A long time ago I had a carburetor synchronizer that used mercury to measure engine vacuum. The synchronizer had four plastic tubes about twenty inches long, each tube was about a quarter-inch in diameter. On top of the rig were four rubber tubes that connected to your intake ports using the little hose barbs molded into the motorcycle’s intake manifolds. This machine worked well, the heavy mercury kind of dampened the vacuum pulses so you could get an accurate reading.
One problem with the mercury gauges was that sometimes, if you revved the engine too much, mercury would get sucked into the engine. The mercury never seemed to hurt an engine but it would pop and miss a bit, then toxic smelling fumes would come out the exhaust pipes for a while. The cure for this problem was to not rev up the engine with the mercury sticks connected.
My mercury gauges went under water in one of the floods that were common in the Florida Keys. Who knows where the mercury that was inside the gauges ended up. Probably in my fish sandwich 6 months later.
Another type of vacuum gauge uses four needle-and-dial mechanical gauges to measure the vacuum. These gauges would jump all over the place reacting to each vacuum pulse. The mechanical type came with plastic clamps that you would fit over each vacuum hose. By adjusting the pinching action on each hose you could slow down the needle reaction enough to be able to read the dials. I didn’t like this type so I threw them away after they sat on my shelf 27 years.
The Carb Tune vacuum gauge uses steel rods and springs inside plastic tubes. They are mostly like mercury gauges except the rods replace the mercury. There are clones of the Carb Tune available (maybe the Carb Tune is a clone of some other brand) but the clones are near enough in price that I sprung the ten bucks extra for the brand name unit.
The Carb Tune need a little work before you can use it. Included in the set is a blue plastic tube with a tiny hole through it that serves the same purpose as those hose clamper deals on the dial gauges. You have to cut the tube into four pieces and then cut each vacuum hose about 100mm from the end to insert the plastic tube into the hose. It’s no big deal but I wonder why Carb Tune didn’t go ahead and complete this final assembly step.
Once put together the gauges work great. The Carb Tunee is very stable and easy to read. Get all four rods even across the top and your carbs are synched. The third rod was stuck on my set but a light tap freed it up to move in the tube.
Normally the actual number of the gauge doesn’t matter; you are going for even-ness. If you had a bad cylinder or your valves were way out of adjustment it might show up on the vacuum gauge so that’s another good use for the Carb Tune.
I rate the Carb Tune gauges 5 stars even if it is a little expensive for what you get. I’m guessing the things are made in China and probably cost Carb Tune a couple bucks a copy. Still, that’s the capitalist system and I was a willing participant for many years so it would be hypocritical of me to bitch about it now.
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