Engine revs high but lower speed

[calgarygringo]

Have been struggling to figure this problem out to no avail. Maybe someone has an answer.
I have a 45 hp Chrysler outboard and motor starts and runs great. Where I get into trouble it seems when the water gets rough and boat is bouncing. When travelling at higher speed and hit a big wave etc. the motor almost feels like it comes half way out of gear. The speed cuts in half but the engine revs high and makes a loud sound as it does when out of water and you are revving in the backyard with just the water attachment on the back. We thought maybe a gas issue but have cleaned and done all the usual stuff.
Is there something in the bottom end we should be checking or something else we are missing.

Thanks

 

[ck85abc3]

Re: Engine revs high but lower speed

I don't know your motor well, so this might not be applicable to you, but...

The propeller spinning on the hub? Mark with a marker and if the two marks don't line up anymore, the hub is slipping.

 

[Jack Daniels]

Re: Engine revs high but lower speed

Are you sure the chop isnt causing the propeller to come out of the water?

 

[calgarygringo]

Re: Engine revs high but lower speed

That is what I first thought as well. I checked and it seems to stay in that mode until we stop then put into neutral then back into forward. That is why I started thinking something down below instead of fuel issues. That is what it sounds like that I was trying to describe.

 

[maxum247]

Re: Engine revs high but lower speed

How does the boat run in smooth water?
Sounds like the propeller is looseing contact with the water in rough waves!
Where does your anti-ventillation plate (flat plate above prop) set in relation to the bottom of the boat, above or below?
Should sit about level with the bottom.

Ventilation, air entering the blade area.
As the blades meet the air, the propeller momentarily looses it's bite with the water and subsequently looses most of it's thrust.

An added complication is that the propeller and engine over-rev, causing very low pressure on the blade back causing massive Cavatation.

 

[calgarygringo]

Re: Engine revs high but lower speed

It does run perfect on smooth water as I thought about the air idea. What confuses us is that even when we slow right down if it has done it's thing it does not return to normal until I stop and put to neutral then forward...
I have been boating all my life and never have run into this. That is why I was wondering if maybe something slipping in bottom end.

 

[maxum247]

Re: Engine revs high but lower speed

Only other thing I can think of is, that when shifted into gear the the shift mechanism in the lower unit isn't locking in all the way, causing the unit to partially disengage.
Once stopped and in neutral resets things, but never really locks in tight so to speak.
Wonder if it's a shift cable adjustment, or if tiller a shift rod adjustment?

 

[eurolarva]

Re: Engine revs high but lower speed

If RPMs jump and speed stays the same do as mentioned above. Test for a spun hub on your prop. This is a very common problem. Typically when a motor jumps out of gear due to a bad clutch or gear it will lurch and feel like you hit something. Put a mark on the prop to the prop nut and then take the motor out and try and duplicate the problem. Once it happens look at the marks you made. If they dont line up you need a new prop or you need to get the old one fixed.

 

[calgarygringo]

Re: Engine revs high but lower speed

I maybe will check the shift cable and prop.
Thanks for the tips.....

 

[eurolarva]

Re: Engine revs high but lower speed

My chrysler is probably different from yours but from the photo you can get the idea. Draw a line with a marker like a sharpie like in the pic. When motor surges check to see if the line still lines up. Props now a days no longer use shear pins. Hardenend rubber holds the prop to the shaft so if it hits a rock or something the rubber will break and hopefully not damage your gear housing ( I prefer gear housing because I dont like talking about lower units ) The end nut that holds the prop on spins the same as the prop does unless the rubber is broken. Because of the torque of the motor it is real hard to test this by hand. Best way is to warm the motor up real good then goose it real hard and see if you can get it to slip

 

[TireFryer]

Re: Engine revs high but lower speed

eurolarva:
what model motor is this one in the pic? Is this a shear/drive pin version or the hardened rubber hub version? My 1969 55HP Chrysler looks the same (in terms of prop in the pic)... prop spins freely and trying to see if pin is broken or hub needs replacing... thanks