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Carb synch...wow!

Discussion in 'Bling and Appearance' started by typhoon, May 21, 2007.

  1. Why didn't I do this when I got the bike!
    It's one of those jobs on a bike that is shrouded in mystery, and has had a lot of "black art" stigma assigned to it.
    Basically it is pertty straightforward, with the hardest part removing the fuel tank (if needed for access to carbs on your bike) and setting up a fuel supply for the carbs when the tank is gone.
    What you are trying to achieve is to have each carburettor drawing the same amount of air for each individual cylinder at the same time. This means that each cylinder is sharing equally in producing the same amount of power at the same time, so the cylinders are not "fighting" each other (ie, one working harder to keep a stable rpm/load than another cylinder).
    For a clearer understanding of why motorcycle engines have a carburettor for each cylinder, look up "ram tuning" "independent runner" and "tuned length" on Google. It is all about resonant frequencies.
    Anyway, teh process is fairly simple, and all you need is a simple two gauge vacuum gauge set. You can buy four gauge sets, but they are completely unnecessary.
    On a two cylinder bike, you just attach each gauge to each carb vacuum port, but on an inline four or triple, you attach the two gauges to the left two carbs. Start engine and warm up, and adjust the damper screws on the gauges tso the needle flickers one grauation on the gauge scale or less. Don't over damp, as you'll end up with an unresponsive gauge set. Then you adjust the synchronisation screw to equalise the vacuum reading between carbs. A small adjustment goes a long way, and blip the throttle lightly betwen adjustments, to settle the linkages back into place.
    Once you have cylinder one and two synched together, you leave cylinder one attached as a reference, and move the other gauge to cylinder three and repeat process, then onto cylinder four.
    You'll probably have to adjust your idle speed after this process, and it is a good idea to recheck your carb synch at this higher idle speed (you still have the gauges out anyway!).
    What you get out of this is a much smoother engine, not just at idle, but through the bottom third of the rpm range, you'll get more torque, and smoother delivery of that torque. You'll also get improved fuel consumption and if your bike has flat spots in acceleration crusing around, you'll probably eliminate these too.
    I thought my bike was reasonably close to being in tune, but the carbs were way off, with a 40% difference across the set of four carbs!
    Anyway, if you are thinking of having a go at a carb synch, it's not that difficult, it just requires patience and basic mechanical skills. I'd say if you can change your own spark plugs, you can synch carbs.
    Of course, consult your bike workshop manual for any special techniques particular to your bike's model.

    Regards, Andrew.

  2. I've been meaning to do mine for a while, haven't synced them since I cleaned them and dad adjusted the screw before I could tell him what he was doing. Its been snatchy on and off the throttle since then, rather annoying.
  3. mine keeps dying when i try to do it as it needs the suckion points to pump fuel into the carbs.

    So im planing on using a large diameter gauge on the airbox end.
  4. Andrew, just wondering ..did you check the carb float levels before syncing the carbs? Doing so went a long way to making my A1 run smoother too.
  5. thanks for the write up typhoon, untill now, that was the only thing that i thought i couldnt do in the ordinary service schedule. looks like ill be off to repco to by a vacuum gauge set.

  6. Intewesting. Vewy intewesting. Cheers.
  7. yea also dont forget to be 100% sure, connect all your gauges to the same source for calibration ie the same carb via a T connector or similar so you know each gauge reads the same.

    most dial type gauges will have an adjuster on the front or back to manually adjust the needle.

    +1 for a good write up
  8. That won't achieve anything, you need to measure vacuum at each carb, downstream of teh throttles.
    You just need to run the tank on prime, or use some other fuel supply.

    Regards, Andrew.
  9. Float levels good, just checked recently with a piece of clear hose over the end of float bowl drains and held up alongside the carbs.

    Regards, Andrew.
  10. Bah, a good single carb setup is superior to multiple carburretors. At least that's what Burt Munro always said...
  11. Excellent guide there Typhoon. Something I'm just about to embark on myself. Will probably find out when I get to a repco store but how much did the vacuum gauge set you back?

    Just did a google search for a more model specific process and found this. Very helpful and easy to understand also.
  12. My gauges were around $60 for the pair.
    Don't get sucked into buying the four gauge set, they are unnecessary, and only take off a minute or two at the most off the procedure.

    Regards, Andrew.
  13. So how do you tell when you need to rebalance?
  14. Good write up Andrew.
    Zbike, There is no reason why you cant buy a length of fuel hose and connect it to the tank a long as the tank is higher than the carbs as in fitted to the bike.
  15. I made up a fuel tank out of an old old can.
  16. i use and old lawnmower fuel tank for doing the sync :)
  17. can't it be done with just one vacuum gauge?..... measure the vacuum on each throttle body and adjust them to the reading on the reference cylinder (usually cylinder 1)... is it possible?
  18. No, because as you adjust the carbies the revs rise and fall and thus vacuum. So even by the second carbie setting it will mean the first vacuum is different to when you saw it last.