The $1.55 Manometer
Fine Tuning the Carbs

By: John Ratliff AKA "Rat"

WARNING: This procedure requires running your engine at high RPMs while making adjustments to the engine. Please see our engine run-up article for suggested safety measures before proceeding. Please see our engine run-up article for suggested safety measures before proceeding.

The carburetors on the Rotax 503 DCDI can be a chore to synchronize. Getting the temperatures between the cylinders even can be even more elusive. Proper carburetor balancing requires a manometer however, these can be very expensive for a tool that is rarely used. After researching several manufactures I did locate a few inexpensive ones offered by some of the motorcycle supply houses but found that they were not recommended for two strokes. Bing actually sells several devices on their site however the least expensive one cost $34.00, still too expensive for a tool you might use once a year. There's gotta be a better way. One night while searching the web I came across an idea written by Marty Ignazito. He claimed that a manometer could be built for $1.55. Now that's better! While he hadn't actually tested it, it sounded logical so I decided to give it a try.

Parts required:

  1. 1 Yardstick (or small flat stick about three feet long)
  2. 9 Feet of 1/8th I.D. Clear Tubing
  3. 4 or 5 Zip Ties
  4. “Some” Two Stroke Oil

Building The Manometer

1. Start by drilling 4 or 5 holes in the center of the yardstick about 1 foot apart.

2. Fold the tubing and secure it through the holes with the zip ties to the yardstick with about 1-foot of each end of the tubing extending from the top of the yardstick and enough on the bottom to form a “U“.
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3. Next s-l-o-w-l-y pour the oil into the tubing, enough to fill the tube so that when it settles you will have about 1 foot filled at the bottom of the manometer. You may have to thump, or blow slowly into the tube to remove the bubbles in the oil or if you have the patience just wait a couple of days and they'll settle on their own. The bubbles must be removed in order to give an accurate measurement.

Before you can use the manometer to fine tune the carbs you will need to manually sync your carburetors in any fashion you prefer. You may find Dave Beckstrom's method to be very easy. After manually syncing the carburetors leave the throttle cable locknuts loose with the boots pulled up.

Secure the tail of the aircraft as to prevent it from moving, start the engine and warm it up to operating temperature then shut it down. If you have a primer you will have to remove the lines connecting it to the carburetors, if not remove the primer port cover. Connect each end of your manometer to each primer port taking care to not cross the tubing. Both tubes must be connected before starting the engine to keep the oil from being sucked into the carburetor.

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Fine tuning the carb synchronization.

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1. Start the engine and advance the throttle to the highest level of RPMs you are comfortable with, but not less than 3,000 RPM up to cruise RPM (~5,500). The oil in the manometer will rise in the tube connected to the carburetor with the most vacuum.

2. While staying well clear of the prop, adjust the carburetor with the highest level of oil down by raising the slide in that carburetor. This is done by turning the throttle adjusting screw out 1/8th of a turn, wait for the oil to settle then continue until the oil is level in both tubes. The oil will move with the movement of the slide but give it a second or two to settle before adjusting further. The adjustments must be made in small increments; a small adjustment makes a big movement of oil in the tube.

3. While performing the adjustments monitor the temperatures as they will rise and fall with the adjustments. Keep an eye on the tachometer, if the RPM drops below 3,000 increase it by advancing the throttle lever.

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4. If by chance the carburetor with the highest level of oil cannot be adjusted down, adjust the other carburetor slide down to raise it's level using the same procedure. Use small adjustments and give the engine time to react to them.

5. Once the oil levels are equal your engine should be showing equal EGTs and CHTs for each cylinder. Tighten the throttle cable adjustment locknuts and again check the oil level. If it remains level move to the next step, if it has changed adjust as needed.

6. Reduce the throttle to idle and check the manometer. You are likely to notice that the oil has again risen in one of the two tubes. This is because your idle adjustment screws are set at different levels. DO NOT adjust the throttle cables, as they should be loose allowing the slides to rest on the idle adjustment screws.

Adjusting the idle.

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1. Find the carburetor with the highest indicated vacuum.

2. Slowly turn the "idle screw" of this carburetor clockwise (in) 1/8th of a turn and observe the level of the oil giving it time to react to the change. It should drop as you make the adjustment.

3. If the RPM increases above 2,000 while doing so, use the other carburetor to make the adjustment by screwing its idle screw counter-clockwise (out) to bring the oil level up in that side of the manometer.

4. When the oil is level in the tube and the idle is at least 2,000 RPM you're done. If the RPM is too high for your comfort (say ~2,500 RPM) then turn each idle screw counter-clockwise (out) 1/8th of a turn until you achieve 2,000 RPM.

5. You should notice a smoother idle with even temps.

The next time you fly monitor the temps to insure they are within specs. Although you didn't change the mixture of the carburetors, the temperature should be even at the RPM used to make the adjustment. I have found that because of unequal cooling it seems to be impossible to get the temperatures even across the entire range of throttle.