Mag 350 MPI won't reach full rpm

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by Coz, Jun 29, 2020 at 9:17 AM.

  1. Coz

    Coz Member

    194
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Port engine gets to 4800 wot. Starboard only gets to 4200.
    What are some typical causes?
     
  2. blaster

    blaster Well-Known Member SILVER Sponsor

    May 23, 2011
    potomac river
    2010 sea ray 205 sport
    F-250 CCLB 6.2 4x4 3.73 rear.
    Mercruiser 4.3 mpi alpha 1.62
    How long since a full tune up? Plugs, wires, cap/rotor, clean flame arrester, fuel filter.

    observe the throttle cable movement at the engine. Is it binding or stretched?

    What year 350 Mag is this?
     
  3. Coz

    Coz Member

    194
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    2008. I bought last year. No idea on the tuneup. I checked a cpl plugs this spring and they looked ok. I changed fuel filters and cleaned flame arresters this spring. I have dts if that matters.

    distributor is buried under a pile of wires
     
  4. blaster

    blaster Well-Known Member SILVER Sponsor

    May 23, 2011
    potomac river
    2010 sea ray 205 sport
    F-250 CCLB 6.2 4x4 3.73 rear.
    Mercruiser 4.3 mpi alpha 1.62
    I’d start with checking and replacing plugs along with checking cap and rotor. Tune up can give a real boost of performance. Enough gas makes a big difference too. Sounds like hopefully just a little maintenance.

    just get the proper parts and not Chinese crap.

    BTW can you remove the pile of wires by removing a couple small bolts, along with the control cables? Picture say 1000 words
     
  5. Coz

    Coz Member

    194
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    You would think plugs, rotor would effect other rpms. I wonder if the throttle cable could need adjusting. Seems to me that’s a pita
     
  6. dan313

    dan313 Member

    190
    Sep 27, 2009
    Long Island
    2005 270 Amberjack
    350 Mag Bravo3
    My 05 350 mag never got more than 4200-4400 RPM. When I had the engine rebuilt I still only got 4200-4400 RPMs. A guy on the dock gets the same RPM range as I do with the same motor. I have asked a number of mechanics about it. And they all say the same thing. That's what that motor is rated for.
    Please let me know if you get an answer is to the low RPM range.
     
  7. Espos4

    Espos4 Well-Known Member

    Jan 1, 2017
    Long Island NY
    2007 240 Sundeck
    350 MAG Bravo 3 W/DTS
    Good advice except DTS motors are drive by wire and don’t have a throttle cable.
     
  8. Coz

    Coz Member

    194
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    I was thinking there was an actuator back at the engine where it went back to cable. Maybe that’s the shift linkage.

    could there be an issue with the synchronizer?
     
  9. aerobat77

    aerobat77 Active Member

    559
    Aug 12, 2014
    germany
    2006 240 DA Sundancer
    5.0 MPI / Bravo III
    i do not think its plugs . a good plug affects nice starting , idle , trolling . but at WOT the best plug cannot do more than to ignite the mixture , how hard it makes boom inside the cylinder is not in control of a plug EXEPT its so bad it fires only on 7 without realizing by you .

    assuming all 8 are firing you will not get another 600 rpm by new plugs.

    the throttle body fully opens ?
     
  10. mike15

    mike15 New Member

    5
    Mar 30, 2015
    Lake Erie
    300 Sundancer 2005
    5.0 Mercruiser w/Bravo III Drives
    Do yourself a favor and do a compression test. At minimum you will sleep better knowing you have a healthy engine. If you have 1 or 2 weak cylinders you won’t get full rpm.
     
  11. Espos4

    Espos4 Well-Known Member

    Jan 1, 2017
    Long Island NY
    2007 240 Sundeck
    350 MAG Bravo 3 W/DTS
    There is only one mechanical cable, and that is the shift cable that goes to the outdrive. Everything else is drive by wire.

    your low rpm could easily be caused by one cylinder not firing, either a bad plug, corroded cap, wires etc.

    Marine engines have the ability to “hide” a misfire as they do not shake or vibrate like it would in a truck. One cylinder misfiring in your Chevy truck is instantly recognizable but somehow it’s not noticeable In a boat.

    without any maintenance history, I’d do a complete tune up plugs wires cap and rotor. See what happens. A compression test is a good baseline too. If all of that doesn’t help, consider it may be a Cool Fuel 3 paint issue. There’s lots of info on this site about it. Do some searches.

    If possible, do some research and try to get as much prior maintenance info on the boat as possible.
     
  12. Coz

    Coz Member

    194
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Is interesting that I'm off 600 rpm which is exactly 1/8 of wot. Be putting in new plugs this weekend. Ordered the rest.

    Mercruiser recommends NGK, any thoughts on AC Delco?
     
  13. blaster

    blaster Well-Known Member SILVER Sponsor

    May 23, 2011
    potomac river
    2010 sea ray 205 sport
    F-250 CCLB 6.2 4x4 3.73 rear.
    Mercruiser 4.3 mpi alpha 1.62
    Get the best distributor cap you can find. Brass not aluminum. Makes a big difference.

    I can’t provide specific part numbers but pay for the extra quality so you can enjoy your time on the water.

    You might need a tapered socket to get all the plugs out. Probably should check if what you have fits before heading out for the auto parts store.
     
    Last edited: Jun 30, 2020 at 10:55 AM
  14. aerobat77

    aerobat77 Active Member

    559
    Aug 12, 2014
    germany
    2006 240 DA Sundancer
    5.0 MPI / Bravo III
    i quess ac delco will work but i use ngk and never had an issue , i would go ngk .

    take the iridium ones - i tried iridium and non iridium . non iridium works good ( tr55) but to my surprize the iridium ( itr4a-15) results in a bit smoother idle and run at low loads . the iridium are expensive .
     
  15. Coz

    Coz Member

    194
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Changed all plugs today, didn't fix the problem. Have cap, rotor and wires on order.
    Here's pics of my distributor caps. Looks like I have to take off the metal plate that supports all the wiring to get the cap below. Should I disconnect all that wiring first or try to leave it connected? Looks like a pita. Temping to cut that part of the plate off.
     
  16. blaster

    blaster Well-Known Member SILVER Sponsor

    May 23, 2011
    potomac river
    2010 sea ray 205 sport
    F-250 CCLB 6.2 4x4 3.73 rear.
    Mercruiser 4.3 mpi alpha 1.62
    Unscrew the mount and move the entire thing out of the way. You don’t have to unplug everything, only a couple plugs. Don’t start hacking it all up.
     
  17. Coz

    Coz Member

    194
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Buddy at the dock has rinda scanner. I checked rpm history on both engines. Wot is 4800. Port has 1.4 hrs between 4500-5000. Stbd only has .1 hrs Engine hours are 360

    looks like this has been a problem since it was almost new in 08.
     
  18. aerobat77

    aerobat77 Active Member

    559
    Aug 12, 2014
    germany
    2006 240 DA Sundancer
    5.0 MPI / Bravo III
    i would wait . as i wrote earlier plugs, wires etc . can affect starting , idle , misfiring etc but not so much WOT . i do not think a new rotor will fix it as new plugs didn,t fixed it when your engine runs fine beyond low wot .

    you made sure its the same propeller on the stbd ?
     
  19. Coz

    Coz Member

    194
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Prop are same. Visually they look fine.
    To clarify, the stbd engine lags in all rpms. Never noticed it before due to the synchronizer. I’m gonna run misfire test today
     
  20. aerobat77

    aerobat77 Active Member

    559
    Aug 12, 2014
    germany
    2006 240 DA Sundancer
    5.0 MPI / Bravo III
    you made sure the throttle body fully opens on that engine ?
     
    WV 320 Dancer likes this.

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