John,
At higher temperatures Fuel evaporates faster.
The carb bowls can get quite low sitting a day at 100° F.
You should not expect Vapor Lock on an engine that has not been operated in the last few hours.
At 1000MSL, 100LL fuel will Boil (Vapor Lock) at ~140°F.
Summer MoGas will Boil at ~130°F, and Wither MoGas will Boil at ~120°F.
So unless you are baking the engine under a Dark Cowling in direct SunLight, anything under 130°F is considered a Cold engine.
Why Do We use a Choke???
Liquid gasoline does not ignite. Only Fuel Vapors will burn.
If it is cold enough Gasoline will not vaporize. But this is only a problem in Antartica. We are in the temperate zones.
The carburetor sprays fuel into the carburetor airstream similar to a spray paint can.
Some of it evaporates, Some of it wets the inside of the intake manifold.
If it is cold, the wet gas does not evaporate much so we need to provide some extra gas to get a rich enough mixture that the spark plugs can ignite it.
In winter, with a cold engine, only some of the fuel evaporates the rest gets blown out the exhaust.
As the Engine, Actually the Intake Manifold, warms up, Eventually, all the fuel on the walls evaporate and the choke is no longer needed.
On an idling engine with ~10" MAP, the Pressure Altitude in the Intake manifold is ~28,000 ft.
At 28,000ft even 100LL will boil at ~80°F, MoGas even lower temperatures.
So once the Engine reaches ~80-90°F, there is no longer the need for the Choke.
You will find a point where the Engine will idle nicely, but misses when the throttle is opened.
The pressure increases when you open the throttle lowers the Pressure Altitude in the carb and the cold fuel stops boiling causing a lean condition and the misfire.
This is why you need to idle a little longer until the intake manifold gets warm enough.
Now you know the reason that thermostats are set no cooler than ~180°F.
Cooler is not Better.
In Summer, the Engine is already above the Choke temperature and Not needed.
It will start and idle, but it may still misfire if revved up until the temps get above ~120 (50°C).
That should sound like a familiar number to a Rotax Owner.
The AUX Pump is an AUXILIARY Pump, not a MOMENTARY Pump!
It does the same thing that the Main Mechanical Pump does except it does not need the engine to be running to do it.
It is Not there just to Prime the engine for 10 seconds. Leave it ON!
Opening the throttle while cranking, bypass what we commonly call a choke, but is really an enricher fuel circuit in the carbs.
The 912 does NOT have a Choke as we usually think of it.
Opening the throttle also raises the pressure (Lowers the Vacuum) in the intake manifold, causing LESS fuel to evaporate.
Opening the throttle will clear out a flooded engine by leaning it out. The opposite of what you need.
If you have to crank the engine for 10-15 seconds with the mechanical pump before it starts, that means you needed another 10-15 second with the AUX pump to complete the Carb fill. Leave it ON!
If you want to shut it OFF after starting and while taxiing, OK, but leave it on until the engine starts.
The engine will require up to 2 revolutions of cranking for the ignition to charge.
That should take well under a second.
Turn ON the AUX Pump.
Close the throttle.
Wait 20-30 seconds. (Adjust as you get experience.)
"Start"
Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
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