Can't get full throttle

[garyp9999]

I have a 1970 7.5 HP Mercury outboard. Runs great except that when I put it in gear and increase the throttle, at about 1/4 speed, it starts to decline as I increase the throttle. Anyone have any ideas? Thanks

GaryP

 

[mthieme]

Re: Can't get full throttle

When you say decline, you mean the rpms drop? What happens after that? Does it conk out or want to? Do you have to idle for a little bit before going through the same thing?

 

[garyp9999]

Re: Can't get full throttle

Yes, the rpm's drop, but the motor doesn't conk out, it runs and pushes the boat, just at a slower speed. As I let off on the throttle (from full throttle), it doesn't change much until I get back to about 1/4 throttle, then it actually gains some rpm's. Through the whole range of throttle, the engine purrs. No misses or uneven firing. The only other thing I can describe is that the tone of the motor changes as I increase the throttle. Hard to describe, but there is a sound that makes me think the air mixture is changing. Butterfly opens fine. Any ideas? Thank you.

 

[mthieme]

Re: Can't get full throttle

Yup...got a couple ideas.
Sounds like a fuel system issue.
Things I would check...
Fuel pump diaphram
carb kit &/or saturated float
primer bulb / fuel lines cracked (whatever) - sucking air.
Fuel Filter(s) (partially) clogged

 

[garyp9999]

Re: Can't get full throttle

Thanks for the input. When it happened, I tried pumping the bulb, nothing changed, so I think the fuel pump is not the issue (do you agree?). The carb looks good and the float is dry and light. Fuel filter is also OK. But, I think you may have something with the sucking of air. The tank is very old. The fuel hose and bulb are bran new, but there could be a problem in the tank. My friend has a merc also, so I think I will borrow his tank and try it. If any other ideas come to mind, please let me know. Thanks much!

 

[mthieme]

Re: Can't get full throttle

Sounds like you've already got your hands in there.
The only other thing that comes to mind is a high speed stator coil. DOubtful this is your issue, but may be worth mentioning (?).
I had one go out on a 25 Mariner once - the motor just went about 1/3 and that was it. It ran good, just didn't me full rpm.

 

[Jack Spratt]

Re: Can't get full throttle

Did you change props? If not fuel, it sounds like it may be overpropped.
But I have very little experience w/ small gas engines.

 

[BIGcarpy50]

Re: Can't get full throttle

I was going to say the high speed stator.....search testing stator on here im sure you will find something....The same problem happened to me last summer...50hp rude ran great in barrel and at dock but when i put the throttle down it would gain speed then lose power after about 1/2 way...

 

[garyp9999]

Re: Can't get full throttle

Problem solved. Turns out to be pretty simple.....blocked tank vent. Thanks for all the ideas.

GaryP