Hi all,
Some mates and I have a 25HP yamaha 2 stroke, model is 25NE so think I have worked out that puts in somewhere in the 94-97 range.
We bought this as a bit of a project and have recently given it a bit of a service (cleaned the carby out, replaced the thermostat, replaced the impeller, didnt replace the water pump housing as it didnt arrive in time but have one now). It mostly runs pretty well, however if we are driving at full noise for a little while, maybe 2-3 mins it will all of a sudden drop power. It then kind of ramps up and down in speed. If we take it back to half throttle for a few minutes, then go back to top speed we can get another couple minutes of full power before it slows down again.
I have been suggested 2 possible causes:
1. Fuel issue. Either the primer bulb, hose or something is not getting enough fuel through it to run at full speed for a while. throttling down lets it catch up. I have ran it with the cover off and the fuel filter is the clear type. The fuel level in the filter doesn't visibly drop so it seems like there is at least enough supply getting to that point. I think if we run it for a while with someone working the primer bulb constantly we might be able to force enough fuel through and determine if this is the issue. So think this will be relatively easy to check.
2. The engine is getting too hot so after a while at speed the pistons heat to the point they expand a bit and are getting a bit stuck. slowing down allows them to cool enough that it goes away. The tell tale has good flow out of it and the water coming out doesn't get that hot, but someone said it could still have a blockage somewhere in the head and that water is not actually getting to the points it needs to cool and basically short circuiting back out. Our next plan is a flush (with vinegar or something commercial), but if that does nothing we will pull the head off. This is potentially getting a little serious for some part timers, is there an easy way to tell where the water is flowing? Would we only need to take the head cover off to see the water path, or the whole head too?
Has anyone else had this issue and have the best starting point or any other things to check? Maybe the thermo switch? We are 3 guys in our 30s so lining up time to tinker is always hard so hoping to streamline the process
Some mates and I have a 25HP yamaha 2 stroke, model is 25NE so think I have worked out that puts in somewhere in the 94-97 range.
We bought this as a bit of a project and have recently given it a bit of a service (cleaned the carby out, replaced the thermostat, replaced the impeller, didnt replace the water pump housing as it didnt arrive in time but have one now). It mostly runs pretty well, however if we are driving at full noise for a little while, maybe 2-3 mins it will all of a sudden drop power. It then kind of ramps up and down in speed. If we take it back to half throttle for a few minutes, then go back to top speed we can get another couple minutes of full power before it slows down again.
I have been suggested 2 possible causes:
1. Fuel issue. Either the primer bulb, hose or something is not getting enough fuel through it to run at full speed for a while. throttling down lets it catch up. I have ran it with the cover off and the fuel filter is the clear type. The fuel level in the filter doesn't visibly drop so it seems like there is at least enough supply getting to that point. I think if we run it for a while with someone working the primer bulb constantly we might be able to force enough fuel through and determine if this is the issue. So think this will be relatively easy to check.
2. The engine is getting too hot so after a while at speed the pistons heat to the point they expand a bit and are getting a bit stuck. slowing down allows them to cool enough that it goes away. The tell tale has good flow out of it and the water coming out doesn't get that hot, but someone said it could still have a blockage somewhere in the head and that water is not actually getting to the points it needs to cool and basically short circuiting back out. Our next plan is a flush (with vinegar or something commercial), but if that does nothing we will pull the head off. This is potentially getting a little serious for some part timers, is there an easy way to tell where the water is flowing? Would we only need to take the head cover off to see the water path, or the whole head too?
Has anyone else had this issue and have the best starting point or any other things to check? Maybe the thermo switch? We are 3 guys in our 30s so lining up time to tinker is always hard so hoping to streamline the process