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  1. #1
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    91 Mercury XR4 low rpms

    Hello all, I got a 1991 Mercury XR4 serial number 0D038009. Had the boat out on the lake today and I don’t normally hammer WOT all over the place, but opened it up and only got 4300 rpms or so at WOT. It should be more like 5500-5800 if I’m not wrong. Bought a new 26 tempest prop for it this year after having to replace the old small (1.78) lower unit with a SEI 1.87 this year. Used mercury prop calculator and got 25/26 pitch calculated for 64 mph at 5500 rpm. Boat is a 1991 Ebbtide Dyna Trak 181. Weighs 1150 dry, 26 gal tank, calculated 600 people n gear. Got a total weight of 2200 lbs. Merc recommended 25 or 26 tempest. Bought the 26 and here we are. Had a heavy tournament load (about 800lbs or so people n gear) but didn’t think that extra 200 lbs would bring the rpm down so much. Would that extra weight affect it that much? Or is something else wrong with it that I’m not seeing. To me it doesn’t add up but figured I let the pros see if they find something. Thanks everyone

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    #2
    Also forgot to add: Only have 5 hours or so on break in for the new lower, not sure if that changes anything

  3. Member
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    #3
    xr4 is a 2.4 liter mtr no way it will turn a 25 or 26 tempest enough rpm 23 would be best choice , way overpropped
    .................................................. ...the scariest thing in life is the unknown ...................................

  4. Member
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    #4
    I just put on a new 22 tempest plus, can only seem to get 5K rpms out of it with just me, was turning 5400-5500 with a smaller diameter 24p 5 blade. Not sure what’s going on. Had the 26 before and got 4300-4500 out of it, only 500 rpm for a drop in 4 pitches seems off to me. Any ideas?

  5. Member lpugh's Avatar
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    #5
    Compression test and plug inspection are first things to check. No need to chase a problem if the motor is not sound
    Thank You Leon Pugh

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    #6
    Quote Originally Posted by lpugh View Post
    Compression test and plug inspection are first things to check. No need to chase a problem if the motor is not sound
    Had work done to it this winter, between 130-140 on every cylinder, I did run some power tune through it that can sometimes foul plugs so will check those tomorrow but something is still not adding up.

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    #7
    what kinda work
    .................................................. ...the scariest thing in life is the unknown ...................................

  8. Member
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    #8
    New switch boxes, water pump, filters, fuel lines, voltage regulator, charging system, new lower unit, I’m sure there is some stuff I’m forgetting, didn’t have any”custom” work or anything like that done to it, just replaced stuff that wasn’t working. Don’t think it should be having this hard a time spinning a 22p

  9. Member
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    #9
    Do you know if Mercury parts were used or aftermarket parts, especially the electrical components, such as the regulators, stators, switch boxes? Sounds like it’s possible it’s not firing all cylinders.
    Mercury 2002 200hp Carbed Saltwater
    OT566776
    2002 Cape Horn 22' Bay

  10. Member
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    #10
    All electrical parts that were replaced were CDI, had the issue of one cylinder not firing before, which is why I replaced switch box. After replacement my mechanic said all 6 were firing and good to go. (think #5 was the one not firing before). Merc stuff was either not available or like 3x the price. I did power tune it yesterday so there is a possibility that one of the plugs is fouled up, but no black carbon or anything like that came out the exhaust so I think it was a pretty low hour motor.

  11. Member
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    #11
    I’ve read about plenty of new electrical parts not working out of the box, happens to Mercury parts but most are aftermarket parts. I would check the plugs first, definitely a possibility.
    Mercury 2002 200hp Carbed Saltwater
    OT566776
    2002 Cape Horn 22' Bay

  12. Member
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    #12
    Ok will do, what’s the best way to check them? I don’t have a tester or anything like that. Would taking the plugs to a auto shop be the easiest?

  13. Member
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    #13
    Just look at them, if the tips are super black then replace, or post a picture of them, and the brain trust here can tell you if they looked fouled up. Or if they are over a year old I would just replace them, shouldn’t be too expensive.
    Mercury 2002 200hp Carbed Saltwater
    OT566776
    2002 Cape Horn 22' Bay

  14. Member
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    #14
    Just looked at all the plugs, all seemed good, have some oil on them and around the spark plug hole, which I assume is normal and the reason it smokes a good bit at the first start. What’s the next thing to look at? Merc prop selector has let me down twice now. It’s on a light 18 ft bass boat, feel like it should have no issue turning 5600+. It now recommends a 19 or 20 pitch, which if I really need to could have my 22 repitched to 20, but feel like something is still missing here. I’m pretty sure my motor is mounted pretty low, I can confirm when I get back to the house

  15. Member
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    #15
    correct height would benefit for sure
    .................................................. ...the scariest thing in life is the unknown ...................................

  16. Member
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    #16
    There is 5 mounting holes, it’s currently mounted on the second from the top (if top is hole 5, it’s on 4), so there is a decent amount of room to move it up. I also just noticed that the throttle is hitting my livewell levers, not sure how much farther it would be able to go without those in the way but will look tomorrow.
    Attached Images Attached Images

  17. Member
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    #17
    might better measure prop t pad height first before moving
    .................................................. ...the scariest thing in life is the unknown ...................................

  18. Member
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    #18
    Make sure your throttle cable and carb linkage are adjusted correctly and that the carbs are actually achieving wide open throttle.

  19. Member
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    #19
    Ok, will put a plank from bottom of hull And see how it lines up on the lower and measure prop shaft to pad as well. Throttle and cables seem to be in the right spot. How many rpms can I expect to gain for each hole I go up (if it even needs it) ? I can trim it to the max that the “slow” trim goes and it won’t throw a rooster tail in the air

  20. Member
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    #20
    Ok, forgot to get prop to pad but got pad to AV plate, couldn’t get motor all the way trimmed down on the trailer but got to to almost level, AV plate was just under 1.5 inches above pad. How many rpms could I expect from going up 1 hole? I believe each hole is .75 inches from one to the next, so it would be just about 2.25 above pad. Going to double check the tach reading but I was only getting 52 mph at 5K rpm.

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