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JonathanMilbank
Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area
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 Posted: Thu Feb 28, 2019 4:08 pm Post subject: I'm puzzled, again! |
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This time it's the old topic of 912 ULS kick-backs on start up. After a couple of days during which our group members pondered possible causes of this recent phenomenon, following a long period of relatively trouble-free starting, we reached the conclusion that the battery has become elderly. The gradual slowing of cranking speed while engaging the starter, creeps up unnoticed over a period of years.
A new battery fixed the problem most convincingly, whizzing the propeller around with great alacrity. But one of my group members raised this question "Why should there ever be any kick-backs at all, even with a tired battery, considering that we have the latest CDI boxes in addition to all the other measures designed to prevent it? After all, the built-in temporary retardation of timing to 3 degrees ATDC while starting, should guarantee that kick-back can't happen even with an old battery!"
The only "explanation" I can conjure up is that when overall battery voltage reduces with age and while it drops even lower during cranking, the built-in deliberate delay in spark being triggered until after TDC while starting, either doesn't function or only works partially.
How's that for a half-baked theory?
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Rick Moss
Joined: 21 Sep 2014
Posts: 96
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| Posted: Thu Feb 28, 2019 11:56 pm Post subject: Re: I'm puzzled, again! |
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My first question would be: Do you just have the latest CDI boxes, or do you have the later rotor also? If it’s a new engine, you presumably have both. If the CDI’s are retrofits, I’ve seen a couple where the original rotor has remained, which renders the ignition retardation feature inactive.
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Remi Guerner
Joined: 14 Dec 2010
Posts: 284
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| Posted: Fri Mar 01, 2019 1:22 am Post subject: Re: I'm puzzled, again! |
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Jonathan,
I was unfortunate to have one of those engines prone to kicking back and have been trying everything possible to fix it over the years: new battery, battery moved forward to shorten the cables, preheating when cold. These provided some improvement but did not fix the issue. Then I installed a Soft Start module and the new flywheel hub with delayed timing. Then I had to replace the ignition units with the last version with integrated soft start. This again provided a great improvement but there were still an occasional weaker kick back. I finally replaced the sprag clutch (that was 3 years ago) and then the problem was completely solved.
I asked myself the same question as you did: Why should there ever be any kick-backs at all, even with a tired battery, as there is no spark possible before 3 degrees ATDC while starting? I do not have a sure answer but I believe what we then perceive as a kick back is not a kick back. It would call it a kick-stop: during cranking, the sprag clutch slips occasionally, then the prop stops turning for a fraction of a second, giving the false impression of a kick back. However this situation will not last long, as these repeated quick-stops will for sure accelerate the damage to the sprag clutch, ultimately causing a constant slipping and preventing to crank the engine.
One thing is sure: with an engine fitted with the delayed/retarded ignition system and an undamaged sprag clutch there is no kick-back nor kick-stop at all, even with a very weak battery.
Remi
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area
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| Posted: Fri Mar 01, 2019 2:40 am Post subject: Re: I'm puzzled, again! |
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Remi and Rick,
The aircraft has EVERYTHING known to man to prevent kick-backs, including the flywheel with extended lugs. The CDI boxes seem to be working as advertised, not least because the timing remains well retarded for several seconds after start until the built-in advance takes over.
My engine seems to be one of those which is just simply "accursed" by this problem, to the extent that I fitted an additional separate cranking switch to facilitate overcoming the inertia of the HEAVY Airmaster propeller hub and letting it spin for a couple of seconds, before adding choke when cold or sparks when warm. This separate switch and techniques have proved very effective to overcome the problem during most starts.
I'm too embarrassed to tell you how many times that the sprag clutch has been changed, or more specifically how few hours have been flown between changes, but please believe I'm keen that it shouldn't happen again. So now I've bought an additional lead-acid battery booster pack to assist the aircraft's lead-acid battery particularly with the first start of each day during cold weather.
I'm not planning to move the battery forward into the engine compartment again, due to C of G considerations and also because when it used to reside there the kick-backs still happened sometimes.
Remi, your explanation of "kick-stops" due to a poor battery is new to me and seems to be a reasonable hypothesis. So from now on I'm going to change the battery more frequently and use the booster pack as appropriate. Many thanks to both of you for the quick responses.
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Remi Guerner
Joined: 14 Dec 2010
Posts: 284
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| Posted: Mon Mar 04, 2019 9:27 am Post subject: Re: I'm puzzled, again! |
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As an addition to my previous post regarding the kick-back problem, I believe it is useful to add the following regarding the sprag clutch replacement. After reviewing the different issues of the 912 IPC, from 2017 up to now, I discovered that Rotax has made several changes in that area over the years. The sprag clutch itself has not been changed but the sprag clutch housing was modified several times. Housing P/N 852405 was introduced in May 2008 with the following comment: "new housing with expanded space for sprag clutch". Then in January 2017, P/N 852406 was introduced with the very same comment. I guess the changes were designed to improve the behaviour of the sprag clutch.
My conclusion: when replacing the sprag clutch, it is critical to replace also the housing and make sure the P/N is the last version.
Remi
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area
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| Posted: Mon Mar 04, 2019 12:07 pm Post subject: Re: I'm puzzled, again! |
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Thanks also for this latest observation, Remi. Next time that I need to replace the sprag clutch, I'll keep the housing issue in mind. But "next time" will hopefully be a very long time!
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budyerly(at)msn.com
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| Posted: Tue Mar 05, 2019 4:49 pm Post subject: I'm puzzled, again! |
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Remi and Jonathan,
Of course the early 912ULS were happier running backwards than forward.
If you have done all you say you have, the list gets pretty small and expensive.
Things which helped the kickback was most importantly, the soft start module and the new ignition modules (six wire). Careful assembly of the pickups on the electrics was also essential to assure the correct pickup went to the correct box. An early spark just as the valves close will kick right back. See your Rotax Heavy Mx manual. I have only seen this once on a very early Rotax 912ULS where the A&P found some broken wires (another problem we all discussed before) and repaired the wires with new wires and didn’t quite get everything wired back up right. What a nightmare. We ended up going to two new ignition boxes paying extreme attention to the pickup wiring and all was fine. I was chastised by Kerry at Lockwood for not changing both boxes initially, as tinkering with one old box and putting only one newer one initially with the old one, really made a mess. Note that the timing controls changed as did the flywheel pickups in the early 2000s then again around 2009. The new boxes even with the original flywheel really helped this plane.
The high torque starter was also a help on some of these older engines with soft start or with starting on one ignition technique. On a 912/914 low compression, the low torque spins at about 300 RPM with a supercharged battery or booster charger at 50 amps. The high torque starter spins at nearly 500 RPM on the 914. On the 912ULS I found the RPM still hit 400 plus which is more than enough for quick starts. The Skytec starter is OK but I went with the Rotax. I was impressed and you know what a cynic I am!
I would also check for a spun crank shaft or a valve timing problem. I don’t do that in my shop. I pull the engine and go to Lockwood and have Kerry speck out the problem. A spun crank is nasty expensive and is usually better to replace the entire short block. Really expensive. $14,000 min with labor.
I had a sticky valve problem on two cylinders causing a problem which was just that, sticky valves. We never could figure what actually caused it but the kickback was spectacular. We redid the heads and that cleared up the problem. Changed a slightly bent valve and a pushrod, lapped the valves, replaced valve springs (Service bulletin) and checked the crank and ignition modules which were OK. Heck of a thing. And only about $4300 which was a bargain considering two guys for 3 days plus engine runs at Lockwood. I suspect that the engine was over revved and hit a valve as there were some tell tale signs on the piston carbon of the valves hitting.
I’m in Florida so cold engine starts occur only one month of the year for about 2 days. If the engine is really cold (below 0C) the cold oil and tight oil pump can slow the engine crank some 50 RPM on even a semisynthetic oil. Warm your engine with a blanket or something. I’m not a fan of bock engine heaters that use the crankcase bolts however, they make heat pads for the oil tank and crankcase that work if the cowl is wrapped fairly tight outdoors or in a hangar. A 100 watt bulb in the intake works if you leave it for an hour and wrap the cowl tight. Mobile 1 MX4T or Motorcycle racing full synthetic oil helps, but warmth is always nice for quicker starts.
Good luck. Remember you will find the problem. It is always the last place you check.
Bud Yerly
Sent from Mail for Windows 10
From: owner-europa-list-server(at)matronics.com <owner-europa-list-server(at)matronics.com> on behalf of Remi Guerner <air.guerner(at)orange.fr>
Sent: Monday, March 4, 2019 12:27:46 PM
To: europa-list(at)matronics.com
Subject: Re: I'm puzzled, again!
--> Europa-List message posted by: "Remi Guerner" <air.guerner(at)orange.fr>
As an addition to my previous post regarding the kick-back problem, I believe it is useful to add the following regarding the sprag clutch replacement. After reviewing the different issues of the 912 IPC, from 2017 up to now, I discovered that Rotax has made several changes in that area over the years. The sprag clutch itself has not been changed but the sprag clutch housing was modified several times. Housing P/N 852405 was introduced in May 2008 with the following comment: "new housing with expanded space for sprag clutch". Then in January 2017, P/N 852406 was introduced with the very same comment. I guess the changes were designed to improve the behaviour of the sprag clutch.
My conclusion: when replacing the sprag clutch, it is critical to replace also the housing and make sure the P/N is the last version.
Remi
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=487913#487913
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rayfitton(at)virginmedia.
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| Posted: Wed Mar 06, 2019 5:46 am Post subject: I'm puzzled, again! |
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| Quote: | On Mar 6, 2019 at 00:48, <Bud Yerly (budyerly(at)msn.com)> wrote:
Remi and Jonathan,
Of course the early 912ULS were happier running backwards than forward.
If you have done all you say you have, the list gets pretty small and expensive.
Things which helped the kickback was most importantly, the soft start module and the new ignition modules (six wire). Careful assembly of the pickups on the electrics was also essential to assure the correct pickup went to the correct box. An early spark just as the valves close will kick right back. See your Rotax Heavy Mx manual. I have only seen this once on a very early Rotax 912ULS where the A&P found some broken wires (another problem we all discussed before) and repaired the wires with new wires and didn’t quite get everything wired back up right. What a nightmare. We ended up going to two new ignition boxes paying extreme attention to the pickup wiring and all was fine. I was chastised by Kerry at Lockwood for not changing both boxes initially, as tinkering with one old box and putting only one newer one initially with the old one, really made a mess. Note that the timing controls changed as did the flywheel pickups in the early 2000s then again around 2009. The new boxes even with the original flywheel really helped this plane.
The high torque starter was also a help on some of these older engines with soft start or with starting on one ignition technique. On a 912/914 low compression, the low torque spins at about 300 RPM with a supercharged battery or booster charger at 50 amps. The high torque starter spins at nearly 500 RPM on the 914. On the 912ULS I found the RPM still hit 400 plus which is more than enough for quick starts. The Skytec starter is OK but I went with the Rotax. I was impressed and you know what a cynic I am!
I would also check for a spun crank shaft or a valve timing problem. I don’t do that in my shop. I pull the engine and go to Lockwood and have Kerry speck out the problem. A spun crank is nasty expensive and is usually better to replace the entire short block. Really expensive. $14,000 min with labor.
I had a sticky valve problem on two cylinders causing a problem which was just that, sticky valves. We never could figure what actually caused it but the kickback was spectacular. We redid the heads and that cleared up the problem. Changed a slightly bent valve and a pushrod, lapped the valves, replaced valve springs (Service bulletin) and checked the crank and ignition modules which were OK. Heck of a thing. And only about $4300 which was a bargain considering two guys for 3 days plus engine runs at Lockwood. I suspect that the engine was over revved and hit a valve as there were some tell tale signs on the piston carbon of the valves hitting.
I’m in Florida so cold engine starts occur only one month of the year for about 2 days. If the engine is really cold (below 0C) the cold oil and tight oil pump can slow the engine crank some 50 RPM on even a semisynthetic oil. Warm your engine with a blanket or something. I’m not a fan of bock engine heaters that use the crankcase bolts however, they make heat pads for the oil tank and crankcase that work if the cowl is wrapped fairly tight outdoors or in a hangar. A 100 watt bulb in the intake works if you leave it for an hour and wrap the cowl tight. Mobile 1 MX4T or Motorcycle racing full synthetic oil helps, but warmth is always nice for quicker starts.
Good luck. Remember you will find the problem. It is always the last place you check.
Bud Yerly
Sent from Mail for Windows 10
From: owner-europa-list-server(at)matronics.com <owner-europa-list-server(at)matronics.com> on behalf of Remi Guerner <air.guerner(at)orange.fr>
Sent: Monday, March 4, 2019 12:27:46 PM
To: europa-list(at)matronics.com
Subject: Re: I'm puzzled, again!
--> Europa-List message posted by: "Remi Guerner" <air.guerner(at)orange.fr>
As an addition to my previous post regarding the kick-back problem, I believe it is useful to add the following regarding the sprag clutch replacement. After reviewing the different issues of the 912 IPC, from 2017 up to now, I discovered that Rotax has made several changes in that area over the years. The sprag clutch itself has not been changed but the sprag clutch housing was modified several times. Housing P/N 852405 was introduced in May 2008 with the following comment: "new housing with expanded space for sprag clutch". Then in January 2017, P/N 852406 was introduced with the very same comment. I guess the changes were designed to improve the behaviour of the sprag clutch.
My conclusion: when replacing the sprag clutch, it is critical to replace also the housing and make sure the P/N is the last version.
Remi
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=487913#487913
===========
st Email Forum -
pa-List">http://www.matronics.com/Navigator?Europa-List
===========
p; - MATRONICS WEB FORUMS -
ums.matronics.com
===========
p; - NEW MATRONICS LIST WIKI -
matronics.com
===========
p; - List Contribution Web Site -
p; -Matt Dralle, List Admin.
http://www.matronics.com/contribution
===========
|
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