Starting a 125 sport special
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Thread: Starting a 125 sport special

  1. #1
    Junior Member Stubaker58's Avatar
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    Stuart Baker
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    Starting a 125 sport special

    Hi,

    Following my progress with the ignition/lighting switch Iíve moved on to actually trying to start my little Benelli after what I think is a long time laid up. All Iíve done to the engine is clean the points and replace the kick start spindle which was broken off at the lever ... probably in an accident as there were other signs of an off, and probably why the bike was laid up in the first place.

    The good news is that after a lot of kicking coughing and spluttering the bike did start and run for maybe 45 seconds so there is nothing fundamental wrong however it did not start again and there was a lot of backfiring, in fact one backfire blew the end cone clean off the silencer!

    Can anyone tell me:

    a) how best to set up the carb, itís on standard jetting etc but how many turns should I give the air screw?
    b) anything I should consider when going through the timing? and
    c) if there is a knack to starting them e.g. should I flood the carb, use the choke, open the throttle etc?

    Many thanks

  2. #2
    Senior Member
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    Cam Douglas
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    I'm not familiar with the Benelli 125 sport special, but have had a lot of experience with Dellorto carbs. Your bike should have a Dellorto UB22BS carb.

    Firstly, pull the carby off, take it apart and give it a thorough clean with carby cleaner. Make sure none of the jets are blocked and blow through all galleries with compressed air.
    Check that the jets are an appropriate size. For your carb. I'd reckon about a 95 main jet and 42 pilot jet would be somewhere near the mark.
    Make sure the float and float valve are working OK. I don't think the float level is adjustable with that carb (remote float bowl)
    Set the mixture screw 1Ĺ turns out from the closed (fully screwed in) position. Once you get I running and warmed up, you can fine tune it. NOTE: When screwing it in to find the closed position, do it gently. If you screw it in to hard, the tapered tip of the valve needle can jam in the orifice and snap off when you try to open it again. If that happens, it's very difficult to get it out. If it can't be extracted, the carb is pretty much junk.

    Backfiring can also be an indication of bad ignition timing. I dunno whether you have a manual that gives you the correct timing setting. If not, set it statically to a few degrees BTDC. Too much advance (any more than about 10į) will make it hard to start, and can cause backfiring.

    Good luck with the bike.

    Cheers,
    Cam

  3. #3
    Junior Member Stubaker58's Avatar
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    Stuart Baker
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    Thanks Cam
    Iíll give that a go and let you know
    regards

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  5. #4
    Member
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    I have seen the flywheel woodruff key shear- so the flywheel can move on the crank. That causes all sorts of timing issues and certainly could cause backfires.
    I suggest checking your timing- and make sure the key is intact if you can.

    for my 250 egg- I ususally tickle until it floods, then stand beside it - take slack out of kickstarter and give it a kick. Generally first couple of kicks.
    Depending on the time of year, it sometimes starts better with no choke if you tickle it.

    my old 66 200 Sprite was the same way.

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