Balancing carbs the old fashioned way!

[Glyn]

Now don't all laugh at once.

I was reading an old motorcycle tips book at the library today. It suggested that for twin carbs, after the motor is warm, turn the throttle up to a decent rpm (so the bike is say at 2500rpm), with the bike in gear and on the stand and disconnect 1 plug. You now have a single with the rear wheel flying around. Measure the speed of the wheel. This could be done with a pushbike electric speedo. With nothing else changing, run it on the other plug and measure wheel speed.

You get the idea. Synch one carb/barrel to the other so the wheel speed on both cylinders is the same.

Would this work? Sounds like it might. They say it was a common method in the "old days".

[Aelwulf]

I don't see it being as accurate (especially on finicky ones like the V) but I do see it working well enough.  In theory if each is pushing the same power to the wheel, etc they should have the same fuel/air/vacuum ratios I'd reckon on a base level.

[Rick G]

I've seen that approach used on two stroke twins . Its way too complicated . I was taught,  in 1965 , to pull the rubber boots from the carbs  and using a mirror , watch the slides open and get both to open simultaneously. I believe the was a Yamaha factory procedure . Things were simpler then  and  2 stroke a different story.

[Kenny]

  If you are going to try this method,remember you are dealing with an Electronic ignition & you need to ground the plug wire of the unused cylinder.Or you will damage the ign.unit.
;) Cheers Ken S. (overseas file)

[Repairman]

This is an easy, cheap and accurate way to sync. twin carbs.
I got the tip from someone who always used it on BMW boxer twins.
You need: About 3meters of clear plastic hose, about 6mm.(1/4") thick.
               One wooden board  0,5 -1.meter long.
               Two  short rubber hoses. (that will fit the vacuum niples.)
               ATF transmission oil  (or any other "coloured" oil..)
               Some kind of fasteners, clamps to fit over the plastic tube..

   Make a "U" at the middle of the plastic hose.
   Attach the "U" at one end of the board.
   Lead the two hoses to the other end of the board, and fasten them
   so you get two straight  hoses with the "U" in one end..
   Keep the board vertical with the "U" at the bottom end, and fill it approx. one third the length of the board with. ATF-oil..
Slide the two short rubber hoses onto the ends of plastic hose.

...And now you have made a carb. sync. tool.. ;)

  Keep it upright, attach it to the bike and start it up.
  Adjust the carbs.
 
   

[Brian Moffet]

We call that the 4 dollar manometer  ;)

[Lucky]

yup a well worn topic here  :D

[jasonm.]

Glyn's posting THIS IS ALL ASSUMING BOTH CARBS low speed screws ARE SET TO OPTIMUM SETTINGS.Your better off getting a cheap vacuum gauge.

[williamruck]

I just made myself a "manometer" but I think it was more like a $3.50 one. Now all I have to do is finish rebuilding the starter so I can actually "warm" the bike up.  ::)

[Glyn]

Isn't there a danger, even a small one of that ATF getting sucked into the motor, or is the vacuum not strong enough/ counter balanced by the other cylinder, for it to do that?

[h2olawyer]

If the balance is way off, the ATF can get sucked into one cylinder.  No danger, as it will just make lots of smoke.  If your tubing is long enough, you have time to hit the kill switch before the ATF gets that far.

H₂O

[Brian Moffet]

It can.  However, ATF will cause some smoke, but no damage to the engine.  Lucky recommends putting a carb jet in the way to help smooth out the pulses, and if you clamp one side and slowly let it go, you'll pretty much know if the ATF fluid is going to get sucked in by one cylinder.

Brian (balanced mine using a mercury manometer, and then gave that to the hazardous waste people in Santa Cruz.)

[Rick G]

I build mine with a balancing valve in each side , much the same as my quad  set up for 4 cylinders, which is over 50.00 now.