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Thread: new US 41cc


  1. #1
    3D  Flyer's Avatar
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    new US 41cc

    I am a newbie to gas engines but have been flying RC glow and electric for some time. I am trying to set the needles for my new US 41 cc engine.
    I have gotten it to the point that it run good at the high end (full throttle) and will idle, but the problem is it dies every time I try to increase the throttle from idle to go to full throttle.

    I am running 87 octane with 32:1 oil mixture (lawnboy ashless).

    I have tried numerous needle combinations and have not been able to hit the sweet spot.

    Please if anyone could give me some adivse.

    Thanks.
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  2. #2
    Moderator w8ye's Avatar
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    RE: new US 41cc

    Your low speed needle is "in" too far
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  3. #3
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    RE: new US 41cc

    Yep, I thought that too. Too lean right? So I tried turning it out (counter-clockwise) and it did not seem to help. I wonder if somethings wrong with my carb?
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    RE: new US 41cc

    Does this thing need several tanks run through it at idle before I can set the needles properly?
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  5. #5

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    RE: new US 41cc


    ORIGINAL: 3D Flyer

    Does this thing need several tanks run through it at idle before I can set the needles properly?
    Most new engines can be set without issue. More information is needed.

    A gas newby often is unaware of the issues regarding gas engines and cowls. Is the engine cowled?

  6. #6
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    RE: new US 41cc

    It will be eventually, but it is not currently.
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    RE: new US 41cc

    I would say no. As soon as you can get the engine running right, start flying it. No matter how good you think you have it running on the ground, often as not, that's just a place to start. They almost always need to be tweaked for proper performance in the air.

    Regarding the idle adjustment, many guys go for an ultra smooth, even idle, at a speed that's as low as possible - which is guaranteed to quit when you try to add throttle. The engine should have an uneven note to it on the idle when it's right? Most engines run best, most reliably, when run as rich as possible on the idle.

  8. #8
    Moderator w8ye's Avatar
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    RE: new US 41cc

    What is being referred to is the little vent hole in the steel plate on one side of the carb. Air blowing across this little vent hole will sometimes alter your mixture.

    The hole is the atmospheric static reference for the metering valve diaphragm of the carb.

    Often a person will solder a small piece of brass tubing to this hole and run a plastic line to a place inside the fuselage. I have seen the brass tube extended to a point at the entrance to the carb.

    Another trick you see is an extension on the carb inlet or a velocity stack.
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    RE: new US 41cc

    Mine has a velocity stack.
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    RE: new US 41cc

    Ok... that helps as it is impossible to set the needles properly on a cowled engine with the wrong ventilation setup.

    How old is the engine? What prop? Are you using a tach to set the HS needle... tell us how you set the HS needle?

  11. #11
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    RE: new US 41cc

    What is the proper orientation of the velocity stack?
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  12. #12
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    RE: new US 41cc

    The engine is brand new, less than 2 tanks run through. The prop is 18x10. 87 octane fuel, 32:1 lawboy ashless oil mixture. I did not use a tack for the high speed needle, just richened it until it ran smoothly.
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  13. #13
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    RE: new US 41cc

    Mine has a power ram velocity stack (the angled alluminum one)
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  14. #14
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    RE: new US 41cc

    I've had the best results angling the velocity stack into the wind
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    RE: new US 41cc

    Yep, that is how mine is oriented. ??????
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    RE: new US 41cc

    Did you make sure the prop is 90 deg to the coil pickup? on the flywheel?

  17. #17
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    RE: new US 41cc

    I did know about this so I tried to get it correct. It may be a few degrees off.

    Is the proper way to have the flywheel magnets at 6 o'clock with the prop at 9 and 3 o'clock?
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    RE: new US 41cc

    8y , is that the hole right in the middle of the diaphram?

  19. #19
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    RE: new US 41cc


    ORIGINAL: WhiteRook

    8y , is that the hole right in the middle of the diaphram?
    I was talking about the hole in the metering diaphragm steel cover plate.

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    RE: new US 41cc

    low needle out 1 1/2 turns high needle 1 3/4 turns that will get you going then after break in turn in the high needle approx 1/8 to 1/4 turnin ,low end should remain fine , i never run lawnboy or amsoil only stihl synthetic it is great and run it 40 to 1 3.2 oz to a gallon of gas

  21. #21

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    RE: new US 41cc

    One thing to remember with gas motors.. They dont quit when they are too rich.... They quit when they are too lean... If the needles dont seem to have much effect on it, then the carb is probably plugged up.. Dissassembe it and make sure it is clean. Make sure the diaphrams are ok.. Search for a thread on servicing walbro carbs...It will show you how to do it.. set both needles per the manual - probably 1-1/2 out on both. Tune it with a tach.. you want it 2-300 fat on the top end and then adjust the bottom end so you get a good transition... then fly it till it breaks.. Quadras will get better over time..

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    RE: new US 41cc

    I have a Brison 2.4 and I was getting the exact same symptoms. Even with the low speed really rich it still did what you are describing. Everything is tuned in just fine, then when you let it sit at idle for a couple seconds, throttle up, it then cuts. I actually sent the engine in and he said there is a spring in the carb itself that was too heavy. So, the gas was unable to get to the engine fast enough because it wasnt opening up fast enough. Once he fixed that problem it has never given me a problem since.
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  23. #23
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    RE: new US 41cc

    Going the wrong direction. I just did my DLE 20 It will stall if also rich on the low end. You need to peak the high end for most rpm then back it off just a bit. Then turn in the low end till it stops cutting out. Start rich on both needles. My Dle was rich on the low and cutting out but I would go back to idle quick so it wouldn't stall after peeking the high. Just keep turning in the low in small very small increments till the bog stops and it transitions to full without quiting. I started out with the needles set 1/8 or more rich from the start from where the factory set them.
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