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Thread: Checking of Rotax 2 stroke Timing With Strobe Lamp

  1. #1
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    Checking of Rotax 2 stroke Timing With Strobe Lamp

    All,
    As in title anybody got opinions advice etc re checking points ignition rotax 2 stroke timing with a strobe lamp?

    As usual answers on a postcard gratefully recieved.

  2. #2

    Checking of Rotax 2 stroke Timing With Strobe Lamp

    Always relied on accurate static timing.

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    Checking of Rotax 2 stroke Timing With Strobe Lamp

    Thats ok but if you buy an aircraft and want to check the timing and dont want to strip out the back of the engine????

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    Checking of Rotax 2 stroke Timing With Strobe Lamp

    Funny you should mention strobes and 2 strokes.

    I bought my first ever motor cycle some months ago...a very old BSA Bantam 175 2 strojke...re-built the engine and very carefully set the timing with a timing disc and a diy piston stopping device...the same way I set it on my 503.

    The BSA manual goes into quite some detail on the procedure...set to 16.5 degs BTDC...the engine has no timing marks just like my 503 and I used the same tool I made to find exact TDC on my 503...so I am quite confident I got it set to 16.5deg BTDC.

    After I set it I began to think about checking it with the timing strobe lamp I still have in the loft from my Mk 1 Cortina days.

    I made my own marks on the flywheel to test out the strobe...in my dark garage I rigged up the lamp...and was surprised to find it read 12 degs BTDC.

    So I thought...maybe a strobe is just to accurate and the engine makers take this into to account...I left it at my static setting not beiong brave enough to experiment.

    It would be much easier to do it with a strobe as long as the engine maker gave to correct setting and I have no idea if other a/c engine have provision to set with a strobe.

    What are your thoughts Paul.

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    Checking of Rotax 2 stroke Timing With Strobe Lamp

    I have a friend currently restoring a Bantam and was tempted myself but lucked into a 400/four for a good bit less than was being asked for a Bantam. converting the piston depth to a degree value is a matter of a little trig but if the cam comes around under power from the engine, it will advance because when you set it by hand, you are pushing against the bearing slack which will centralise as the engine rotates under power. Half a thou, maybe a quarter but a couple of degrees worth. Strobes are better as long as there is no electronic or mechanical advance mech.

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    Checking of Rotax 2 stroke Timing With Strobe Lamp

    But do I aim for 12 or 16.5 BTDC its not documented and my engine they say is very critacal tio it being within 1/2 deg so I'm told.

    Quite enjoying my Bantam...its a lot more sprightly thast I expected for a 50 year old machine...but does not like going faster than 50....which suits me.

    Just triing to sort out the jetting as I have altered the petrol/oil mix from 25/1 to 40/1 on modern oil....how well up on 2 stroke jetting are you Tom.

    What a 400/four??

  7. #7
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    Checking of Rotax 2 stroke Timing With Strobe Lamp

    Now returning to rotax 2 strokes!!

    As I see the issue there is no advance and retard so variance of strobe point is down to (as in previous post) 'slack in the mechanism'. However, if you look through the timing port you will see a line on the mag wheel and three lines on the outside of the port. The rotax book gives the statement that timing is correct when the mag wheel line lines up with the center port line but also states that the two other lines left and right of the port center line are the limit of variance of the timing. As I only want to 'check' the timing I believe that a strobe lamp would be usable (if nothing else should both cylinders be the same under a strobe light that would suffice).

    I will try it and let you all know the outcome!

  8. #8

    Checking of Rotax 2 stroke Timing With Strobe Lamp

    Depends on whether it's Bosch (points) or Ducati (timing pulse coil).
    On the former you only need to strip out the back of the engine if adjustment is required.
    You can test the static timing with a multimeter set on ohms and a Skydrive bearing tester or digital caliper to check the piston drop before TDC through the plug hole.
    The Ducati, I'd have thought, pretty much is fixed and unlikely to move, unlike points.
    Dave

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    Checking of Rotax 2 stroke Timing With Strobe Lamp

    I was thinking it was like my Ducati ignition 503 Paul...sounds like a strobe may be on the cards...think I would set it static then test it with a strobe. see if they agree...you will know for future then.

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    Checking of Rotax 2 stroke Timing With Strobe Lamp

    The way that the Ducati triggers will naturally fire the motor as the magnet induces into the pick up coil. It won't give any indication of correct timing. For that, it will be necessary to create timing marks to reference the strobe to, related to the piston position. The rotor is about 120mm in diameter from memory so half a degree is no more than half a millimeter on the pickup coil.

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