Challenger Carburetor Synchronization and Idle Adjustment

Rotax 503 and 582 engines with dual carburetors require the carburetors to be synchronized. This will ensure that with the throttle closed both carburetor slides are at the same position. Then when the throttle is advanced both of the carburetors rise together and at any point along their travel they are at equal positions. In addition, the idle must be set so that the engine has an appropriate idle speed with the throttle closed. Finally the throttle lever must be set so that it will not over travel and put an excessive pull force on the ferrules at the ends of the throttle cables when the carburetor slides have reached their maximum travel.

When the carburetors are not synchronized one cylinder receiving more fuel than the other. One cylinder will be doing more work than the other. The engine will not be developing its maximum power or working at its peak efficiency.

Challenger Ultralight Carburetor Installation with Approved Aircraft Grade Fuel Lines

When the carburetors are unsynchronized, this will show up on the engine temperature gauges. The exhaust gas temperatures (EGTs) and cylinder head temperatures (CHTs) will be significantly different for the two cylinders when the carburetors are unbalanced. When the carburetors are properly balanced, the EGT and CHT readings will be nearly the same for both cylinders. The Rotax Owners Manual on page 10-1 specifies that the CHTs should be within 34′F for the 503 and 16′F for the 582 while the EGTs should be within 43′F for both engines.

Carburetor Synchronizing Procedure

1. Close the throttle lever to the idle position.
2. Remove the air filter from the carburetors. You will now be able to look inside the carburetors and see the slide assembly in each one. Rotax calls them carburetor pistons. When the throttle lever is pulled back to the idle position you will here a click sound as the carburetor slides (pistons) land on top of the tips of the idle setting screws.
3. Back the idle screws out approximately 6 turns on both carburetors until the brass screw tip is no longer seen inside the barrel of each carburetor (The idle screws are the 13mm slotted hex head screws on the left side of each carburetor) The carburetor slides will likely drop down until they hit the bottom of the carburetor with a metallic click.
4. Remove the black rubber boots (Rotax calls them grommets) from the vertical adjustment screws at the top of the carburetors. These are the hollow threaded adjusting screws that the throttle cables pass through.
5. Loosen the hex jam nut that holds the vertical adjustment screw in place on each carburetor.
6. Rotate the vertical adjustment screws so that each carburetor slide has an opening of about 3/8″. The adjusting screws may not be at the identical adjusting height to accomplish this. You will not likely be able to get a set of measuring calipers in there to measure the gap, so use a screwdriver shaft or the end of a wrench to measure the slide opening. The openings must be the same in both carburetors.
7. Now open the throttle to about 1/2 way and now measure the two openings in the carburetors to confirm that the slides have lifted the same distance.
8. Now open the throttle until the carburetor slides (pistons) lift all the way up just to the point where they allow the full opening of the throat of the carburetor. The bottom of the slide will be even with the top of the throat of the carburetor. This is your maximum allowable travel for the throttle lever. You do not want it to lift the slide any further. If you allow the throttle lever to pull the slides to the very top it will cause very high forces on the little silver soldered ferrules on the ends of the throttle cables. They may break if this is done repeatedly. That would result in one carburetor dropping to the idle position while in service and thus a partial engine failure. Both throttle slides should reach this top position together.
9. Set the throttle lever so that it cannot be pushed beyond this maximum travel position and will not pull the throttle cables any further. Usually a throttle quadrant has a screw that is fastened to restrict further throttle lever travel.
10. If you open the throttle to full and you hear the slides click, this means they are hitting the top of their travel. You need to back off the throttle lever travel so that the slides do not hit the end stop at the top of their travel. The lever should hit its end stop before the carburetor slides hit theirs.
11. Tighten the two jam nuts on the vertical adjusting screws on the top of the carburetors to lock these screws into the synchronized positions.
12. Slide the black rubber boots back over the two vertical adjusting screws on the top of the carburetors.
13. Screw in the two 13mm idle adjustment screws all the way in and then back them out 2-1/2 turns. Count these turns carefully. The two idle screws must be at the exact same adjustment. They set your idle speed by lifting the carburetor slides from the bottom position you previously set with the adjustment screws on the top of the carburetors.. To increase your idle speed, turn the screws clockwise. To reduce idle speed, turn the idle screws counter clockwise. They must always have the same number of turns in adjustment.
14. Move the throttle lever back and forth to test and confirm that the carburetor slides are moving freely and together and through their full movement. You should hear the two slides hit the bottom of their travel together when you reduce the throttle to idle. You should hear one click indicating they are landing on the idle screws at the same time.
15. Re-install the carburetor air filter and safety wire it.
16. Run the engine and you should see the two CHTs and the two EGTs within the required temperature differential ranges as quoted in paragraph three.
17. Adjust the idle speed as necessary.

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The Carburetor at Idle - The Piston Almost All The Way Down	The Carburetor Idle Adjustment Screw is the Hex Head Screw it in the center of the photo.	The Carburetor Piston Adjusting Screw.
Measuring the Idle Gap for Comparison with the end of a small box wrench	Carburetors at Half Throttle - Both are at the same travel.	Carburetors at Full Throttle - The bottom of the pistons are even with the top edge of the carburetor throat.