Rpm's and top speed, is there somthing wrong?

[Nutron]

I am running a 1975 2 stroke 115hp Johnson. The boat gets out of the hole good but the top RPM'S that I can get are 2500 max with a top speed of 40. The motor sits on a 1988 Glastron 16'. Would the prop be wrong or is there an adjustment that needs to be made to make the engine run higher rpm's? I figured on this light of a boat it should run alot better? I use to have a 90hp and it seemed to run better, I had someone tell me the 90 had more torque, the engine has the original prop on it, real good shape no damage at all. Please let me know what is going on or whether this is correct, also could you put a new and different pitch prop to get the horsepower and rpms out of it?

 

[metriccrescentwrench]

Re: Rpm's and top speed, is there somthing wrong?

Are you sure the tach is correct? 2500 rpm at 40 mph sounds way, way off.

It kinda' sounds like you may need to go to a different prop, but before you can make an informed decision on that you need to figure out what the real WOT rpms are.

 

[Nutron]

Re: Rpm's and top speed, is there somthing wrong?

How do I find out what the real rpm's are? When it was a the shop they had it up to 4000 to 4500 rpms in the tank, when I dumped it in the water the tach read 2000 to 2500 rpms, do you think the tach is off? and I currently have a 13x19 prop, if I went to a 15 or 14 pitch would that change the rpms and top speed, or do you think 40 mph is all the 115 will do?

 

[hidef]

Re: Rpm's and top speed, is there somthing wrong?

I would say your tach is experiencing technical difficulties you might need to change the pulse setting on it to read the engine RPM correctly.. If you are hitting 40 MPH you have to be turning somewhere around 4600-5000 RPMs. I would think your top speed should be right around 45 MPH so you might be slightly under propped but not by much. You might be over propped with the 19" but not knowing your exact RPMs it is hard to say.

 

[Nutron]

Re: Rpm's and top speed, is there somthing wrong?

How do you go about changing the pulse setting? And do you do this while at full throttle running down the lake? Thank you for your help. Also what is a good pitch to run, should I go to a 17?

 

[194269]

Re: Rpm's and top speed, is there somthing wrong?

According to my cal. at 40 MPH your engine is turning 4950 RPM. A little slow, but not bad, would like to see it at 5300 to 5500 RPM. You might want to change to a 17" pitch prop. The only way to change the plus setting is with everything shut off. There should a selector switch on the back of the tach. reading 4, 6, 8, and 12 I belive. not sure what number your engine uses, but I think its 12.

 

[wilde1j]

Re: Rpm's and top speed, is there somthing wrong?

Setting on back of tach is 6P. You need to know correct WOT RPM before giving any thought to messing with props. You need to prop for 5800 RPM @ WOT. 2500 RPM and 40MPH are simply not possible.

 

[Nutron]

Re: Rpm's and top speed, is there somthing wrong?

So your saying at 40mph the rpms are higher than 2500?

 

[hidef]

Re: Rpm's and top speed, is there somthing wrong?

That is what he is saying, it is impossible for your boat to go 40MPH with that engine turning 2500 RPM. You have to change the pulse setting on the back of the Tach. Take a look at it and find out what it is currently set at 6P is likely the correct setting.

 

[Nutron]

Re: Rpm's and top speed, is there somthing wrong?

I looked on the back of the tachometer and cannot find anywhere where that setting would be, There are just wires connected to it, is one of the wires where you are talking about you have to move?

 

[194269]

Re: Rpm's and top speed, is there somthing wrong?

Go back and read what I said before. With the speed you are running, the engine speed calculates to be 4950 RPM. Now thats twice the speed your tach reads, or you can say your tach reads at half engine speed. So you do have some idea of how fast your engine is turning but you still need to fix the tach.