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  1. #1

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    Motor Timing Question

    Hello:

    I haven't been able to find a good explanation of the way a simple 2-stroke motor with flywheel and coil (I have a Poulan 25cc off a craftsman weedwacker)
    handles timing as the motor increases RPM. My flywheel seems to have two magnets embedded a few degrees apart. Is this how the motor creates a spark
    sooner at high rpm's ?? My motor may have timing issues at high rpm's is why I'm asking.

    Thanks for any input !!

    Mike C.

  2. #2

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    RE: Motor Timing Question

    Usually timing is set at around 28 degrees.(ie no advance /retard)
    Motor usually runs at or near a rev range where this is about the best setting.
    I don't know about your particular engine but most magnetos will run a magnet to induce the coil with a charge and the remaining magnets helps to collapse the charge field as the spark is required.

  3. #3

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    RE: Motor Timing Question

    Thanks for the reply!

    Ok, so no advance / retard features on these simpler 2-stroke motors.

    Interesting about the
    2nd magnet collapsing the field for the spark I was wondering about that.

    Here is a pic of my magneto setup. There is an iron "arm" that sticks out to the left of the coil and gets near to the
    flywheel. Iwas wondering what that did. Any thoughts?

    [img][/img]
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  4. #4

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    RE: Motor Timing Question

    hello
    at what rpm you have problems???
    what is the prop you are using , pitch and diameter??
    the engine is good for a sunday flyer plane, I am using gas with no alcohol, no problems with deposits in the carb.
    Good starting, my carb have only one needle, High speed; enlarged throath with round file to near 3/8, opposite side to small holes in the venturi.
    Using standard timing in the coil.
    Happy landings
    Jose
    The experience is a comb the life gives you when you are bald. \"Ringo\" Bonavena.

  5. #5

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    RE: Motor Timing Question

    Seems the motor plateaus at about 6,000 rpm. I'm using an 18x6 prop for just testing on the bench. Maybe a smaller prop would bring the rpms up some.

    I just want a nice easy flying "Sunday" airplane I only do sport flying anyways. I thought it would be rewarding to take a thrown out weekwacker and use the
    motor on an airplane.

    I'm asking about the ignition system because I was wondering if the timing on these motors limits the rpm's.

  6. #6
    propellantchem's Avatar
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    RE: Motor Timing Question

    18x6 at 6k RPM is about right. Smaller prop will increase engine speed but tends to reduce thrust. Engine will pick up RPM in air as you unload the prop. Lots of info on here about getting set up with gasser engine. Standard mods are carb, intake, and exhaust porting. Depending on engine weed wacker can go from mild to wild your choice. Spark advance is not a big issue especially at high RPM. Charge turbulence takes over at high speedI like a large prop at moderate speed for a nice relaxing day at the field.

  7. #7

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    RE: Motor Timing Question

    Thanks for the input!

    I'm fairly happy with the motor so far and I think it will work. I wish I was more mechanically inclined to cut away some of the 'dead weight' on the motor like
    the mounting bracket, cooling fins, etc. but that would be opening another can of worms for me I think. The new carb I bought on ebay last year seems to
    work great. I was going to buy another Walbro carb off ebay this year and noticed the supply of carbs dried up....I wonder why...

  8. #8
    av8tor1977's Avatar
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    RE: Motor Timing Question

    They've been selling "new old stock" carbs, a lot of them, and since so many new engines come with unsuitable single needle adjustment, there has been a high demand for the older, twin needle carbs.

    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

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    RE: Motor Timing Question

    ORIGINAL: mjcelli

    Thanks for the reply!

    Ok, so no advance / retard features on these simpler 2-stroke motors.

    Interesting about the
    2nd magnet collapsing the field for the spark I was wondering about that.

    Here is a pic of my magneto setup. There is an iron ''arm'' that sticks out to the left of the coil and gets near to the
    flywheel. IĀ*was wondering what that did. Any thoughts?

    [img][/img]
    Iron "arm" is part of the armature laminated core.
    Many instances you will see two, one each side of the coil.
    Purpose is to "conduct" the magnetic field from the rotor through to the coil at the appropriate time, ie when it lines up with the rotor magnet.

  10. #10

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    RE: Motor Timing Question

    Thanks for the insight Bluejets! One more question and I promise to put this to rest. Does anyone know if the coil assembly contains a transistor or capacitor to help time the spark??
    I think the transistor is known as a Darlington transistor if it's used. Not sure if these simple 2 strokes need such a thing.

    Thanks all!

  11. #11

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    RE: Motor Timing Question

    HELLO
    for my weedeater I search in Phelon , the magneto maker, but no results, near nothing specs.
    The magneto was working well ever, remember gap only .
    I dont know if it is transitorized, or simple coil, never reach specs to read, searched with google but no results.
    No problems , only fouled plug from time to time
    happy landings
    Jose
    The experience is a comb the life gives you when you are bald. \"Ringo\" Bonavena.

  12. #12

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    RE: Motor Timing Question

    ORIGINAL: mjcelli

    Thanks for the insight Bluejets!Ā*Ā* One more question and I promise to put this to rest.Ā* Does anyone know if the coil assembly contains a transistor or capacitor to help time the spark??
    I think the transistor is known as a Darlington transistor if it's used. Not sure if these simple 2 strokes need such a thing.

    Thanks all!
    magneto coil does not normally have any embedded transistor, no.
    it simply operates on induction from the magnet.

  13. #13

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    RE: Motor Timing Question

    Great thanks for the responses all !! Lots of good information here!

  14. #14
    Moderator w8ye's Avatar
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    RE: Motor Timing Question

    The magneto coils are usually set at a fixed advance of about 22 degrees on the weed whackers

    Whereas RCEXL and C-H battery ignitions are usually set at 28 degrees even if they have auto retard or fixed timing.
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
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