Motor over revving

[shaunknee]

Hi all

I have an ?85 Mariner 2S 25HP. I?m thinking the pitch is insufficient for my motor. At full throttle I think my rev-limiter is kicking in.

My current prop is a 13P (48 4356 A40L 13P stamped)

I did the timing on the back of a large boat and the motor runs great at full throttle. It even gets a 17 foot boat up on plane. The problem is when I put the motor on the 14f inflatable I want to use it on. At full throttle it stops firing / runs rough on the small, light boat.

I know I should get a tack, however I was wondering if I increased the pitch by one or two more I might solve my problem.

I was hoping for a pitch suggestion.

Thanks

 

[rndn]

Re: Motor over revving

It's really hard to tell if you need 2 inches of pitch or 8 inches of pitch. If you want to buy a prop with 2 inches more of pitch and find out that you still hit the rev limiter and then buy a prop with 4 inches more of pitch, etc, etc. Even if you can get a loaned prop you really still will not know if your motor is over-revving. Try to find a local marina that will hook up a test tach for you and also suggest the right prop. For your reference for every 1 inch of pitch change you should see about 200 a rpm change. Add 1 inch drop 200 rpm's, reduce your pitch by 1 inch and add 200 rpm's.

 

[shaunknee]

Re: Motor over revving

Thanks for the response rndn

If I wanted to try a prop with 2 more inches of pitch off ebay am I looking for a 15P with a certain diameter and spline size ?

Thx

It's really hard to tell if you need 2 inches of pitch or 8 inches of pitch. If you want to buy a prop with 2 inches more of pitch and find out that you still hit the rev limiter and then buy a prop with 4 inches more of pitch, etc, etc. Even if you can get a loaned prop you really still will not know if your motor is over-revving. Try to find a local marina that will hook up a test tach for you and also suggest the right prop. For your reference for every 1 inch of pitch change you should see about 200 a rpm change. Add 1 inch drop 200 rpm's, reduce your pitch by 1 inch and add 200 rpm's.

 

[rndn]

Re: Motor over revving

As long as the prop is designed for your motor you should be fine with spline and diameter. A larger or smaller diameter has very little effect on the performance, so be concerned that it will fit and that the pitch is what you are trying to get.

 

[rndn]

Re: Motor over revving

As a rough guess, I would think the motor would still over-rev by adding only 2 inches. A 25HP pusing a 14 foot inflatable is a lot of motor for a little bit of boat weight. If you're over revving at 13P, if you're just going to try something, I would jump to 17P. Most outboards have a 500 to 1000 top rpm range so you do have a little wiggle room should you need while still keeping the motor operating within the suggested guidelines.

 

[shaunknee]

Re: Motor over revving

Thanks for the advice. I'll try a 17P. This motor seems to explode to full power. It is amazing how such an old motor can produce so much power.

As a rough guess, I would think the motor would still over-rev by adding only 2 inches. A 25HP pusing a 14 foot inflatable is a lot of motor for a little bit of boat weight. If you're over revving at 13P, if you're just going to try something, I would jump to 17P. Most outboards have a 500 to 1000 top rpm range so you do have a little wiggle room should you need while still keeping the motor operating within the suggested guidelines.