Mag drops with new engine

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Mag drops with new engine

Postby mmcmillan2 » Thu Nov 24, 2022 1:05 pm

My mechanic installed a fresh Airworx rebuilt O300A on my 170B. Initial run up had a 10-20 rpm drop on right mag and 110-120 rpm drop left mag. I’m also noticing a hair more vibration than with old engine.

Mechanic said run it 5 hrs then see if it needs to be looked at. Possibly oil fouling on lower plug(s) from unseated ring(s). Fly it and seat the rings. Airworx had told me they run on dyno so it will be pretty much broken in at arrival.

I have about 5 hrs on it now. Still getting 100-120 mag drop on one mag. Should I be concerned? I did look at the spark plug wires to confirm they are going to correct cylinders. Power seems normal.
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Re: Mag drops with new engine

Postby voorheesh » Thu Nov 24, 2022 7:12 pm

Your difference in drop between the 2 mags seems to be a bit more than the limitations I have always followed (approx 75/100 rpm drop from either mag and 50/75 difference). Maybe someone else knows actual mag drop limits for Continental O300. If the mag drop is immediate/rough, that usually means fouled plugs. If a lean ground run does not correct that, then removal and cleaning should follow. In my experience, cleaning fouled plugs should result in an acceptable mag drop. Otherwise, it might be mag timing or a short somewhere in your ignition system. To answer your question, I would be concerned about it and and get it corrected/resolved, including getting help from the company who overhauled the engine. It is not unreasonable for you to insist on acceptable performance in a newly overhauled engine or any serviceable engine, for that matter. (Just my opinion, others may have more knowledge on mag drops)
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Re: Mag drops with new engine

Postby GAHorn » Thu Nov 24, 2022 8:36 pm

Firstly, let me remind that the left and right magnetos are timed differently…so it is not uncommon for the magnetos to have different “drops”.

Secondly, remember that the Owners’ Manuals for these airplanes are Not Approved by anyone other than the Marketing/Publications Dept at Cessna back in the early 1950s.

I have found that the same engine in different year-models of this airplane…have Owner’s Manuals which are not consistent..they give different “run-up” advice in the manuals from year-model to year-model. The ‘54 OM tells us to use 1600 RPM and 100 rpm “drops” are acceptable…but the ‘50 manual uses 1600 RPM and gives NO leeway at all..specifying 50 RPM drop on the Right Mag and 75 RPM on the Left Mag…. (which if followed probably means half the fleet should not fly if anything else is observed.) :?

But the engine mfr’r tells us to use 1700 RPM…and provides completely different advice. 8O


The FAA APPROVED OPERATORS MANUAL for this engine is given in this screen-shot:

2AD4B18E-DACB-44B1-99E6-989E1D631C96.png
Click to ENLARGE


So a mag drop on either mag can be up to 150 RPM…but the DIFFERENCE between the mags should be 75 RPM or Less.

I suspect your mags may be improperly timed. The Left mag should be 28-degrees BTDC and connect to the Lower plugs, and the Right mag should be 26-degrees BTDC and connect to the Upper plugs. (You’d be surprised how many are improperly wired.)
I’d suggest you first check that magneto timing.

BTW, when you received your engine from Airworx were the magnetos already overhauled/installed? (I hope so and that older mags were not installed when it was put on your airframe… I beleive in overhauling or rebuilding all accessories when the engine is overhauled.)

Also, be aware that the high-tension leads (the spark plug wires) can be deteriorated and should be replaced at engine/magneto overhaul….or at least tested and approved for return to service. I high-tension Tester is a box similar to a magneto Timing box…. but it puts a high-voltage on the center conductor and checks for voltage leaks and/or breakdown. (I.E., simply checking they go to the correct spark plug isn’t a sufficient test.)

Hope this helps…and Hope you’ll let us know what you find and what the fix was. :wink:
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Re: Mag drops with new engine

Postby mmcmillan2 » Thu Nov 24, 2022 9:03 pm

Yes, mags were overhauled and the leads are all new. Its a certified rebuild.

I’m going with to get the installer to look at it again. I’m thinking it ran properly on the dyno but maybe something was mixed up when mounting the engine.

My previous engine consistently had a 40 rpm drop on both mags.
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Re: Mag drops with new engine

Postby mmcmillan2 » Thu Nov 24, 2022 9:07 pm

Could a plug be so fouled that a 1 hour flight wouldn’t “burn it off”? Has to be removed and cleaned?

I’ve been leaning mixture to rich of peak on ground and air.
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Re: Mag drops with new engine

Postby voorheesh » Thu Nov 24, 2022 9:52 pm

Excessive lead fouling, in my experience, is best fixed by removing the plugs and cleaning out chunks of lead followed by proper sand blasting. But lead deposits usually take time to accumulate and your engine is low time. Oil fouling typically burns off with normal engine operations unless oil continues to collect in a cylinder for some reason. Not too likely in your case. Plug fouling is almost always accompanied by roughness and abrupt RPM drop. So, like George says, your issue may be timing. It also may turn out that it’s not a problem (engine makes acceptable power and runs smooth within mag drop limits). But you should find out. I can sympathize with frustration of being told to “fly it for 5 hours and see if it’s still a problem”. After investing in an overhaul, you deserve a solid answer on this IMO.
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Re: Mag drops with new engine

Postby ghostflyer » Thu Nov 24, 2022 10:55 pm

Make sure all the “new” plugs are all of the same heat range . I would be checking all the bottom plugs for a “wet”cylinder as some one could have dropped a plug before fittment .
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Re: Mag drops with new engine

Postby GAHorn » Thu Nov 24, 2022 11:04 pm

mmcmillan2 wrote:Could a plug be so fouled that a 1 hour flight wouldn’t “burn it off”? Has to be removed and cleaned?

I’ve been leaning mixture to rich of peak on ground and air.


Highly unlikely…. Unless an early failure of some sort has occurred. For example, a new plug may have failed. A new high-tension lead may have failed. (both also highly unlikely ….but possible.)

Also a new engine…may have a broken ring… :cry: …. and letting oil up to foul a plug.

Check your mag timing first. THEN you can begin to question other things.
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Re: Mag drops with new engine

Postby mmcmillan2 » Fri Nov 25, 2022 2:22 am

Thanks, I’ll keep you posted.
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Re: Mag drops with new engine

Postby mmcmillan2 » Wed Nov 30, 2022 2:34 am

One of the lower spark plugs was fouled. Cleaned them then reinstalled. Still getting 120+ on left mag drop. Reset timing and now getting 75 drop on that mag. Airworx told me that it is common to need to reset timing during breakin process.
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Re: Mag drops with new engine

Postby GAHorn » Wed Nov 30, 2022 4:11 pm

mmcmillan2 wrote:One of the lower spark plugs was fouled. Cleaned them then reinstalled. Still getting 120+ on left mag drop. Reset timing and now getting 75 drop on that mag. Airworx told me that it is common to need to reset timing during breakin process.


Mag timing can be affected by mag-gear-to-crank-gear “wear-in”…. but it’s not “common”…those gears are hardened. If they substituted or changed (swapped) gears perhaps there is a “wear in” going on…. but I’m only speculating.
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Re: Mag drops with new engine

Postby Joe Moilanen » Wed Nov 30, 2022 5:46 pm

GAHorn wrote:
mmcmillan2 wrote:One of the lower spark plugs was fouled. Cleaned them then reinstalled. Still getting 120+ on left mag drop. Reset timing and now getting 75 drop on that mag. Airworx told me that it is common to need to reset timing during breakin process.


Mag timing can be affected by mag-gear-to-crank-gear “wear-in”…. but it’s not “common”…those gears are hardened. If they substituted or changed (swapped) gears perhaps there is a “wear in” going on…. but I’m only speculating.

My guess is that gear wear would have to be extreme to produce a timing change enough to warrant a differential mag drop as much as the one in question. A timing change from "wear" in a magneto would more than likely come from the follower on the points wearing down where it contacts the cam lobe on the shaft. With the point gap becoming narrower the timing would advance. Here is something to consider for anyone having new Bendix magnetos or recently overhauled Bendix magnetos...Last year while flying with only 300 hours SMOH (newly rebuilt Bendix mags also) I experienced extremely rough running. It was sporadic but enough for me to have my head on a swivel looking for a place to set down. I isolated it to the right mag with several mag checks. Figuring that it must be a plug problem since the mags were fairly new, I cleaned all the plugs (new since SMOH also) and tested them on a tester, all good. I decided to take the mags back to my re-builder for a pre-mature 500 hr. inspection. They couldn't find anything in the right mag that could have caused the problem. HOWEVER, he chose to just dress and adjust the points in my mag instead of replacing them because they were having LOTS of problems with a rather large batch of new Bendix points wearing out extremely fast. back up in the air...and same extreme sporadic roughness on the right mag! As I was doing the mag check while circling the airport I noticed that it started running rough once just as I barely touched the mag switch. AH-ha! By wiggling the mag switch in either "both" or "right", the roughness would occur. i removed the switch and took it apart. Sure enough, wear and carbon arching. $600.00 and some odd dollars later all is fine. Although my problem wasn't a mag the bad batch of points is something for consideration.

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Re: Mag drops with new engine

Postby mmcmillan2 » Thu Dec 01, 2022 2:55 am

Mine do have the Bendex mags. Type S6LN-21. Overhauled 8/22.
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Re: Mag drops with new engine

Postby Joe Moilanen » Thu Dec 01, 2022 6:24 pm

mmcmillan2 wrote:Mine do have the Bendex mags. Type S6LN-21. Overhauled 8/22.

I've never had my timing drift that much from new or rebuilt mags breaking in before. I don't think I've ever experienced more than 1 or 2 degrees drift for any reason, YMMV. I think I'd monitor the points gap if the trend continues?
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Re: Mag drops with new engine

Postby n2582d » Thu Dec 01, 2022 8:30 pm

Might want to check the magneto capacitor.
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