 The 912 series
of engines use an altitude compensating Bing carburetor. These are very
reliable, and literally trouble free if properly set up and maintained.
To properly set them up requires several tools.
A feeler gauge.
A double vacuum gauge.
Two 9 mm open end wrenches to adjust the M5 nuts on the throttle cables.
The following are the recommended steps to adjust the carbs.
IMPORTANT: make sure cables are not effected by ANY engine movement,
and that they move FREELY back and forth with no stickiness.
- First step is to allow the engine to come up to operating
temperature, this should be in the 200 degrees F water temperature, and 1200
EGT.
- Make sure that the engine has been loaded with a properly adjusted
propeller that limits the rpm tied down on the ground to 5600 rpm.
- Make sure that both throttle cables are allowing full opening of the
throttle valve. This can be done visually with the engine turned off and
full power applied.
- With the engine turned off bring the throttle back to an idle.
Adjust the idle screws back until the carb actuating arm is
against the carb body.
- Adjust the idle screws in until they touch the actuating arms, now
adjust them each an additional 1 1/2 turns.
- Gently adjust the air mixture screws in all the way and then back
them out
1 1/2 turns.
- Have someone start applying throttle very slowly, adjust the cables at
the fittings so both cables begin opening the throttle arms at the same time.
- Start the engine and adjust the idle screws equally until
you obtain 2,000 rpm for an idle speed.
- This procedure will normally synchronize the carbs at an idle.
- Now with the engine still turned off have someone advance the
throttle slowly, adjust the throttle cables so that they are both taunt,
and so that both throttles start opening at exactly the same time.
- Once you have done this bring the throttle to half throttle and
measure the distance between the throttle activation arm and the carb
body, they should be the same.
- Advance to full throttle, both throttle arms should be in the full
open position.
If there is a problem with the synchronization of the cables it can
usually be
traced to movement in the outer throttle casing. That is when you are
applying throttle instead of it moving the cable the outer casing is
flexing or bending.
If this is the case you MUST secure, or route the
cable so that all of the throttle movement is directed to the throttle
arm.
In the standard configuration you would use #60 and & #58 to adjust
the throttle and choke set up on a Rotax 912 equipped with Bing
carburetors.
Using a vacuum gauge
First perform
a manual synchronization adjusting the Bowden cables so there is full
travel from full open to full closed of the throttle valve arm, as
described above.
Then
remove the compensator tube and install the gauges. Rotax provides a small
plug screw, left of the idle mixture control screw on the carburetor to
connect the hoses from the vacuum gauges.
However, you still have to
remove the compensator tube and plug the passage way before the carbs can
be synchronized.
With this in mind, it's really easier to just connect the
vacuum hoses where you disconnected the compensator tube.
After you adjust the carbs, the vacuum
gauges should read the same at all engine settings.
Remember, it's the length
and the tightness of the cables that adjusts the carb synchronization. When you're adjusting
them, you're simply lengthening or shortening a cable, and making each
throttle valve arm move the same distance at the same time.
Click here for video showing how to properly set carbs. |
