. Might gain. You will most certianly lose speed if you lower the motor.<br /><br />Now, back to what I asked above -- did it ever perform right?

 

[mochosla]

Re: Johnson 150HP revs up

I just bought this boat so i don't know if it ever worked fine. i assume it did at some point.<br /><br />Thanks.

 

[rdgallo]

Re: Johnson 150HP revs up

DHadley, remember the guy with the 175 hp Merc that was wanting to get more speed out of his Ranger 17+ footer? He would probably want to talk to this guy who is getting 60 mph out of a 150 Johnson on a 21 foot boat, huh?

 

[Dhadley]

Re: Johnson 150HP revs up

Yeah, I thought about that too Ron......... just another reason to have a Johnson?? <br /><br />Mochosla, lets look at the prop. What is it? How worn? I'm not as concerned about the little nicks as I am the cup. And where the cup ends.

 

[mochosla]

Re: Johnson 150HP revs up

I'm sorry. what do you mean abou the cup and where it ends?

 

[Dhadley]

Re: Johnson 150HP revs up

What brand prop is it?<br /><br />Aluminum or stainless steel?

 

[mochosla]

Re: Johnson 150HP revs up

Okay, I can't see the prop brand but it looks like it's the original that came with the motor. The size is 14 1/2 X 19" and it's aluminum (black). One thing I noticed is that the prop does not clear the bottom of the transom. I mean that only about half of it will come under the the boat. I don't know if that's how it's supposed to be.....???? The prop does not look very worn at all.

 

[walleyehed]

Re: Johnson 150HP revs up

I'm a bit slow some days, but I think I see one of two issues here..???<br />#1., If the boat is equipped with a jackplate, it's at the top of it's travel....<br />#2., If we don't have a jackplate, it would appear (to me) we have a 20" shaft on a 25" transom...????<br />DH, whatcha think???

 

[mochosla]

Re: Johnson 150HP revs up

I don't have a jackplate but now that you mention it, and based on the fact that my motor seems to be short, can I install a jackplate like this one:<br /><br /> http://www.iboats.com/mall/index.cg...k plate&session_id=62861665&*******=731431265 <br /><br />Is there a rule of thumb as to how low the motor is supposed to sit on the transom? <br /><br />Thanks for your help!!

 

[walleyehed]

Re: Johnson 150HP revs up

From what you have explained, the prop blade when at it's highest point, is 6" above the bottom of the boat...is this correct???<br />Where the motor sits on the transom is not as important as where the anti-ventilation plate is in relation to the bottom of the boat.<br />We need to know where that plate above the prop is in relation to the boat bottom...can you post that info??

 

[mochosla]

Re: Johnson 150HP revs up

I will take a picture and post it here (if I can figure out how to do it). Can I e-mail it to you?

 

[walleyehed]

Re: Johnson 150HP revs up

Yes,....<br />ktsander@st-tel.net