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Thread: RPM reduction?

  1. #1

    Default RPM reduction?

    What would cause my 150 Merc Optimax to drop in rpm's? There seems to be no other problems with the motor it but I can only get about 4200 rpm's out of it. I can't remember what the top end was last year but all I know is I'm running about 10mph slower and it's a bugger getting her on step.

  2. #2
    Member
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    Default Prop or Jet?

    5500 is where you should be at if its a Jet. You should check the Sleeve and inpeller, Might need a shim.

    350SD

  3. #3
    Member kodiakrain's Avatar
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    Default Air Supply ?

    Checked the Air Filter, right? my first suspicion on RPM drops
    Ten Hours in that little raft off the AK peninsula, blowin' NW 60, in November.... "the Power of Life and Death is in the Tongue," and Yes, God is Good !

  4. #4

    Default

    Quote Originally Posted by 350SuperDuty View Post
    5500 is where you should be at if its a Jet. You should check the Sleeve and inpeller, Might need a shim.

    350SD
    It's not a jet unit. I guess I should have said that from the start.

  5. #5
    Member fullbush's Avatar
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    Default

    i bet you have a clogged injector. the computer would compensate for the loss of the cylinder making it run smooth so you never know except power loss

  6. #6

    Default

    Quote Originally Posted by kodiakrain View Post
    Checked the Air Filter, right? my first suspicion on RPM drops
    I checked the air filter and it looks new.

    Fullbush, you could be right. I was hoping I didn't have to have a real mechanic take a look at the motor. Do you think it would be an injector problem if the motor runs fine other than the rpms?

  7. #7

    Default Prop

    Did you change props, that can drop RPM or not let it get up to higher RPM's buy have changed that.
    Beleive it or not, it happened to me in one of my expermenting with pitches

  8. #8

    Default

    Quote Originally Posted by alaskapiranha View Post
    Did you change props, that can drop RPM or not let it get up to higher RPM's buy have changed that.
    Beleive it or not, it happened to me in one of my expermenting with pitches
    Good idea but I haven't changed my prop. The pitch can sure make a difference but I was getting 5500 rpms last season and I haven't changed a thing.

  9. #9

    Default

    Hey everyone thanks for the input. I took it in to a mechanic and my rpm reduction was from a nick in my spark plug cord. He taped it up and charged me $49. I told him to order a new one for me because I'd sooner have a new cord rather than chancing any type of corrorsion. I got her back to 5500 rpm's today.

  10. #10
    Member kodiakrain's Avatar
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    Default Right On

    Good Deal, thanks for posting the final solution, glad you're "Revvin' Away Again,"

    Gonna be a Great Summer, eh?
    Ten Hours in that little raft off the AK peninsula, blowin' NW 60, in November.... "the Power of Life and Death is in the Tongue," and Yes, God is Good !

  11. #11
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    Default exact same thing for me!

    I Had the exact problem with my motor (2003 merc 150 opti) but i was running a jet. Same situation. bad sprk plug wire, I had a new one put in, and a new plug aswell. In the tank I got full RPM out of it. I haven't put it on the water yet to see if shes back to her old self, but i bet it is. Need some of these lakes to get ice free!!!

    My other problem was I was getting about 30:1 gasil consumption. From running the silty rivers, one of my block sensors was clogged as well(with sand/silt). The mecahnic cleaned that out, and with the new spark plug wire thinks I should be back to "normal" oil consumption.

    Can't wait for the water to soften up to go see how it runs now!!

    just thought I would post what i found with my engine!!

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