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Weber Dual-choke on XJ900F

Discussion in 'XJ Modifications' started by Dave in Ireland, Oct 2, 2019.

  1. Dave in Ireland

    Dave in Ireland Well-Known Member

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    Thought I'd better start this off a seperate thread to detail the eventual install, including the cussin' and swearing that comes along with one-off projects like this and the trials and tribulations of getting it right.

    ...and, to update, things are afoot.
    A recent descent into bad running had me examining the carbs, and by the time I buy new rubbers (hard and broken/leaky) front and rear and new diaphragms, plus the basic cost of a minor carb refresh kit for the float chamber, it all adds up to more than I feel like paying.
    A Weber is the weapon of choice, this time.
    I had a close look at the room available and Yamaha having moved the alternator and starter inboard and upwards means there's no real room for an SU in there unless I have it poking out the right hand side, meaning there's going to be a torturous passage for fuel/air mix, so rough measuring up revealed just about enough room for a simple branch manifold with a downdraught Weber sitting directly on top and it can poke up between the two frame rails. Hopefully there will be enough space above it to allow a smooth flattened elbow to get air into the top of it and a cool air hose can be run to the front of the bike, with a filter cone on the intake end (details to be finalised).
    I've scored a Weber 32 DIR, which is a close relative of the DGV and even though it's a production-line carb originally fitted to small Volvos, the jetting in it is changeable enough to allow proper running on a 900. I'm fairly sure it will be an improvement on the SU, as the first choke is 23mm, the second choke is 24 mm, and there are a couple of extra enrichment circuits that might (or might not) help out.
    To start with, the manifold will be roughed out using 1.5" mild steel thin-wall tube going into each port stub (I'll use the original port stubs for fitting, then replace them with a set of the cheapies from China), The feed tubes will be welded onto a short section of 2" square box , mounted longitudinally, centred on the rear of the head. Cylinders 1 and 4 will be fed via slightly longer curved elbows and cyls 2 and 3 via shorter elbows.
    This is to attempt in some way to give each cylinder a chance of getting an even feed, both from the primary choke and the secondary choke in turn.
    It won't be be perfect, but it will work.
    I expect I will drop some power, but the mere fact of having a decent choke area I might not lose too much - I foresee a loss of at least 10hp and likely 15.
    However, it it gets to 115 and can sit there without being too rich or too lean, I'll be happy enough.
     

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