to match by eye or even with a feeler gage as outlined in the manual
is out of the question. The mechanical synchronization is a good
starting point only and not a basis to rely on for a good carb sync.
Two matched vacuum gauges is a must and will be an accurate means of
evaluating your adjustments made to the carbs. Before adjusting my
carburetors my engine ran smooth only at a few rpm settings. The goal
is to have equal vacuum on both gauges during idle and throughout the
rpm range of the engine. If you can get the vacuum gauges to read the
same at idle and the levers are advancing evenly when the throttle is
applied you will have no problem achieving the right setting. After
hooking up the vacuum gauges to the engine and doing the mechanical
sync as outlined in the manual start the engine and warm it to 200
degrees or so. I used duct tape on the oil cooler and most of the
radiator to achieve this temperature on the ground without running
the engine at 4000 rpm. As the engine is warming you will and I
repeat you will see two different readings on the gauges. The thing
to keep in mind here is the vacuum at idle will be about 14 hg and
less than 5 hg at 4500 rpm. So the carb that has the highest reading
will be closed more than the other. simply close the carb with the
lower vacuum reading until it matches the vacuum reading of the
higher carb. You will have to reset the idle stop screws accordingly.
Make small adjustments, mine was only out less than 1/16 th of an
inch. You can adjust the cable anchors or the cable adjusters which
ever. Keep in mind that the balance tube helps to equalize any small
difference between the carbs, so when you remove that each carb is on
its own and any difference between the carbs will be very noticeable
when the vacuum gauges are attached in its place.
I left the idle mixture screws at the recommended setting of 1.5 turns.
My engine runs great and a lot smoother.
Tools:
I purchased two vacuum gauges from sears $19.95 each and checked them
separately on my auto engine to see if they matched. They did.
you will also need to hook in line for each gauge a small valve to
dampen the flutter of the needle. I used small water valves purchased
from the local hardware like what is used on ice makers. They also
worked great.
8 feet of vacuum line, so you can place the gauges in the cockpit.
Some rotax dealers sell a kit for about $100.00
Please note: I am only sharing my experience with trying to do this
and not directing proper methods.
I hope this may help anyone who has not yet synchronized their
carburetors.
K.F