Drop in RPMS when throttling up from Idle

[Odyssey121]

Hi All

Looking for some ideas or assistance on what might be causing this issue. We have 2x 250HP Yamaha DETU 4 stroke 2013 outboards on our boat.

The issue is only on the starboard outboard. We had fuel issues where our tank had to be replaced because of small particles blocking the filters. After numerous services, clearing the filters and running diagnostics, we couldn’t find anymore issues with the fuel and engine was running fine.

Our past few trips when throttling up to 3900rpm or higher from idle the starboard engine still struggles to get to 3900rpm. The rpms jump all around and we struggle to get on plane especially after a long drift. I have to throttle back to neutral and slowly throttle up, playing around with it a bit till eventually the starboard engine manages to maintain the same rpms as the port one.

It sounds like the engine is running at much higher rpms when it gives this issue.

Have had our service agents check the engine again and fuel filters etc but the issue still seems to be occurring and we a bit lost on what could be causing it. Never happened prior to the fuel contamination but since then it’s not gone away.

Any help or suggestions on what It might be or to get our service agents to have a look at that they might have missed or not checked? They also cannot explain what might be causing it.

Thanks

[99yam40]

Hi All

Looking for some ideas or assistance on what might be causing this issue. We have 2x 250HP Yamaha DETU 4 stroke 2013 outboards on our boat.

The issue is only on the starboard outboard. We had fuel issues where our tank had to be replaced because of small particles blocking the filters. After numerous services, clearing the filters and running diagnostics, we couldn’t find anymore issues with the fuel and engine was running fine.

Our past few trips when throttling up to 3900rpm or higher from idle the starboard engine still struggles to get to 3900rpm. The rpms jump all around and we struggle to get on plane especially after a long drift. I have to throttle back to neutral and slowly throttle up, playing around with it a bit till eventually the starboard engine manages to maintain the same rpms as the port one.

It sounds like the engine is running at much higher rpms when it gives this issue.

Have had our service agents check the engine again and fuel filters etc but the issue still seems to be occurring and we a bit lost on what could be causing it. Never happened prior to the fuel contamination but since then it’s not gone away.

Any help or suggestions on what It might be or to get our service agents to have a look at that they might have missed or not checked? They also cannot explain what might be causing it.

Thanks

might want to explain what I bolded up above.
if both motors have tachs, how can the motor sound like it is runniing higher RPMs than what you are reading?
Could a slipping prop hub be the problem?
or disturbed water causing prop to not bite well in the water/

[zenoahphobic]

might want to explain what I bolded up above.
if both motors have tachs, how can the motor sound like it is runniing higher RPMs than what you are reading?
Could a slipping prop hub be the problem?
or disturbed water causing prop to not bite well in the water/

My initial thought too, but that contradicts “struggling to get to 3900rpm.”
Therefore this suggests a problem with the engine producing power. No1 culprit would be fuel supply, the fuel issue not resolved.

[zenoahphobic]

Also the sounding of running at higher revs could come from the engine changing it’s timing to deal with load. When ignition timing is advanced an engine can go “quiet”

[Keithbaja]

Run a auxiliary tank up to the motor...( with primer bulb). All new,, good brand. Not China junk.....

[Odyssey121]

might want to explain what I bolded up above.
if both motors have tachs, how can the motor sound like it is runniing higher RPMs than what you are reading?
Could a slipping prop hub be the problem?
or disturbed water causing prop to not bite well in the water/

Thanks for the suggestions so far much appreciated. Sorry it’s a bit tough to explain correctly. Going from idle straight to 3900 the tachs show 3900 for port engine. The problem Starboard engine jumps revs so goes up to 3400 then quickly drops revs but is working hard to try get to 3900 sound wise but theres no bite. Almost like it’s cavitation but that’s never been an issue since we got our boat in 2013.

Problem only started in 2019 when fuel tank had issues and fuel was contaminated with black particles so we would have trips where engine would cut out when running on plane. We got technicians in and they replaced the tank, fuel lines, all fuel filters and gave the outboard a full service with diagnostics etc. Since then we constantly checked filters and everything has been clear after services one we had 2 weeks ago.

It’s very frustrating because would do test runs in the bay and wouldn’t see any issues. When we head out to the tuna grounds engine runs fine. Starboard engine does this when we head back after a long drift where we keep engines on. Usually it’s worse if there’s a decent swell on the starboard side or we heading into the swell. Once I manage to get some speed with a following sea and the port engine then I can get starboard engine to 3900rpm and running fine again.

[99yam40]

I would think if the prop hub slips or a prop cavitation is occurring the motor RPMs on the tach would go up not down.

for the motor to struggle to get to 3900 you may be dropping a cylinder or 2.
or maybe the rail pressure is dropping(not feeding enough fuel to the injectors to get the power needed)

could the motors idling for long periods of time cause that one to have a low level in the VST for some reason.
very strange and hard to troubleshoot if they cannot reproduce the problem while testing.

have you tried pumping the primer bulb before taking off again to see if that helps?(make sure the VST is full)

[Odyssey121]

I would think if the prop hub slips or a prop cavitation is occurring the motor RPMs on the tach would go up not down.

for the motor to struggle to get to 3900 you may be dropping a cylinder or 2.
or maybe the rail pressure is dropping(not feeding enough fuel to the injectors to get the power needed)

could the motors idling for long periods of time cause that one to have a low level in the VST for some reason.
very strange and hard to troubleshoot if they cannot reproduce the problem while testing.

have you tried pumping the primer bulb before taking off again to see if that helps?(make sure the VST is full)

I think this could be the issue, thanks will try this next trip and see how it goes. Because going from mooring like I said no issues only after idling for long periods. We thinking of switching off engine when on the drift and then seeing if issue reoccurs or not and pumping primer bulb. Will post back after our trip and trying that.