DIY Home made vacuum synchronizer / manometer

Building a manometer or vacuum synchronizer is a great addition to your shop and arsenal of tooling. The tool is simple in design and fairly inexpensive to build.  If you add the feature to stop the transmission fluid in the tube from being pulled into the engine, the build will be a bit more complicated.  Even though pulling transmission oil into the engine probably wouldn’t have been a big deal, I still wanted to prevent it from happening.  That is why I added the 3/4″ X 6″ PVC reservoirs. Another benefit of the PVC reservoirs is that they smooth the reading of the fluid.  They dampen the pulsations from the engine and cut down on fluid oscillation. The meter will operate as it should because it works  by the means of vacuum. The key is to equal inside tubing volume on both sides, this will result in a more accurate reading.  I feel this testing setup gives me a more accurate reading than most commercial vacuum synchronizers.

I also have a 3cylinder and 4cylinder setup shown at the bottom of this post.

Parts Used to build all 3 (2cyl, 3cyl, and 4cyl)
1  25ft length Oil-resistant Black Buna-n Rubber Tubing, 1/8″id, 1/4″ Od, 1/16″ Wall
1  25ft length Masterkleer Pvc Tubing, 3/16″ Id, 5/16″ Od, 1/16″ Wall Thickness
2  6″ lengths of 3/4 PVC
4  caps
2   For 3/16″ Tube Id barb to 1/8 pipe fittings (plastic)
2  1/8 barb to 1/8 pipe fittings (brass, I couldn’t find them in plastic)

If you can do it cheaper that’s great. I just wanted to keep it under half price of a new set of vacuum gauges or a new manometer.

I was able to get everything from McMaster-Carr and I had a few bits leftover.
I have seen simpler setups using a yardstick but, this is my take on it.


Home made Carburetor Synchronizer, manometer

I had some scrap lumber laying around and I ripped 2 grooves down  it.  The lumber itself is 50″ long.  A small stand was also built so the meter could stand on its own. The total height is around 55″ to 57″ tall.


Home made Carburetor Synchronizer, manometer

I cut a relief in the bottom to allow for the hose to make a nice bend. I tried to keep everything as close as I could to length from one side to the other.  The black 1/8″ vacuum lines are 10ft long each.  This gives me plenty of line to put the meter where I want it.


Home made Carburetor Synchronizer, manometer

I filled the tubing half full of transmission fluid.  2 cycle oil can also be used, or regular motor oil.


Home made Carburetor Synchronizer, manometer

These two 6 inch sections of 3/4 inch PVC tubing is used as a reservoir if the cylinders are so out of sync that it wants to pull oil all the way to the engine. There is enough room in these reservoirs for all the oil in the tubing plus a couple inches to spare for air. If this happens the oil in the reservoir should just bubble and will not go into the black 1/8″ tubing.  Along with keeping the oil from being pulled into the engine, they also smooth the pulsations or oscillations from the engine running, giving clearer reading.

With this setup I don’t calculate how far off one cylinder is to the other. I try to get the carburetors or throttle bodies set as close as I can (if not dead on). If I feel that this meter is not responding the way I think it should I just swap the sync tubes going to the engine from one side to the other and check to see if I get a similar reading but, the opposite.


This is the three cylinder version of the vacuum synchronizer.
This is the three cylinder version of the vacuum synchronizer.


This is the four cylinder version of the vacuum synchronizer. If the tubing on the bottom is followed, it is easily seen that all the tubes are tied together in a nonstop or infinity loop.
This is the four cylinder version of the vacuum synchronizer. If the tubing on the bottom is followed, it is easily seen that all the tubes are tied together in a nonstop or infinity loop.


A few things to Note

  1. The lines will connect between the throttle slide or butterfly and the intake valve.  Most engines that need to have the carburetors or throttle bodies synchronized will have a small barbed fitting that is capped off.
  2. Before starting to synchronize the carbs, the engine needs to be in good mechanical health.  If there is a cylinder that is low on compression, a valve seat that is leaking, valve lash is out of spec, valve lash setting is different from one side to another, an exhaust port or pipe is blocked, the carburetors have a leaking diaphragm, or there is a leaking intake manifold the reading will not be correct. All cylinders need to be the same mechanically if they are going to run the same. If the engine is not in good mechanical health, the carburetor synchronization will be all for nothing.
  3. Use a fan to cool the engine while its running. The engine can get very hot and if it gets to hot, the engine will need to be shut down. The synchronization will then need to be started again after the engine has cooled down. A Hot engine can ruin readings, so keep the engine cool with a fan.
  4. Make sure all connections are tight. Air leaking past any of the hoses will ruin readings. (reason I used 1/8″ lines)
  5. At this point you will want to consult the owners manual to synchronize the carburetors.
  6. Make sure the vacuum ports on the engine is clear and free of debris. A clogged line will make the reading non responsive.
  7. Some manufacturers recommend a specific vacuum reading when syncing carbs.  The whole Idea of syncing carbs is to get the cylinders to run as equal as possible for a smooth idle and correct throttle response. There are some exceptions though.  If the intake tracks are of different length, the manufacture will recommend a difference between the two cylinders. If this is the case a digital vacuum synchronizer or gauge cluster is recommended.
  8. Don’t forget to put the caps back on the vacuum / synchronizing ports on the engine.


Riding motorcycles and wrenching since my preteen years, I have moved from motocross to street bikes. Being a teen back in those days it was tough to get me off of the bike. Now days even though I am very busy being a dad I still have my weekends and go to the track to race and on occasion will do a track day or two.

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