Excessive mag drop on Rotax 912ULS

[rodgerc]

This info has been posted elsewhere and is shared here at JG3’s prompting. 
 

For the past 6-1/2 weeks my 37 hr total time 912ULS powered RANS Raven has forlornly sat in the hangar due to a persistent 350-400 RPM drop at 4000 RPM. 
 

I had become so frustrated that selling her off cheap was becoming a serious consideration….so was burning it. 😡

Last Friday  I finally discovered and rectified the fault, tipped off by a Vans RV12is engine installation instruction. 

 

Cutting to the chase, one of the 4 ignition trigger coils’ air gap was 0.002” (0.05mm) below the minimum spec set by Rotax. Yep, about half the thickness of a human hair is the difference between a barely detectable mag drop and one that made the engine rattle and shake….and it was clearly set that way in the Rotax factory.

 

The specified air gap range is 0.012” to 0.016”. My gaps were 1 x 0.016”, 2 x 0.012” and 1 x 0.010”….The problem trigger coil was the 0.010” gap.

(I had previously thought that closer trigger coil gaps were always better, provided there was no actual contact. Those with electronic expertise may be able to explain why that is not necessarily the case….?)
 

I now have 4 x0.014” gaps (actually 5 counting the tacho) and a very smooth running engine. 

 

So if you’re suffering a mag drop in the 300-400 RPM range on a Rotax, buy yourself a set of brass feeler gauges and check the trigger coil air gaps.

[IBob]

Thank you RodgerC, that is unexpected and good to know.

I have seen too wide a gap cause poor starting on an older 912. And as you say, non ferrous feeler gauges are a must: the steel ones stick to the magnets and feel like a snug fit when they are not at all.

 

I also found that my 912ULS idle speed adjustment and idle mixture screw settings were a bit out and not identical from the factory. There was no obvious impact on idle, and the settings are easily found under Basic Throttle Adjustment in the Line Maintenance Manual.

[skippydiesel]

2 hours ago, rodgerc said:

So if you’re suffering a mag drop in the 300-400 RPM range on a Rotax, buy yourself a set of brass feeler gauges and check the trigger coil air gaps.

 

Have been looking for a Brass (non magnetic) Feeler Gauge Set that goes down to 0.3048mm  (0.012") - 0.4064mm (0.016") almost as rare as "hens teeth" in Australia - can you recommend a supplier???

[IBob]

Skippy, some steel gauge sets seem to come with a selection of brass ones as well. So maybe they're hiding in plain sight?
If all else fails there is a selection on aliexpress: I don't know how it runs there but I recently got a set of (non aircaft) bearings and they arrived in under 2 weeks.

[pluessy]

some on ebay:

this one seems to be 006 to 016 in 002 increments: https://www.ebay.com.au/itm/264092846959?epid=2295842872&hash=item3d7d290b6f:g:X6AAAOSwubRfoxxz&amdata=enc%3AAQAHAAAAwJv5UrkQn98de5bchTVWZNfEwmMXCu0KOylz6vCToVdj6Y6krrUi4x5AbGC1ookaLWl6crNapl%2BT4OsAV9XPkk3n6tLHtWnjsdTZ15FUaXAkjYo49HOsXVbDKAAxyY0VXJRrwlKpJ9Nioy3%2BSHHbXY58m1hXIcav4XbLVS5EhGwwhxQg5zt8Msc1frL4vL6tsWtmdJRPltYf7NpYNoJdUq59HC3PGSZQVjUVkJH%2BfQjVfBL%2BwR%2FiTOyteMVJ8XyJlw%3D%3D|tkp%3ABk9SR4y8qI_oYQ

this is a full set, 0.05-1.00mm in 0.05mm steps: https://www.ebay.com.au/itm/155393994522?hash=item242e342f1a:g:QaIAAOSwMthj32NR&amdata=enc%3AAQAHAAAAwHqtnRRWgt4WvHiNEfCed%2BjRc7At7LfGwuwGzceT3DLOOx66Erlzqw6n3yOpieenIfrqHEE%2Bb8LY%2BC6mmb3o2lqJ2GG0jozTHeqB6UYZl1pfppv3V1dLW9tdg4s5ymwA9Qhk%2BBTJyPBoxRJAHO0BIcK54svlyARf88c5mtwBAxi8SIAbG%2BTpAs0Kna8k7pvk3k%2BEI6WPkxejzoKD7HBYkLJ3xlDQuMZUb4IkfTwgWdsrCVaBUexJhHuiPhYWSt%2B3Ig%3D%3D|tkp%3ABk9SR86QrY_oYQ

 

[facthunter]

Type that without error and  throw a party.   Nev

[skippydiesel]

Thanks Pluessy  - I haven't gone through all your web sites, however after an afternoon search (between other activities) I only found one Australian (based) supplier, of brass feeler gauges, that will measure the required range of 0.3048mm  (0.012") - 0.4064mm (0.016")

 

 https://www.henchman.com.au/J000BR/Proto-J000BR-Feeler-Gauge-Set-6-Blades-Non_dash_Magnetic/pd.php   (Please Request Lead Time)

 

https://www.googleadservices.com/pagead/aclk?sa=L&ai=DChcSEwiPu-HP34r-AhWDWw8CHXsSAj8YABACGgJ0Yg&ohost=www.google.com&cid=CAESauD2QSgEOFSR50iyTcyKHP1cMAD0T_Q9M-eKr6F7pXBrho61AYahUgvmozaNWI6TC2gZ61mriCz55IAzWZ2EzcikHClmx7IHJ7Dvl9Pv6iwliyrY9ZZeb_fYaG46tk8FU0MdoFCkcvJ-U0o&sig=AOD64_2YZMyYutQ7lGjSCMN2R7oldsqkjg&q&adurl&ved=2ahUKEwjXkdvP34r-AhUut1YBHRVtBgEQ0Qx6BAgCEAEhttps://www.googleadservices.com/pagead/aclk?sa=L&ai=DChcSEwiPu-HP34r-AhWDWw8CHXsSAj8YABACGgJ0Yg&ohost=www.google.com&cid=CAESauD2QSgEOFSR50iyTcyKHP1cMAD0T_Q9M-eKr6F7pXBrho61AYahUgvmozaNWI6TC2gZ61mriCz55IAzWZ2EzcikHClmx7IHJ7Dvl9Pv6iwliyrY9ZZeb_fYaG46tk8FU0MdoFCkcvJ-U0o&sig=AOD64_2YZMyYutQ7lGjSCMN2R7oldsqkjg&q&adurl&ved=2ahUKEwjXkdvP34r-AhUut1YBHRVtBgEQ0Qx6BAgCEAEhttps://www.googleadservices.com/pagead/aclk?sa=L&ai=DChcSEwiPu-HP34r-AhWDWw8CHXsSAj8YABACGgJ0Yg&ohost=www.google.com&cid=CAESauD2QSgEOFSR50iyTcyKHP1cMAD0T_Q9M-eKr6F7pXBrho61AYahUgvmozaNWI6TC2gZ61mriCz55IAzWZ2EzcikHClmx7IHJ7Dvl9Pv6iwliyrY9ZZeb_fYaG46tk8FU0MdoFCkcvJ-U0o&sig=AOD64_2YZMyYutQ7lGjSCMN2R7oldsqkjg&q&adurl&ved=2ahUKEwjXkdvP34r-AhUut1YBHRVtBgEQ0Qx6BAgCEAEhttps://www.googleadservices.com/pagead/aclk?sa=L&ai=DChcSEwiPu-HP34r-AhWDWw8CHXsSAj8YABACGgJ0Yg&ohost=www.google.com&cid=CAESauD2QSgEOFSR50iyTcyKHP1cMAD0T_Q9M-eKr6F7pXBrho61AYahUgvmozaNWI6TC2gZ61mriCz55IAzWZ2EzcikHClmx7IHJ7Dvl9Pv6iwliyrY9ZZeb_fYaG46tk8FU0MdoFCkcvJ-U0o&sig=AOD64_2YZMyYutQ7lGjSCMN2R7oldsqkjg&q&adurl&ved=2ahUKEwjXkdvP34r-AhUut1YBHRVtBgEQ0Qx6BAgCEAE

[facthunter]

Proto was USA made. You're allowed to use any combination of gauges to get other sizes. Just keep them clean and they tear easily.   Nev

[rodgerc]

3 hours ago, skippydiesel said:

 

Have been looking for a Brass (non magnetic) Feeler Gauge Set that goes down to 0.3048mm  (0.012") - 0.4064mm (0.016") almost as rare as "hens teeth" in Australia - can you recommend a supplier???

Supercheap have a $29.95 set with a selection of brass sizes included

[skippydiesel]

2 hours ago, rodgerc said:

Supercheap have a $29.95 set with a selection of brass sizes included

Ah! but what sizes?

[onetrack]

It appears the Supercheap ("Toolpro") feeler gauge set has brass feeler gauges in the sizes .006, .008, .010, .012, .014 and .016".

 

https://www.supercheapauto.com.au/p/toolpro-toolpro-feeler-gauge-combination/12584.html

[skippydiesel]

On 02/04/2023 at 11:38 PM, onetrack said:

It appears the Supercheap ("Toolpro") feeler gauge set has brass feeler gauges in the sizes .006, .008, .010, .012, .014 and .016".

 

https://www.supercheapauto.com.au/p/toolpro-toolpro-feeler-gauge-combination/12584.html

Thanks Onettrack - It appears I have been a bit hasty with the SCA offering.

[rodgerc]

Jump forward 7 months and 40+hours of flight time and the 350-400 RPM mag drop issue reappeared a few weeks ago. 
 

Perhaps it was not the trigger coil gaps but rather the possibility that back in April, the weather had cooled sufficiently for a marginal ignition module to stop misbehaving. 
 

For the past couple of hot and sweaty weeks in SE Qld my 350-400 RPM mag drop has occurred, but only after ~10 minutes of taxiing. 
 

Long story short, a set of Mark Kyle’s aftermarket ignition modules has “fixed” the issue.
 

It’s only a hypothesis, but perhaps when the “faulty” module reached a certain operating temperature, it would revert to the Rotax retarded “soft start” timing and thus, when selecting that module only for a mag check, the engine dropped 350-400 RPM in protest to retarded timing. 
 

Did somebody just mutter, “correlation does not imply causation”?

[johnm]

Did somebody just mutter, “correlation does not imply causation”?

 

............................ if you dropped a rock on your foot - the above statement holds logic