Gurus. What would happen if I...

by Mike Hoare in 912 / 914 Technical Questions 10 years ago

... hit the starter button after the engine (912ULS) is already running? It's a hypothetical at this point, but I ask because my shiny new starting technique involves leaving the starter engaged a fraction longer than I did before. This gets butter smooth starts with no kick back, but am I damaging anything?

Re: Gurus. What would happen if I...

by Roger Lee » 10 years ago

Roger Lee

125 5.12k 4.53k

Hi Mike,
That fraction of second as you say shouldn't make a difference and yes it could be better for the start to prevent kickback. That said you need to take care of whatever is causing kickback because that will absolutely damage the sprag clutch to the starter and it's a PITA to fix. Don't let this go as many have found out it isn't fun to replace the sprag clutch. The starter sprag clutch will disengage when the engine over takes its rpm and then the starter at that point is not turning the engine any longer. Allowing kickback stretches the sprag spring which in turn won't allow the locking lugs to engage when the starter engages.

Fix the kickback.

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

Thank you said by: Mike Hoare

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Re: Gurus. What would happen if I...

by Rob Seaton » 10 years ago

Rob Seaton

3 1.32k 240

I agree with Roger, it not a concern. Once the engine is running, say at 2000rpm, the freewheel gear is "freewheeling" so unless the starter is going faster than the crank the starter is not under any load. If the starter goes faster than the crank it will only add some assistance to the motion as there is no bandix to jam or wear.
How fast does the starter go with no load? Anyone know?
Leaving the starter engaged for longer periods would cause wear to the bearings and eventual destruction of the starter. It is not designed to go so fast. I hear that the UK requires a "Starter engaged" warning light in case the relay of start switch may be faulty. Good idea, is it true?
Does it also detect if the sprag is locked up and the starter is now being driven (turns into a electric motor?)

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Re: Gurus. What would happen if I...

by MARK JACKSON » 10 years ago

MARK JACKSON

12 332 11

I tell all my students that the button is a starter button and not a door bell. The worst thing you can do is give it a quick press and release it before you know whether the engine has actually started. To start the engine properly you need to push and hold the starter button until the engine is running. It will not do it any harm as has been said by Roger and Rob ( I mean they both said it won't harm it as well ), because as the engine starts the sprag will release the starter motor. I don't think powering the starter motor when it is unloaded will damage the starter motor. It is an electric motor, that is its job. As long as it doesn't overheat, which it won't if its not loaded it will be OK. Having said that, if the starter motor is attached to the engine, it could put more wear on the intermediate gears. Stabbing at the starter button and releasing it before the engine has started properly can be a cause of kick-back and as has been said will damage the spring on the sprag. Kick back is the sprag not releasing properly as the release spring is damaged. As the engine fires it tries to turn the starter motor as well, which it can't, so it stops with a bang. The engine cannot drive the starter, if the sprag engaged while the engine was running it would stop with a bang and destroy something in a big way.

Leaving the starter button engaged a second or two longer when starting the engine is a good thing.

As Roger says, Fix the cause of the kick back, it won't fix itself and it will only get worse.

Mark

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Re: Gurus. What would happen if I...

by Roger Lee » 10 years ago

Roger Lee

125 5.12k 4.53k

Hi Mark,
The sprag should release as the engine rpm over takes the starter generated rpm which happens quite quickly during a normal start.
Kick back doesn't come from the sprag not releasing.

The sprag kickback is the inability of the sprag to carry the crank through the compression stroke causing the piston to reverse direction as the spark fires 4 degrees BTDC. The kick back or what we see is the reversed direction.
Kick back should also be reduced with the new soft start ignition as it fires 3 degrees after TDC.

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

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Re: Gurus. What would happen if I...

by Mike Hoare » 10 years ago

Mike Hoare

10 16 0

Great feedback, thanks guys!

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