choke while running

by Jan-Willem Honders in 912 / 914 Technical Questions 6 years ago

Hi all,

What should a Rotax 912ul do when you pull the choke with the engine running?
Mine speeds up about 200RPM, except at wide open throttle.
Rough running only happens at the very end of the stroke (of the choke cable).

To me this seems to indicate a lean mixture, or am I seeing things?

Thanks, Jan

Re: choke while running

by Roger Lee » 6 years ago

Roger Lee

125 5.12k 4.53k

Hi Jan,
In the idle rpms it may make the engine run a little rough and rich. At cruise rpms (5K+) it shouldn't do anything.
There is no reason to mess with the choke for a normal running engine if there is no problem. You may create a problem so I would leave it stock.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: choke while running

by Jan-Willem Honders » 6 years ago

Jan-Willem Honders

2 4 0

The point is sadly that I don't have a normal running engine, suspected low power and mixture problems.
(It's also in another thread on this forum, rpm Rise with carb heat)
I was hoping that the effect the choke has on the running engine, making it run better generally, would aid in troubleshooting.

Or I am just chasing my tail here.....

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Re: choke while running

by Roger Lee » 6 years ago

Roger Lee

125 5.12k 4.53k

How many hours on the engine? Last time you did a carb sync? Is this something new as a problem or has it been this way?

There is a center punch mark on the end of the choke shaft. Make sure it points toward the air intake side or towards where it looks like a tube is coming out the side of the choke housing. I don't see the choke causing your problems at higher cruise rpms.
So long as you fuel pressure is okay, the fuel filters are clean, The carb throttle arms open equally as you advance the throttle, the re-circulation hose jet size isn't too big or a Banjo bolt or any other orifice after the fuel pump is clean and non obstructive then I would say you may need to have the carbs stripped down and someone either overhaul them or at the least look at them internally.
Does putting a set of carb sync GAUGES tell you anything?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: choke while running

by Rob Seaton » 6 years ago

Rob Seaton

3 1.32k 240

by pulling the "choke" (actually a small carburetor) you are adding more air and fuel, so the RPM should increase.
The mixture does not change, so it is not a useful tool for troubleshooting for lean/rich. (we used to teach this troubleshooting method until Conrad showed us the light)
You can prove this by running the engine at say 2000 rpm and then slowly pull the choke and watch the RPM and EGT rise.
Once they have stabilized (peaked) write down the RPM & EGT numbers.
Now push in the choke and advance the throttle to the same RPM as you have written down; the EGTs will be the same, so the mixture is the same.

Thank you said by: Jan-Willem Honders, Andrew Dunning

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Re: choke while running

by Jan-Willem Honders » 6 years ago

Jan-Willem Honders

2 4 0

I was certain I read somewhere that you can use the choke for troubleshooting as you described, thanks for clearing things up!

I actually pulled the carbs apart and found a burr on one of the choke shafts, preventing the disc(?) from properly seating.
This apparently caused an improper mixture.

The engine runs fine now.

Jan

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choke while running

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