Merc 15 too high RPMs? Overheating?

Discussion in 'Outboard Maintenance' started by Dillusion, Sep 23, 2012.

  1. Dillusion

    Dillusion devilray snob

    Took the Merc 15 (2002 2-stroke) our after a few weeks of not using it and it was working fine, but about 3-4 minutes into my runs at WOT, the motor will jump from 5200 to 5800 RPM and slow down considerably in speed. I am currently using a SS 10 pitch prop. At 5200 RPM at WOT I am getting about 17-18kts, and at 5800RPM at WOT I am only getting 13-14kts top speed. The motor sounds audibly revving higher, but the speed drops down like that on my GPS.

    It was also the first trip out on the new hull, so the prop was blowing out on turns that I was testing purposely. After it 'blew out' it would rev high like that and stay there as well.

    I have sincle lowered the jack plate about 1" and I will be re-testing it today.

    I tried the outboard in all 3 tilt settings and nothing changed. Is it going into protection or something at that high of an RPM? it seems that 5800-6000rpm is the max for this motor.
  2. Creek Runner

    Creek Runner Well-Known Member

    That motor does not have a protection mode, your prop is slipping causing your engine to rev higher becuase it's not pushing you forward. Keep playing with your set up, also check your hub.

    When I say it doesn't have a protection mode I mean there is no slow, or guardian protection. It does have a rev limiter which you may be hitting but that your set up or hub.

  3. Dillusion

    Dillusion devilray snob

    Right, so in theory something as simple as lowing the jack plate by and inch could fix the issue of slipping?
  4. Dillusion

    Dillusion devilray snob

    Tested again today, dropped it 1"-1.25" from previous setting and it works like a charm. No more spinning.

    When some weeds build up though it causes an air pocket which allows it to spin again making the RPM's shoot up.
  5. el9surf

    el9surf Well-Known Member

    Are you in the Orlando area? Send me a pm if you live in the area. I would like to check out your boat with the jackplate setup.

  6. Your motor has the water inlet on the bottom of the cavitation plate and not on the lower unit. Not the best motor for a tunnel and a jack plate.

    I would take the motor off the jack plate and hang it on the transom and it should stop sucking as much air.
  7. Dillusion

    Dillusion devilray snob

    It was pissing fine at the highest setting on the jack plate. I know where the water inlet is and what it looks like when it's sucking up too much air (I did it on the old gheenoe with JP).

    It's not ideal, but it works fine.
  8. AfterHours2

    AfterHours2 Stripper in my own Mind!

    Eventually, I would measure the foot print of an atlas micro jacker and go from there. Being able to adjust on the fly is a great thing on tunnels. For the most part hanging the motor, shimmed upon the transon, is probably the best thing for now. Your set back is not helping you at all. Other than the plate looks cool on the back...
  9. Dillusion

    Dillusion devilray snob

    I'm getting an aftermarket cav plate soon so I will be doing more screwing around.
  10. AfterHours2

    AfterHours2 Stripper in my own Mind!

    Was just pondering, an Atlas, 25 two stroke with a cupped 4 blade would be the hotness on your Shadowcast. That thing would run super skinny. I don't have your same rig but if you want to test out the motor setup hit me up..