Slick Mag Drop

flickroll · #1 05-03-2009, 06:45 AM

What is an acceptable mag drop for Slicks on a Lyc O-360? I'm seeing 100 RPM, +/- 10-20. Seems excessive - used to get about 75 or so on Slicks on a Cont. IO-520. If excessive, what is a likely culprit? The engine has 300 hours TTSN. Thanks

Sheldon · #2 05-03-2009, 07:10 AM

I see 45-55 rpm drop with my newley overhauled Slicks...

BlndRvtr · #3 05-03-2009, 07:13 AM

Jim,

Lycs seem to always have a bigger mag drop than Contis, regardless of mag maker. A smooth 150 RPM drop (max) with no more than 75 RPM diff. is the common acceptable standards for Lycs. Your 100 drop sounds fine, as long as it's relatively smooth. Your little red Lyc. Operators Manual should verify that.

Geico266 · #4 05-03-2009, 07:13 AM

How many hours are on your Slicks? Maybe time for an overhaul.

flickroll · #5 05-03-2009, 07:54 AM

Thanks All. The engine runs smoothly on one mag. The engine and mags have 300 TTSN. Probably OK, just a little larger drop than I am used to seeing.

az_gila · #6 05-03-2009, 10:18 AM

Quote:

Originally Posted by BlndRvtr

Jim,

Lycs seem to always have a bigger mag drop than Contis, regardless of mag maker. A smooth 150 RPM drop (max) with no more than 75 RPM diff. is the common acceptable standards for Lycs. Your 100 drop sounds fine, as long as it's relatively smooth. Your little red Lyc. Operators Manual should verify that.

...Lycoming O-360 Operators manual.

It says a 200 rpm max. drop with a max. 50 rpm difference - starting at 1800 rpm (2000 max)

Just to confuse things, my Tiger certified O-360 aircraft manual calls for 175 max drop and a max. difference of 50 rpm.

Mechanics have told me that the difference is the important number, as both magnetos really should be operating/wearing evenly.

I would stick with a 50 rpm max. difference, rather than the 75 rpm you quoted.

DickDe · #7 05-03-2009, 11:11 AM

Lycoming says mag drop is supposed to be 50 and 75 RPM. In 300 hours I would expect the timing has drifted off slightly, the plugs have eroded slightly with some lead build up. All this will cause a larger mag. drop but this is all normal. I would pull the plugs, check for lead, clean & regap. Then check my mag timing with a buzz box and retime if needed. I have 300 hours on my 0-320 and I do this every year during condition inspection as routine maintenance. It appears to me that mag drops get a bit greater as I put time on the engine between annuals

Oh, I also rotate the bottom plugs to the top as the lower ones usually get leaded up faster than the upper ones. I use a cheap automotive spark plug sand blaster to clean them.

Dick DeCramer
RV6 N500DD flying
RV8 fuselage
Northfield, MN

az_gila · #8 05-03-2009, 11:55 AM

..numbers come from?

My Lycoming O-360 Operators Manual gives the numbers above.

Quote:

Originally Posted by DickDe

Lycoming says mag drop is supposed to be 50 and 75 RPM. ......
Dick DeCramer

DickDe · #9 05-03-2009, 04:40 PM

I stand corrected and Gil is correct...the 0-320 Lycoming Operators Manual says " Drop off should not exceed 175 RPM and should not exceed 50 RPM between Magnetos". The 0-360 may differ a bit and no reference is made towards any difference between Slick or Bendix Mags.

Dick DeCramer
RV6 N500DD
Northfield, MN

flickroll · #10 05-03-2009, 05:35 PM

Quote:

Originally Posted by DickDe

In 300 hours I would expect the timing has drifted off slightly, the plugs have eroded slightly with some lead build up. I would pull the plugs, check for lead, clean & regap.

Oh, I also rotate the bottom plugs to the top as the lower ones usually get leaded up faster than the upper ones. I use a cheap automotive spark plug sand blaster to clean them.

Dick - this was all done two weeks ago. Timing was reset, plugs cleaned and gapped.