1986 Yamaha 25hp doesn’t run good under a load

mattkline205

Cadet
Joined
Jan 12, 2021
Messages
25
I have a 1986 Yamaha 25hp it’s the European model and it has a new fuel pump all new fuel lines under the cowling with a new filter new coil and I have cleaned the carb twice. It will idle perfect but when it’s full throttled it doesn’t seem to reach the rpms that it should it sounds bogged down. I figured it was a fuel pump because at full throttle I squeezed the primer bulb and it took off and the rpms sounded right so I replaced the pump but it won’t do it again even when pumping the bulb. Compression is 120psi both cylinders
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Was the motor running strong before or it's something that developed afterwords ? Being a 35 year oldie the current prop must be shot or in the process to become so. Start with a prop hub test to determine if the prop is working right. The primer bulb will remain fully firm only when it's fully primmed, once motor is running will soften a bit which is normal.

Happy Boating
 
Last edited:

mattkline205

Cadet
Joined
Jan 12, 2021
Messages
25
I actually just bought the boat and the guy that had it before me said the motor was running great then all of a sudden stopped running good under a load so he replaced the fuel pump and cleaned the carb. How do I do a prop hub test?
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Remove the prop,

Spun HubTest.JPG

draw 2 straight lines at each side or 2 dots as seen, re install the prop, go for a wot run, remove the prop and check if both lines or dots have moved from where original were set. See if can draw a white line or dots with a marker or something appropriate. Hub slip failure it's very notorious specially when the prop powers heavy loads when at throttle...

Happy Boating
 
Last edited:

bbakernbay

Seaman Apprentice
Joined
May 3, 2012
Messages
34
Sea Rider - Thank you so much for sharing your immense experience, not only does it assist the original Poster but many others searching the same issue.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Sea Rider - Thank you so much for sharing your immense experience, not only does it assist the original Poster but many others searching the same issue.
You're welcome,

To the OP, report if with prop issues after the hub slip test to go from there.

Happy Boating
 
Last edited:

mike_i

Master Chief Petty Officer
Joined
Jun 28, 2017
Messages
879
Check the gas tank vent, is it open and clear? If you use a portable tank pop the gas cap open and try it. When you tested the prop as Sea Rider suggested did you do it in whatever body of water your boating in? The reason I ask is because you tested it and replied the results in less than an hour, just want to be sure you didn't run the motor in a tank or on muffs.
 

mattkline205

Cadet
Joined
Jan 12, 2021
Messages
25
If the prop hub was messed up wouldn’t it just act as it was in neutral cause the prop would be slipping
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
If the prop hub was messed up wouldn’t it just act as it was in neutral cause the prop would be slipping
Depends, if 100% shot yes, on its way to become spun no, the hub usually takes some time to be fully kaput. As Mike 1 asked, where did you wot test the motor, looking at it, on muffs, open water ? What slips is the internal rubber hub, turns bad over time and use...

Happy Boating
 

mike_i

Master Chief Petty Officer
Joined
Jun 28, 2017
Messages
879
You want to do the prop test with the boat in the water, that's the only way you'll have the full resistance of the water and boat on the prop and hub. Do you know if your prop has a shear pin?
 
Last edited:

mattkline205

Cadet
Joined
Jan 12, 2021
Messages
25
I’ll have to check about the shear pin. It’s seems to me like either a fuel issue or timing issue. Could the reed valves be messed up
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
If you squeezed primer and it picked up RPM it not a prop problem. Nor will a spun or slipping hub cause it to be bogged down, just the opposite. You have a carb or fuel delivery problem.
 

mattkline205

Cadet
Joined
Jan 12, 2021
Messages
25
If you squeezed primer and it picked up RPM it not a prop problem. Nor will a spun or slipping hub cause it to be bogged down, just the opposite. You have a carb or fuel delivery problem.
That’s what I was thinking. I was just taking all the input I could get. I cleaned the carb again yesterday this time I used my air compressor along with some break clean so I’ll give it another go this afternoon
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
If it's a non through exhaust prop should have a shear pin, remove the prop and check if in good condition, then check whatever comes to mind..

Happy Boating
 

mike_i

Master Chief Petty Officer
Joined
Jun 28, 2017
Messages
879
When he said he "squeezed the primer bulb and it took off and the rpms sounded right" that made me also think it was a fuel problem and suggested he check the vent and open the gas cap. Question to the OP, why did you replace the fuel pump, fuel lines, coil and clean the carb? Did you have the problem before you did all of this work or did you do it because of the problem?

If you squeezed primer and it picked up RPM it not a prop problem. Nor will a spun or slipping hub cause it to be bogged down, just the opposite. You have a carb or fuel delivery problem.
 

mike_i

Master Chief Petty Officer
Joined
Jun 28, 2017
Messages
879
OK now I'm back to my original guess that it's a fuel problem. What condition is the primer bulb in when the motor bogs down? Are you running the motor on a portable tank? If so have you run the motor with the gas cap off or confirmed the vent is clear?
I replaced all that stuff because of the problem to see if any of that would fix it
 

mattkline205

Cadet
Joined
Jan 12, 2021
Messages
25
The primer bulb looks brand new. And the gas tank is a portable tank but there is no vent buiilt in to it so when I run it later I’m gona try with the cap off
 

mike_i

Master Chief Petty Officer
Joined
Jun 28, 2017
Messages
879
When I asked about the primer bulb I should have been clearer. By asking the condition it's in I should have asked if it's puckered as in sucked in or compressed or hard when squeezed or soft where you're able to pump fuel thru it until it firms up.
 
Top