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  1. #1
    Speedling's Avatar
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    MR-1 Engine at 9800 rpm

    Ok, first, this is really in a boat.
    I started with one engine at 10,200 rpm, and the other at 9800.
    I pulled the air filters, added in velocity stacks, Iridium plugs, and even pump wedges (relatively ineffective).
    I now sit at 10,200 rpm and 9800.
    This is with both sides at 14/20.
    I can deprop... but I would think the engine rpm would change at least a LITTLE.

    Is this just too much prop, take it down to like 19? Or is there something wrong with the computer telling it to stop at 9800?

    Thanks for the insight.


  2. #2
    powerstroke specialist mikegp's Avatar
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    Did you check both props with a gauge before installing them? I have seen them out of the box with differences .not because they said 14/20 mean they are a true 14/20 ,unless you have check them before installation .

  3. #3
    Speedling's Avatar
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    Yup, I bought a gauge beforehand and they are both exact.
    I am gonna try and pitch the one that is lower rpm's UP to see if the rpm's stay anyways or drop. I dunno what to do exactly. I mean, if the engine is just a little worn out with 260 hours, lol that's pretty sad, but I doubt that's the case.
    200 rpm isn't huge, I'm guessing that's just where the engine wants to be.

  4. #4
    Speedling,
    Here is an excerpt from a Group K discussion of yamaha jet boat twin engine applications. While it may be "normal" to have lower rpms on one side, they recommend a staggered (different pitch on one engine) approach due to the way the pump is loaded differently on one side. Read the whole article for some other tips for your boat. The bottom line is that there are some differences in how the same engine performs in a boat vs a ski. While I have and have had both 4stroke and 2stroke skis, I have done a bit of research on the yammi boats for a good friend that just bought one. You need to be well versed in all the nuances. Do your tach's still work? Anyway here is the excerpt and the link.

    "Twin Engine Pump Loading

    The phenomenon described above affects the twin motor jet boats in an entirely different way. The water intake surfaces on the bottom of twin motor hulls are on angled surfaces on each side of the hull. This “angled water entry” gives an effective entry angle that mimics the pump loading of turning the boat. This means that (while driving in a straight line) the drivers side pump (starboard) is receiving water at an angle that mimics a left hand turn (thus loading the engine rpms down). At the same time, the passenger side (port) impeller is receiving water at an angle that mimics a right hand turn (resulting in higher rpms and a closer cavitation threshold). This is why the left (port) motors of most twin-engine Yamaha jet-boats tend to run higher rpms than the right (starboard) engines.

    Getting past the whole theoretical aspect, the functional result is what’s important.. Because of the phenomenon described above, the right side pump is generating more actual thrust than the left pump at any given rpm. This causes the steering wheel to constantly be “tugging” toward a left hand turn, and at the same time causes the left side engine to rev higher than the right engine in order to generate the same thrust.

    After weeks of testing, we eventually resolved all these problems (on twin motor Yamahas) by fitting better design impellers on the pumps, and applying a noticeably steeper pitch to the left hand side prop. The end result is engines that turn virtually identical rpms all the way through the throttle movement range, and an end to the steering wheel constantly “tugging” to the left. In most cases, the “off-the-shelf” pitch of the props we chose were not right on the money, so we custom pitch each pair that we sell to the specs that worked best during our on-water tests. Installing these “staggered pitch” aftermarket impellers is one of the most effective improvements that you can make to your twin motor Yamaha jet-boat."

    http://www.groupk.com/yjetboat.htm

    Best of luck with your boat. You may want to pitch the higher rpm motor to run lower instead of the other way around. Try calling the folks at Group K for their opinion on your specific year and model. They may have dealt with it before. Be sure to put floats on your clean out lids.

  5. #5
    Speedling's Avatar
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    Yes, I am well aware of the pump loading stuff.
    I understand that the one side will be lower rpm due to having the same pitch.
    I don't understand, however, how doing mods, the engine is unresponsive in rpm's at all.
    So yes, I would bring that side to a lower pitch in order for the engine to go to full rpm's.
    I"m wondering if it's just not going to go past 9800 no matter what. Is there something to limit it?
    If I test on the hose, and it's stopped at 9800, then it's something OTHER than the impeller limitation due to water pressure. I will be testing this tomorrow hopefully.

  6. #6
    Speedling's Avatar
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    Oh, and btw, you're looking at the 2 stoke boats, of which I had, and had over 65 mph!!!!
    And THAT had bad tach's.
    This... I don't know. I have to check the tachs somehow I guess, but I don't know how to do that other than looking for another tach and wiring it up.

  7. #7
    Hey,
    Good point on the tachs. On the earlier (two stroke) boats, the faria tachs are junk. I would invest in a "tiny tach" that many use for diagnostics and carb tuning. It is 47 bucks and uses a spark plug wire for pickup. At least you can verify if they are reading the correct rpm's before going completely nuts. Yeah, after I wrote the post, I realized I forgot to point out that the article was mainly for the two strokes but the pump stuff is still valid.

    http://tinytach.com

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