Excessive RPM drop - Lycoming O-540

[Grandslam]

Hi Guys, I am after some advice or ideas please.

I have a Lycoming O-540 which had a rough running engine. The fault appeared to be with the R/H magneto, which has been replaced and now the rough running has been eliminated, however now...

During magneto checks the L/H now has a RPM drop of 200-300 RPM. It doesn't run roughly with the drop, but it does have a distinctive change in engine note.

Work c/out so far:

1. Engine and magneto's timed (we have a team fairly well versed with timing).
2. Harness checked (with a lead checker).
3. Plugs cleaned and tested on a rig, gaps checked.
4. P-leads swapped (fault still on the L/H ie. apparent on R selection during this test)
5. Magneto's removed and bench run to check for timing drift/failed capacitor. Came back Serviceable.

Has anyone else had similar experiences, or has any other ideas? Believe me our heads are getting worn out with all the scratching now.:confused:

Many thanks in anticipation PPruners

[Malcom]

Try new plugs.

[stevef]

Strange... a smooth drop usually indicates a timing problem. Bendix or Slick magneto?

[SNS3Guppy]

Magnetos need to be timed to each other, to the engine, and require internal timing adjustments, as well. Further, internal issues with a magneto may mean a weaker spark.

[Zulu Alpha]

Have you checked the shower of sparks unit that is sometimes fitted to these engines.

If you remove the wire to the Slick start shower of sparks unit and the problem stops, then you have a faulty unit. I believe it gives a shower of sparks to assist starting but cuts out when the engine is running.

If it goes faulty it appears like fouled plugs or a faulty magneto.

Good luck

ZA

[paul m]

All plugs changed (REM40E), mags removed (bendix) and bench run. All ok. Harness tested and passes. Mags internal timed, engine and mags timed together.
I am working with Granslam on this and loosing hair over this :ugh:

[stevef]

Two suggestions:

1/ Probably not at all relevant but was the the left magneto removed for any reason when the initial rough running problem arose? If so, it might be worth checking that one of the drive cushions hasn't fallen out of the holder when it was replaced.


2/ Run the engine on the left magneto only from cold, idle for a couple of minutes, shut down and feel for any cold cylinders. That should give more indication as to where the fault lies.

I still get a feeling that the problem is timing-related somehow.
Incidentally, what aircraft?

[stevef]

Something else to consider - I'm assuming that the left magneto has the impulse coupling system rather than retard points. Are you sure that the coupling 'snapped' when you were setting up the timing (by moving No1 past TDC, then back)? If it didn't, you won't get correct timing.
Interesting case!

[Grandslam]

Well, the problem seems to be solved. Many thanks for the advice all.

It appears that the L/H magneto was removed (by someone else) during the initial invest, and the timing gear within the accessory housing 'may' have come adrift, and just 'maybe' replaced in the incorrect position!:suspect:

Nevertheless, after ensuring the timing gear was correctly positioned (dots on the teeth) and the drive cushions replaced, the engine fired into life. L/H drop now 75 rpm, R/H 100 rpm. Ground run and air test c/out satis.

Incidentally, the aircraft is a PA-25.

Thanks again.:)