Chris and Kingsley,
Keep in mind there is no choke per se on the Bing. It is an enrichening circuit
(also known as the starting carb) that only works correctly when the throttle
is closed.
Once above about 3500 RPM the enrichening circuit hole in the carb does not have
enough suction to pull fuel from the bowl and it actually leans the carburetor,
not enrichen it when the throttle plate is open.
So, two things to remember: To start the engine cold, throttle closed and
starting
carb engage (choke to most of us although it is not a choke valve). Once
the engine hits we have two choices, slightly advance the throttle and or hold
the choke out until the engine begins running smooth or: For those who have
their idle set at 1600 RPM hot, and fine pitch, this means the throttle is not
set for idle per the Rotax manual. (You know that initial setting when
rebuilding
the carb!) With this combo the throttle is closed hard, and little fuel
and air gets by the throttle plate and starting can be problematic unless the
throttle is cracked open a bit on start. When normally set up with an 1800
RPM fixed pitch prop pitched for a full power static of 5200 RPM (an average
pitch
between climb and cruise) all starting procedures are by the book. Throttle
closed, choke out (enrichening circuit/starting carb engaged) and the engine
start is quick. (Provided the starter is cranking above 300 RPM anyway.)
Pulling the choke at higher RPMs tends to cause the engine to run lean for the
above reasons but to recap, with the throttle plate open and MP high, the float
bowl is now slightly depressurized and the main jet loses some flow because
of the lowered bowl pressure and consequently the carb leans a bit.
Refer to the Understanding the Rotax series by Phil Lockwood some 20 years ago
and a later set from Rainbow Aviation. The Rotax video series is pretty good.
Better than watching the news, or Hallmark Channel now days.
But first, look at your carbs and exhaust system. A slight jet of hot exhaust
gas can cause roughness at full or cruise power. Pay attention to soot lines
coming off the exhaust staining. If there is any soot on the carb bowl you've
got serious issues. I will fly and put my hand in the cowl access doors and
touch the carb bowls after a test flight (if I can) to see if the bowls are
cool.
Gas evaporates, so even in a hot cowl, the bowls are not hot to the touch.
If one of the bowls is warm....Exhaust leak! Because the CKT exhaust systems
don't seal absolutely tight and sometimes the head to downpipe is not a tight
seal. I've had to actually use lapping compound to get a good seal.
Mid range roughness can be hard to troubleshoot. Normally compare EGT's, keep
the fuel fresh and of proper octane. Remember very old car gas won't burst with
a match, it just burns on the surface. Heck of a thing.
Next thing on roughness is carb cleanliness, balance tube leak, bowl vent leak,
manifold gasket leak / carb rubber connector socket cracked, compressions
significantly
different, but usually, it's a dirty, improperly adjusted throttle
plate, carb or air leak. One culprit if the hoses and interior mechanicals are
clean, is a partially open enrichening lever. If one or both of the carb
"choke"
cables do not allow full closure (on the stop) of the starting carb arm,
the engine runs rough midrange.
On the subject of carb ice in humid Florida, when the temps get into the 40-60F
range, carb ice is a real issue in a normally aspirated engine. If you run a
912 or 912S XS (cold air intake), carb ice is a real issue if the throttle is
fixed at one RPM. The Classic warm radiators preheat the air in the cowl so
it was less of a problem. One reason motorcycles don't get carb ice often is
the throttle is constantly moving. If you are looking for an idea to keep you
busy for a while, why not put in a temp probe in the Bing carb. In the Bing
Carby, there is a manifold tap that can be used to troubleshoot the throat
temperature.
I've never installed a carb temp probe or considered it is because
the 912S XS models I have done all had the water jacket carb heat running at all
times.
Should you want to experiment (I know of no one that has done a true carb temp
probe), there is a screw hole that is plugged for our Rotax Europa's in the carb
I think it is an M3 screw for a manifold pressure tap on single carb engines,
but you can just as easily put in a Harbor Freight thermocouple drilled in
a brass screw and some JB Weld or similar, and check your throat temp but that
is not really an easy thing to do.
Enough of my tomes,
Bud Yerly
-----Original Message-----
From: owner-europa-list-server@matronics.com
<owner-europa-list-server@matronics.com>
On Behalf Of n7188u
Sent: Wednesday, February 01, 2023 2:49 PM
Subject: Europa-List: Re: New engine runs a little rough when weather is cold.
Thanks to all for the feedback. I think that at this point, given that the
engine
behaved perfectly during the last flight with higher OATs I decided to:
1.- Recheck the gearbox clutch friction just in case (even though it was OK 40
hours ago).
2.- Tape both the oil cooler water cooler further to get the temps higher on my
next cold day flight.
3.- Try the choke idea to see if a richer mixture changes anything.
I'll report back.
Chris
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