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Crapping out at 3/4 to WOT

Discussion in 'The Garage' started by lhaluciol, May 18, 2021.

  1. lhaluciol

    lhaluciol XS650 New Member

    Hey Guys!
    Glad to be a new member of the gang’.

    Just bought this 81’ XS650 SH - engine 4M4.
    Currently in a custom bobber frame with straight pipe, cone air filters, BS34 carbs.

    I’m quite familiar with CV carbs but I’m having a bogging issue at 3/4 to WOT.
    Dropping the needle up to 2 clips did not change a thing.

    Both plugs are way too black for my taste, it is way too rich up there.
    (It was already rich on the pilot circuit as the bike idled good only if the fuel screw was 1/2 turn out; went from a 42.5 to 40 to correct it. Now ok at 1 1/2 turns out.)

    Both cone filters are not covering/restricting the air jets (I guess I should try to remove them and go for a ride to be sure).

    Rubber CV membranes are not cracked.

    Ignition doesn’t seem adjustable on the 81’?! But I guess it would crap all over if it was that.

    Next step would be to drop a size or two on the main jets; but I feel like it’s something else!

    Any guru out here with a slight idea?

    Thanks guys!
    Last edited: May 18, 2021
  2. RustiePyles

    RustiePyles Semper Fortis XS650.com Supporter Top Contributor

    I know you say the slide diaphragms look good but have you actually leak tested them to see how fast they drop? And what cone filters are you running? Some of them actually block the air bleed jets in the venturi mouth.
    jetmechmarty likes this.
  3. 5twins

    5twins XS650 Guru Top Contributor

    CV carbs don't like a K&N style pleated pod or a tapered one. Straight foam UNIs work best for pods.


    You mentioned an adjustable needle. The BS34s didn't come with one so yours must be from some aftermarket kit or possibly it's the Canadian needle. If it is the Canadian needle, it's made to be run with it's matching needle jet, not the original.
    nj1639 likes this.
  4. grizld1

    grizld1 Grumpy old man Top Contributor

    Cheap cone air filters usually have an inside flange that blocks the main and pilot air jets in the back of the intake bell, with the results you're seeing. You can cut relief grooves in the flange, but replacement is a much better idea. BS34 carbs on US models didn't have needles with adjustable clips, although they're available as aftermarket items. GOK what the PO installed.
    GLJ likes this.

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