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  1. #26

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    RE: MVVS 120 boxer engine

    your rpm are ball park figures. about 8000 to 9000 rpm static should be fine.
    The carb in downdraught configuration helps a lot.

    Pe, (www.mvvs.nl), MVVS, MOKImotor, RCexl, MTW, Xoar, Mejzlik.
    Blessing in ignorance? There is sanctuary in analysis.

  2. #27

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    RE: MVVS 120 boxer engine

    Hi!
    Try a 17x6 or 18x6 instead! Prefearbly a APC or RAM! The APC clone (JXF) that you now use in the picture is not particularly good!
    Jan Karlsson - Supplier MVVS Products

  3. #28

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    RE: MVVS 120 boxer engine



    As far as I can tell, the two cycle boxer twin engine designs all seem to have the same issues when you run them.
    One issue is air fuel flow isn't balanced or even between the two cylinders (or four or six for that matter). So you have to compromise the adjustments to work with two cylinders. You can't quite lean it out all the way as the right cylinder would run too lean whereas the left cylinder tends to run too rich. The air fuel flow characteristics tend to cause the left cylinder to run more rich and more cool than the right cylinder. You can take athermometer and measure the cylinderhead temperatures to see the difference between them. Also the left cylinder has a unnatural air fuel flow through it in comparison to the right cylinder. This tends to cause the air fuel to not atomize as well and thus it leads to a tendency to drown out the glow plug when you lower the throttle from WOT to idle. Usually the left cylinder tends to be the one that flames out on you.

    Now a MVVS twin uses a common crankcase for both cylinders with the single carb on it. This means you always have to set the carb mixture settings for the leanest cylinder. So it is always a compromise then. With the Fox twin cylinder engines, the two halves are offset and separated and each has its own carb, and the imbalance between the two cylinders is more easy to compensate for, but you still have to adjust everything for the leanest cylinder still. The left cylinder still runs more rich and cooler than the right cylinder does. The Fox Twin still fires both cylinders at the same time, just like the MVVS engine, and although both cylinders have their own carn, they are still connected with each other as the center bearing is a unsealed open bearing. The inline twins don't have this problem though, such as the HP 1.20 twins. They run fine without the weird flameout issues the boxer twins would have,

    I once accidentally did a experiment with a Fox Eagle III .60 engine years ago. Where I took the cylinder and rotated it so the exhaust was on the left instead of the right. I also mounted the engine horizontally on the left too. I was going to use it in a twin engine plane at the time. That was when I discovered it had the same flameout issues that the Fox twin did in that same situation. When you study it some you can see how the air fuel as it flows through the engine is flowing in a opposite direction on the left versus the right, plus the crankshaft and piston causes a difference in the flow and interference too. The air fuel just doesn't like going inthat direction much at all.

    Clarence Lee did a review of the Fox twin years ago, and to solve the problem he simply flipped the cylinder base gasket over 180 degrees to block off the boost ports on the engine. This is the big port opposite the exhaust port on the cylinder sleeve. This results in a loss of 200 to 300 RPMs, but that isn't much considering how much power the engine develops. Mr Fox himself suggested making a balsa plug to do the same thing, but I think the gasket method works good. Another method is to simply use a on board glow ignitor and program it to come on below 1/4 to 1/3 throttle too.

    Anyway, I think if one did this same thing to the MVVS twin engine they would have success in using it as well. As it would have the same problem as the Fox twins I have been using in this area. Either block off the boost ports or use a on board glow ignitor program to come on at lower throttle settings.

    One other thought is to mount the left cylinder so that the exhaust flows up. But then the engine becomes assymetrical and might look odd like that. I have also seen that method used with some homebiult twin engines as well.
    Also what might work better is to mount both cylinders to face to the rear like some of the ROSS engines did years ago. Some people have setup their Fox Twins to be rear exhaust and those engines ran fine and didn't have the flamout issue on the left cylinder.

    One thing is the Fox 1.20 twin was designed to run in the 12,000 to 14,000 rpm range wheras the MVVS 1.20 twin runs upwards of maybe around 8,000 rpms. So one has to prop accordingly for them.

    Here is a pic of a MORIN from 1949 that used the assymetrical cylinder setup too.

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  4. #29

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    RE: MVVS 120 boxer engine

    You just nailed it earl.

    A diverter to direct the flow towards the "lazy" cylinder also works to get balance.
    Pe, (www.mvvs.nl), MVVS, MOKImotor, RCexl, MTW, Xoar, Mejzlik.
    Blessing in ignorance? There is sanctuary in analysis.

  5. #30

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    RE: MVVS 120 boxer engine

    Thanks for the tip, but I already have too little clearance on the prop tips
    I have installed my PIC circuit and a 1S 4000 mAh battery, and two heli glow plug connectors.
    It is effectively a Y-Lead on the throttle output, and anything much below 1/4 throttle, the glow plugs come on.
    Haven't had a flame out since. And I don't need Glow Drivers
    The carburator is now giving problems, two flights, and the diaphragm valves start curling up.
    Leave the engine alone for two weeks, and they lie as flat as new.
    I run the engine completely dry after the last flight of the day, maybe I should do that after every flight ?
    Going to a local Walbro supplier tomorrow, see what he has in the line of alcohol resistant kits.

    And to give credit where credit's due, a very easy starting engine, and actually does sound different from a single.
    And much less vibration !

  6. #31

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    RE: MVVS 120 boxer engine

    Current D10WAT carb kits have a blue mylar membrane that is alcohol resistant. The regulating diaphragm has limited resistance. Running the carb dry  will accelerate the rubber hardening, but running coleman fuel (or equivalent) after a flight day will preserve the carb quite well.
    Pe, (www.mvvs.nl), MVVS, MOKImotor, RCexl, MTW, Xoar, Mejzlik.
    Blessing in ignorance? There is sanctuary in analysis.

  7. #32

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    RE: MVVS 120 boxer engine

    Well, just an update.
    I found out the hard way that this engine is at it's happiest with 20%castor, 80 %methanol.
    NO nitro !
    I'm still using the low throttle on board glow driver, and on startup keep my hand on the RH cylinder, to feel when it fires up, and starts giving power.
    Once hot and running, this motor pulls a Drastik 120 vertical.
    Main needle almost fully screwed out, low needle set up for a slightly rich idle.
    Last flight of the day, I pull the fuel tube, and let the engine run dry.
    This motor isn't too particular about glow plug types.
    Prime it 6 turns with fully open throttle, close to idle, pretty much starts on first or second flip.
    Only flame-out I had, was when the fuel tank was empty !

  8. #33

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    RE: MVVS 120 boxer engine

    When they are treated well, and given what the manufacturer specifies, provided the carb is in good shape, these engines are beasts.

  9. #34

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    RE: MVVS 120 boxer engine

    Hi!
    Sorry! But they are not! And that's why MVVS stopped manufacturing them after a very short period.
    Jan Karlsson - Supplier MVVS Products

  10. #35

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    RE: MVVS 120 boxer engine

    http://www.youtube.com/watch?v=nV0tLu7NflQ

    Some have great success. Not for the beginner though.


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