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Need help with 1999 225 ox66 S225TXRX not getting above 4800 rpm

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  • Need help with 1999 225 ox66 S225TXRX not getting above 4800 rpm

    Asking for help (new to this forum) - I've read several similar posts for other engines and have learned a lot. I appreciate any guidance you can give me on what to do next. I am new to working on outboards when I purchased my boat a few years ago. I've done a fair amount of maintenance but continue to have problems getting performance I should from this engine.

    My boat was running good (I would rarely go above 4600 rpm) but it got to where it would lose power at 4600/4700 rpm. I would then take off and get back to 4600 but if I went above it would lose power. I did a lot of maintenance and things improved significantly. Now, I can run to 4800 rpm typically and sometimes get to 5000 depending on load in boat. A mechanice told me my motor should get to 6000 rpm WOT. I have trouble getting above 4800 rpm now.

    Here's what I have done in the past year:
    -replaced water separator filter
    -replaced small filter under cowling (rear port portion of motor)
    -replaced small screen in VST tank and gasket and cleaned inside
    -replaced low pressure pumps and gaskets (this made a significant different...believe I was pickup on air as gaskets were in bad shape & replaced them later and saw difference)
    -cleaned oxygen sensor
    -replaced spark plugs
    -replaced gas hose bulb & grease fittings
    -replaced water pump and thermostats
    -grease fittings where air could pick up (water separator, small filter screw on top (rear port), low pressure pump gaskets)
    - added ring free on this fuel tank. I have run about half tank through engine.

    Yesterday, my son was driving and when it started to bog I looked at fuel hose bulb and it was fine. I even squeezed it multiple times to make sure fuel was getting to engine and it still bogged down. It drops to about 4400 rpms and then we let off throttle to get back to normal running at that rpm then push back to 4700/4800 rpm and it runs fine unless we push it farther.

    I'm wondering if I should replace spark plug wires?

    From what I've been reading maybe I need to check fuel pressure. I haven't done that before but willing to do whatever to figure out the cause. I'd like to get full performance from this engine.

    your suggestions and help is appreciated.

    Happy 4th of July
    Last edited by Back2bowhunting; 07-05-2020, 11:42 AM.

  • #2
    I recently had a similar issue on my f250. It was low on power and would only get to 4500 rpm. Had a bad spark plug. Check to make sure all cylinders are firing. Run it for a few minutes and pull the plugs. If you have one that is wet you have a dead cylinder.

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