Rotax 912 slow cranking

[ChrisBolkan 130]

Hi!

My Avid has a 100HP 912s. It is not cranking fast and kicks back while cranking. It used to crank much faster and did not kick back. Once it is started it runs fantastic just like always.

I took the battery out (Odyssey PC-612) and had it load tested. It tested good. I have not put a voltmeter on the battery or across the starter to look for voltage drop while cranking yet. I will do that as soon as I get back to the hangar with a friend since it takes two people to do those tests.

My wiring is of plenty adequate size, connections are good and clean. I am willing to bet there will not be excessive voltage drop anywhere when I do check it.

What I am wondering is: Has anyone experienced the starter on the 912 going bad slowly (worn brushes, solder flinging out of the commutator etc.) causing a mysterious slow down in cranking speed? 

It looks like it's going to be a kind of a PITA to remove the starter. It would be nice to hear they can go bad and crank slow before I rip it out and take it apart.

Anyone have an experience like this before?

Thanks!

Chris

[flywise 169]

Hey Chris,

is this happening all the time or only when engine is hot/cold??  Some 912 have had crankcase fretting issues making the crankshaft bind..Here an easy check you can do to rule out that dreaded problem

https://www.rotax-owner.com/en/912-914-technical-questions/2271-rotax-912-uls-fretted-crankcase

http://ctflier.com/topic/1194-crankcase-fretting/

 

[Av8r3400 1179]

How old is the battery?

What is the amp rating on a PC612?

[wypaul 448]

Oil seal leaking into the starter?

[ChrisBolkan 130]

Happening all the time. Hot or cold. A little better when hot, but not any different that the normal difference of hot/cold starting.

Battery is a few years old. Will know if that is the problem when I check cranking voltage and check for voltage drop. Haven't done that very obvious thing yet. 

No idea if crank case id fretting or oil is leaking into the starter. Will find that out when I take the starter off.

I was just wondering if anyone had seen problems with the starter motor itself. Doesn't look like it.

I'll do the voltage drop tests, which will also verify battery condition and if all checks out I will pull the starter.

Thanks guys!

[1avidflyer 1983]

Kind of got me worried when I read about the fretting issue.  My 100 HP 912 is about that same age as was described in the rotax forum link.  About 2000 numbers newer on the serial number though so maybe it's all good.  

[ChrisBolkan 130]

Just reading about the fretting issue now. Haven't got to how to check yet, so need to finish the threads. However it seems like if the engine were tighter one would notice when pulling the prop through by hand. Mine seems fine to me, but I'll keep reading. I sure don't want fretting to be the problem with my engine!

[ChrisBolkan 130]

Thanks for all the input so far!

I went to the plane and ran a couple of tests. My engine serial number is 4425382. It looks like the engines with the fretting problem fall in the 5645xxx serial number range.

The check for fretting is to measure the torque it takes to turn the engine over with one plug removed from all cylinders. If it takes over 150Nm (110ft-lbs) there is a problem. It takes less that 110 ft-lb to turn my engine over with the plugs installed a long as I move slowly through the compression strokes. 110 ft-lb seems like a lot to me but it's what's in the book and my engine takes a lot less that that to turn over so I think I'm good on the possible fretting issue.

Then I jumped the battery with a real solid 12V and tried to start it and it did not crank much if any faster, and was still kicking back (wanting to suddenly stop) so it looks like the battery is OK. 

The Battery is an odyssey PC-680, not PC-612. Senior moment. Anyway it is the same battery model I used to start the 0-320 in my magnum. If it is good it has plenty of power.

So then I thought what if I just ground the ignitions and crank it? If the problem is the ignition firing before TDC grounding the ignitions will eliminate the kick back. So I cranked it with no ignition spark and it STILL cranked slow and kicked back.

I am convinced it has to be the starter motor. I am going to remove it and see what the heck is going on. It will take a while to get this done, but I will report back with what I find.

Chris

 

Edited 20 Nov 2021 by ChrisBolkan

[flywise 169]

Thanks for all the input so far!

I went to the plane and ran a couple of tests. My engine serial number is 4425382. It looks like the engines with the fretting problem fall in the 5645xxx serial number range.

The check for fretting is to measure the torque it takes to turn the engine over with one plug removed from all cylinders. If it takes over 150Nm (110ft-lbs) there is a problem. It takes less that 110 ft-lb to turn my engine over with the plugs installed a long as I move slowly through the compression strokes. 110 ft-lb seems like a lot to me but it's what's in the book and my engine takes a lot less that that to turn over so I think I'm good on the possible fretting issue.

Then I jumped the battery with a real solid 12V and tried to start it and it did not crank much if any faster, and was still kicking back (wanting to suddenly stop) so it looks like the battery is OK. 

The Battery is an odyssey PC-680, not PC-612. Senior moment. Anyway it is the same battery model I used to start the 0-320 in my magnum. If it is good it has plenty of power.

So then I thought what if I just ground the ignitions and crank it? If the problem is the ignition firing before TDC grounding the ignitions will eliminate the kick back. So I cranked it with no ignition spark and it STILL cranked slow and kicked back.

I am convinced it has to be the starter motor. I am going to remove it and see what the heck is going on. It will take a while to get this done, but I will report back with what I find.

Chris

 

Hey Chris,

the kickback is normal when cranking speed is not high/strong enough as the sprag clutch will let go when reaching compression @ TDC

[Av8r3400 1179]

Change out the Left-Right-Both-Start switch to a separate starter and two toggles for the ignitions.  You need to be able to crank the engine without ignition for maintenance and proper starting technique.  These LRBS switches will cause kickback and damage to the spray clutch and ignition modules.

When the contacts start to go bad, this can cause slow cranking, too.

Edited 21 Nov 2021 by Av8r3400