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  1. #1
    Join Date
    Sep 2021
    Location
    Adelaide
    Age
    73
    Posts
    117

    Default Carb conversion.

    The BS38carbs on my bike gave up, holed diaphragms. The diaphragms being 46 years old, started developing tiny pinholes. I plugged a few with Sikaflex, but they just kept coming. I wasn't going to pay good money to fix a couple of carbs I think are rubbish, so I sent off for a couple of Mikuni VM34s. These come with cables and, manifolds and rubbers. I wasn't impressed with the manifolds though: they were well made but the mounting holes were for 8mm bolts and elongated, whereas the mounting bolts are 6mm, amateurish to say the least. Carbs arrived and I bolted them on only to find the cables on top sit right where the fuel taps are. I contacted the seller and apparently, due to Yamaha's manufacturing processes back in the day, no two bikes are the same, so the cables interfere with the fuel taps on some but not others. I did some googling and found a number of like problems.

    I did some checking and found that if the VMs are mounted in the original manifolds, the cables miss the fuel taps, trouble is the VM spigots are 40mm, where as the manifolds are for 48mm spigots. So I had three options: tilt the carbs over at quite an angle to miss the taps, fit the VM spigots with a 48mm sleeve or machine up some longer manifolds to place the carbs behind the taps. Options one and two, went out the window, option one for obvious reasons, option two because I'd have to turn the spigots down a little to make them smooth enough to take a sleeve. That's not something I fancied doing, so, I set about machining up a couple of longer manifolds.

    The mounting flanges measure 90mm across, so I'd need a 90mm round piece of aluminium stock to turn down to 40mm for the spigots and bore through 40mm, that's a lot of wasted aluminium. So, I had a better idea: turn up a couple of 70mm long 40mm spigots with a 34mm bore, machine a couple of 90mm wide flanges and press the spigots into place.

    I machined up one, pressed the parts together and it came up excellent, so then I made another identical one, well, nearly identical, the second one came out 0.20mm longer, but I can live with that. Once finished I polished them, fitted them and fitted the carbs - success, The cables sit 10mm behind the taps and give good access to the adjusting screws for tuning. Another problem! The outlet spigots from the taps sit right up against the carb cables. Easily fixed, swap the taps from right to left and left to right and the outlet spigots now face forward, away from the carbs.

    Next problem: the carbs are normally supported at the rear by the airbox, but as this custom doesn't utilise an airbox, the carbs could possibly stretch and the rubber boots deteriorate. So, now I needed some sort of a support at the rear. So, I made up two U shaped brackets to fit over the slot at the back of the carbs, fitted them with rubber grommets and positive stop T nuts, then I made a larger flat bracket which bolts to the rubber mounted battery carrier and mounted the U shaped brackets to that, then anodised them.

    Worked a treat and not too obtrusive or ugly. Next step, shorten the cables: I machined up a 6mm nipple and soldered that in place, then routed some fuel hose from taps to carbs. Another problem; The taps have a 6mm spigots, whereas the carbs have 8mm spigots. Back onto the lathe to turn up a couple of brass adapters.

    All done, so I wheeled it outside, started it up and tuned it. Runs like a beauty now. But geez, what a lot of work!


    Some pics.
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  2. #2
    Join Date
    Jan 2004
    Location
    Mackay North Qld
    Posts
    6,447

    Default

    Gidday Hoggo1951,
    Welcome to the MetalWork Forums

    We have an top group here who are experienced,knowledgeable and willing to offer a hand if and when it is needed.

    To find the rest of what we have here, do this:

    Goto the rectangular FORUM box in the top LH side,and click on it. A drop down box of 5 lines will appear.

    Select FORUM HOME from the top line of the drop down. Select that and Click.

    At the top is the Terms of Use - effectively our rules- please read them

    Scroll further down down page will show our sub forums and help pages. Note ! lot of pages have a preceding sticky -Good idea to read them to obtain guidance for use of that particular help page or sub forum.

    Should you need further assistance please PM me.

    Welcome again.

    Grahame

  3. #3
    Join Date
    Nov 2007
    Location
    melbourne australia
    Posts
    3,228

    Default

    Quote Originally Posted by Grahame Collins View Post
    Gidday Hoggo1951,
    Welcome to the MetalWork Forums
    I think you mean "welcome back" Grahame.
    Chris

  4. #4
    Join Date
    Aug 2008
    Location
    Adelaide
    Age
    69
    Posts
    1,375

    Default

    Quote Originally Posted by jack620 View Post
    I think you mean "welcome back" Grahame.
    So I'm not the only one who noticed that...

  5. #5
    Join Date
    Jan 2004
    Location
    Mackay North Qld
    Posts
    6,447

    Default

    Quote Originally Posted by Gavin Newman View Post
    So I'm not the only one who noticed that...
    There's more than a few

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