Outboard carb synchronizing

[Plan C]

Any suggestions out there for synchronizing tools for balancing carbs? I know Les uses the Motion Pro Vacuummate, which I have the impression is a great tool, but is pricey for occasional use. I have a set of mercury carb sticks for motorcycle balancing, but haven't tried them on my Honda. How might they compare? Any other choices?

Dave

[Almas Only]

I guess what I have is a home made version of your mercury sticks. Mine uses 10 feet of clear vinyl tubing (Home Depot or Lowes), a three foot long piece of wooden lathe, some clear tape, and a little two cycle motor oil. Loop the tubing at the 5 foot mark, put the lathe in the center of the loop, and run a tube up each side of the lathe. Tape the tubes to the lathe with clear wrapping tape. Then, put one end of the tube in the two cycle oil, suck oil about three feet up the tube, and then elevate both ends of the tube so that the oil runs down into the looped end. Once the air clears, the oil level will be the same height on each side of the stick.

Then, connect one end of the tube to the vacuum port on carb 4, and the other to carb 3. Before starting the motor, put a clamp on the tubes, so that you can stop the oil from being sucked up one side or the other. Once the motor is warmed up, crack the clamp, and turn the adjusting screw on carb 3 as necessary until the oil levels are basically stable. Then loosen the clamp, and make adjustments to the carb 3 screw as necessary to get the two columns of oil at equal height. Jab the throttle at the carbs after each adjustment until you've got it right, then check at about 2000 rpm. Set the idle to about 900 or 950 (with the helm seat back down, you can see the tach from the motor), and move on to carbs 4 and 2. Then, do 4 and 1. Note that carb 4 does not have an adjusting screw, as it's the reference carb against which the others are set.

If the oil starts to take off on you, just tighten down the clamp. If you ever connect just one end of the tube, without the clamp, while the motor is running, you will suck the oil from the tube. For a clamp, I use sink mounting hardware, but most anything will do.

My experience has been that if you try this without the clamp on the tubes, you'll suck the oil up into one carb or the other before you can get them close to balanced. No big deal, but you do have to refill the tube, which is a distraction when you're trying to work on the carbs.

This gives a very precise result, if you're patient. Cost of the instrument is next to nothing, although the little threaded tubes which screw into the vacuum ports are fairly expensive (say, about $15-20 each). They're available through Honda parts, from an outside vendor.

Obviously, do this after you change plugs, set timing, or whatever else you're doing which could affect the carbs.

[Plan C]

I wonder if you avoid the problem of the carbs sucking your fluid if you use the mercury carb sticks. Not nice to think of mercury being sucked into the engine!

Dave

[Almas Only]

I dont' know if the mercury sticks have any design feature which prevents the vacuum from sucking the mercury up (such as a restriction with a small hole allowing air, but not mercury, to pass, or the mere weight of the mercury with respect to the vacuums being measured), but that shouldn't be too hard to do. Even without such a feature, the mercury should be less prone to being sucked into the carb because it is so much more dense than oil. The flip side of the weight/density difference is that the oil should give a more precise reading, although at times, it almost seems too precise, as you can spend a fair amount of time adjusting back and forth with very small increments on the carb screw. It would be interesting to perform the sync with mercury, and then attach an oil unit, and see how close the two systems are. I would think that either would be more accurate than a set of gauges, although much more time consuming. But heck, we've all got lots of time to play with our toys.