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Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 15327
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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MY Mercury OT757204 4 stroke motor is running rough and won't

Customer Question

MY Mercury OT757204 4 stroke motor is running rough and won't run at RPM's over 2000 (It revs up on its own to about 4500 and then goes back down to around 1500-2000) I have changed the Boat filter and now have removed the motor from the boat tank and put it onto a separate tank with clean gas. I had the injectors checked (Doctor Injector) and they are ok. I replaced the Motor fuel filter along with the VSST filter. I check the fuel pressures on the fuel rail and they ran at 41-42 psi. What else should I look at
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.
Hello my name is ***** ***** am I speaking with?Did you measure fuel pressure at idle, or did you measure them with the boat hitting the fastest speed it will go?
Customer: replied 1 year ago.
At idle 41-42 psi. motor won't rev up past 2000 rpms and hold
Expert:  Jason replied 1 year ago.
What you need to do is measure fuel pressure with the engine at the max speed it will hit. Is this something you can do now? The other 2 things is you would have to check compression, and check spark output with a spark tester. Have you done that as well?
Customer: replied 1 year ago.
I will try the spark plug method but not until this weekend the boat is located out of town and I want to line up some things to check when I get back to the boat.
Expert:  Jason replied 1 year ago.
There are 5 things that need to be checked 1. Compression2. Spark out put with either a timing light or adjustable gap spark tester.3. Read the spark plugs, and see if any of them are white, clean, or show any signs of abnormal combustion4. WOT timing5. Fuel pressure with the engine at max speed. Those are all what needs to be checked. Have you done any of those sets of checks, or do you have questions on any of them?
Expert:  Jason replied 1 year ago.
Good morning, just checking in. Any progress?

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