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  1. #1
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    1975 Mercury 150 HP Inline six won't rev up enough.

    Hey guys:

    Thanks for all the past help on this motor. The last problem I found was that the max timing was not set correctly, now it pulls great out of the hole but just won't rev up on the top end. Compression on all six is right where it should be. Spark is great, starts as it should but just won't rev past about 3500 RPMS. If you pull back on the throttle about an inch it makes no difference. Any ideas would be appreciated. I used to think I understood the inline six pretty good but I am a bit stumped on this one. Thanks a bunch, Carter Powell
    Last edited by Carter Powell; 08-05-2022 at 02:29 PM.

  2. #2
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    A few things to look at..... Main jets clean? Use a timing light to look in the venturi while running to see if fuel is spraying out of the blast tube. Fuel pump diaphragm fresh? Any of the spark plugs look clean like new? Inner exhaust plate could be corroded and allowing water in the cylinders.
    Did this start all of a sudden? Double check timing. Could possibly have a plug wire grounding out.
    Too much propeller?

  3. #3
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    The boat and motor set for about eight years. I removed carbs and cleaned all the jets. The timing was set at 25 degrees BTDC. The boat launches hard out of the hole but just does not rev up as it should. Cylinders look good on the inside, no water droplets anywhere. Fuel pump diaphragm has not been checked! Thanks, Carter

  4. #4
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    Apple valley ca
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    I had the same problem but I couldn't go above 2000 rpm and I did everything you could and it ended up being the sator

  5. #5
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    Have you checked the stator? Do a search here or across the web on how to check a stator for prober voltage. From my understanding if the high voltage side is bad it won't have the power for the spark at higher RPM. Also take a good look at all the wires. The old Merc wires really like to crumble after all these years. I had to replace a bunch on my old 1978 blackmax.

  6. #6
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    All of the bad wiring has been replaaced, Darn thing just jumps out of the hole but just won't rev up. I will check the stator . Thanks for the ideas. Carter

  7. #7
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    Lake Keowee, SC
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    I had a friend back in the 80's that brought me his 115 TOP. It would not rev over 3500 rpm either. I tried changing the timing but it made no difference. After a lot of checking, finally dropped the lower unit and the entire exhaust tube was plugged with weeds! Removed the weeds and it was back to normal performance. However, it did not come out of the hole good either and yours does.

  8. #8
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    That story is very familar to my brothers Mariner 45hp he picked up. Beautiful on the outside and inside the cowl. Went through the whole motor because it woldn't break low teens on his 14 foot fishing boat. The convergent end of the exhaust was clogged with chunks of carbon.

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