I've been trying to resolve a problem with my Mercury 225hp for about half the summer and haven't come up with a solution yet. I am hoping somebody could point me in the right direction or at least a new direction.
Details: 1996 Robalo 2120 CC with a single 1996 Mercury 225hp 2 stroke w/ carbs
Background: I've had this Robalo 2120 with the 225hp Merc for about 1 year. It has had some minor issues which I have resolved. Mostly routine maintenance. It generally runs well for an old 2 stroke. I was even able to get the boat up to 45 mph earlier this year.
However, a couple bleed lines came off and one burst/split close to where it connects to the block (starboard side). I didn't think much of it and replaced that line and reconnected 2 others (port side). I had replaced all the bleed lines in April 2021 and followed the diagram in the manual and made sure the check valves were installed correctly and in the right location.
Problem: Since July, the RPMs don't go past 4700 when trimmed down and 5000 when trimmed up. Top end speed is only about 35 MPH. This isn't bad for this size boat, but I know it can do much more.
Trouble shooting:
- Compression checks out fine on all 6 cylinders
- New sparkplugs
- Timing is computer controlled but checked out fine
- Confirmed spark to all cylinders, but not sure how to confirm the strength of the spark. Regardless, the engine starts right up and runs/sounds smooth (at least for a 1996 2 stroke).
- Recently replaced all 6 ignition coils and wires.
- Throttle Position Sensor/Indicator seems to be working. I adjusted it somewhat and it didn't change performance. Interestingly, the volts from the sensor is opposite of what the specs in the manual indicate (~4V at idle and ~0.9V full throttle). Not sure why that may be, but it could be a typo in manual or because manual is for a 1994 and perhaps things changed in 1996. Regardless, if it was that far off, the engine wouldn't run.
- Replaced all fuel filters including the fuel water separator.
- Replaced all fuel lines
- Replaced fuel pump gaskets, valves and hose from block.
- Vent to fuel tank is clear and I even opened the fuel cap to make sure no vapor lock.
- Rebuilt carbs with new gaskets and float needle valves and seats.
- Confirmed throttle cable is not stretched and is giving full throw and opening carbs fully.
- I add StarTron fuel additive to every tank of gas.
- Ran off separate fuel line and tank to completely rule out any fuel supply issues and made no difference
Plans:
- Confirm check valves on bleed hoses are OK.
Suspect:
- Perhaps the bleed lines are being blocked and causing back pressure on the cylinders?
- Computer is causing a limitation to the RPMs due to a faulty sensor?
I know this is a lot but I wanted to give as complete of a picture as possible. I have reached out to a few forums with same information. Unfortunately after several weeks, I haven't received any responses yet.
I would greatly appreciate any thoughts you may have.
Details: 1996 Robalo 2120 CC with a single 1996 Mercury 225hp 2 stroke w/ carbs
Background: I've had this Robalo 2120 with the 225hp Merc for about 1 year. It has had some minor issues which I have resolved. Mostly routine maintenance. It generally runs well for an old 2 stroke. I was even able to get the boat up to 45 mph earlier this year.
However, a couple bleed lines came off and one burst/split close to where it connects to the block (starboard side). I didn't think much of it and replaced that line and reconnected 2 others (port side). I had replaced all the bleed lines in April 2021 and followed the diagram in the manual and made sure the check valves were installed correctly and in the right location.
Problem: Since July, the RPMs don't go past 4700 when trimmed down and 5000 when trimmed up. Top end speed is only about 35 MPH. This isn't bad for this size boat, but I know it can do much more.
Trouble shooting:
- Compression checks out fine on all 6 cylinders
- New sparkplugs
- Timing is computer controlled but checked out fine
- Confirmed spark to all cylinders, but not sure how to confirm the strength of the spark. Regardless, the engine starts right up and runs/sounds smooth (at least for a 1996 2 stroke).
- Recently replaced all 6 ignition coils and wires.
- Throttle Position Sensor/Indicator seems to be working. I adjusted it somewhat and it didn't change performance. Interestingly, the volts from the sensor is opposite of what the specs in the manual indicate (~4V at idle and ~0.9V full throttle). Not sure why that may be, but it could be a typo in manual or because manual is for a 1994 and perhaps things changed in 1996. Regardless, if it was that far off, the engine wouldn't run.
- Replaced all fuel filters including the fuel water separator.
- Replaced all fuel lines
- Replaced fuel pump gaskets, valves and hose from block.
- Vent to fuel tank is clear and I even opened the fuel cap to make sure no vapor lock.
- Rebuilt carbs with new gaskets and float needle valves and seats.
- Confirmed throttle cable is not stretched and is giving full throw and opening carbs fully.
- I add StarTron fuel additive to every tank of gas.
- Ran off separate fuel line and tank to completely rule out any fuel supply issues and made no difference
Plans:
- Confirm check valves on bleed hoses are OK.
Suspect:
- Perhaps the bleed lines are being blocked and causing back pressure on the cylinders?
- Computer is causing a limitation to the RPMs due to a faulty sensor?
I know this is a lot but I wanted to give as complete of a picture as possible. I have reached out to a few forums with same information. Unfortunately after several weeks, I haven't received any responses yet.
I would greatly appreciate any thoughts you may have.
