Yamaha 25hp 2stroke outboard

[will wrenn]

I have a 1988 Yamaha 25hp outboard and I’m having a problem with the motor bogging down past 3/4 throttle. It seems like it’s a fuel issue. I have cleaned the carb, put new ignition coils on, new fuel pump, new primer bulb and line, and all new fuel lines, and new impeller. Any suggestions on what it could be thanks.( The motor runs like a top and fires right up other than that)

 

[mike_i]

Is the gas tank vent clear? Loosen the gas cap next time it happens. Do you have another gas tank you can tempy run it on? Is the primer bulb hard or deflated when the motor bogs down? Have you checked, cleaned or replace any fuel filters?

 

[will wrenn]

Is the gas tank vent clear? Loosen the gas cap next time it happens. Do you have another gas tank you can tempy run it on? Is the primer bulb hard or deflated when the motor bogs down? Have you checked, cleaned or replace any fuel filters?

The fuel tank does not have a vent but I have cracked the cap on the tank while running and I still have the same issue. I have replaced the fuel filter and all fuel lines as well. The primer bulb is not deflated or hard it is in between.

 

[99yam40]

does it help to pump the bulb when the problem shows up.
maybe there is an air leak somewhere, so it is not sucking enough liquid fuel to keep up

 

[will wrenn]

does it help to pump the bulb when the problem shows up.
maybe there is an air leak somewhere, so it is not sucking enough liquid fuel to keep up

no I still have the same problem. Would it help to replace the jets in the carb even though I know they are clean?

 

[99yam40]

sounds like there is not enough fuel in the carb bowl to keep up with the usage.
possible restriction some where

 

[will wrenn]

Should I buy a new carb or a carb rebuild kit. I have messed with the fuel air mixture and I have the screw about 2 and 3/4 turned out. If I turn it in to about 2 1/2 or 2 it starts to run rough. I think where I have it now is perfect. Could my motor be out of time? I did want to note that the motor runs flawless out the water but when the motor goes under load is when it starts acting up past 3/4 throttle thank.

sounds like there is not enough fuel in the carb bowl to keep up with the usage.
possible restriction some where

 

[99yam40]

compression spark, and timing of that spark are all very easy things to check to see it they are in spec.
also, the idle mixture screw should also be where the specs for you motor shows.
if it does not run well with all those in spec there is something wrong
motor needs more fuel under a load
even setting the lower unit into water creates some back pressure that is different than running on muffs

 

[will wrenn]

compression spark, and timing of that spark are all very easy things to check to see it they are in spec.
also, the idle mixture screw should also be where the specs for you motor shows.
if it does not run well with all those in spec there is something wrong
motor needs more fuel under a load
even setting the lower unit into water creates some back pressure that is different than running on muffs

Ok thank you makes sense. I have a video of the boat running in water to see what you think but I am leaning towards a fuel issue. The motor has good spark I haven’t checked the timing yet that is something that I am going to do. Thanks

 

[99yam40]

timing light hooked up to show you what is really happening with spark timing is the best way to go even if you have to do that with help while running the boat at the point the problem shows up.
motor looks like it is jerking around like it has a cylinder not hitting.

if you move the timing light to each plug wire, that should tell you if spark is getting to each individual plugs when it has this problem.
a good white chalk mark at TDC for each cylinder or a small paint mark on the flywheel will make it easier to see and how advanced the timing is.
check everything you can to make sure it is all good, and then look at fuel to air ratio

 

[Nordin]

Looking at the video and it sounds like it is misfiring.
Do as 99yam40 suggest, check with a timing light.
Also check the sparkplugs for crack in the porcelain, change spark plugs, run it in the dark with the hood off and look for sparks jumping to ground (power head). Take a close look at the HT-wires if there are a crack or bad insulation.
It sounds like there is a ignition issue and it is leaking to ground.