Magneto malfunction??

I recently developed an ignition discrepancy issue and can't seem to conquer.

After performing initial engine start on the O-360 A3A, Itypically wait until engine is no less than 100 degrees before making a runupfor magnetos and other systems checks.
This particular morning, while performing mag checks at 1800rpm and making sure all systems were in good working order, the right mag hadit’s usual 30 to 50 rpm drop. Returningselector switch back to both mag allowing rpm to stabilize and subsequentlyswitching to left mag, I experienced a continued backfire. Switch back to both and was experiencing arough running engine. Cylinderscompression check all at 78 and 79 while timing set at 25 º BTDC and both mags sync within 1/2 ºor less.

My first thought was that this mag just went south on me andsince I had a recently 500 hr inspection done on another set of mags, decidedto replace it. After engine start, leftmag continued to run rough (no backfire) and new set of spark plugs installed. Another start and engine continued to runrough. Check the p-leads and all seemed to have the proper resistance,continuity and no short. I purchased a new set ofspark plug leads and engine continued to run rough. Replaced the ignition switch with no changeto the discrepancy.

Because the engine rough running is isolated to left mag, I’veruled out induction system issue. Idecided to remove the newly installed left mag and reinstalled originalmag. Engine start once again resulted incontinued backfire.

Could the issues of left mag backfire be a coincidence and newmag rough running be related to bad condenser and or bad e-gap set? Any suggestions?

In CowTown USA,
DonDDay

Could be bad impulse if you are reusing the old one.

Plug wires crossed on left mag ?????. Check and recheck.

Backfiring? Thru the exhaust? or the intake?
Could be a valve sticking at operating temps & rpm's but not at cranking or testing speeds.
Try some AV Blend and/or Marvel Mystery Oil & see if that helps. Borrow a laser temp gauge & check exhaust temps.
That will tell which cylinder it's on. Lycoming has a "fix" that doesn't require pulling the jug.

John

Thank y’all for the suggestions.

1. Engine was running just fine for well over 350 hoursand problem began at runup prior to flight which did not, nor will it happen untilissue is fixed.
   
2. Impulse coupling checked after removal andreplacement of mag and certainly audible when propeller turned through allcylinders (on two separate mags) during timing and mag sync. One mag backfires, the other one, rough (nobackfire).
   
3. Compression check revealed pressure in the 78-79range with no indication of sticky valve. MMO in fuel religiously used.
   
4. Backfire appeared to be exhaust and not whenright mag selected.
   
5. Exhaust temperatures with laser all in the 320º to 390º range.
   

If the impulse spring breaks it can run rough. It still clicks, but wrong timing.

Pull the valve cover on that low temp cylinder & check the valve lift.
Borrow a BoreScope & take a look inside.
As a last resort, you may have to pull the jug & look for signs of wear on the cam/lifter.

I would try simple before pulling valve cover or certainly a cylinder.
Can you isolate one cyliinder? Run engine from cold for about a minute on bad magneto. Then carefully feel for the cold exhaust pipe. If you find one concentrate on those components.
Id also number plugs for each cylinder as you pull all plugs. Examine each one for a minute crack, hard to see some and for any speck or carbon fouling. Id also test each plug on a Champion plug tester to make sure plugs fire under pressure. Even new plugs can be bad and someone may have dropped on along the way. You can also swap all the plugs from odd bank to even bank to see if problem changes and follows the plugs.
Problems like valve lift or cams lobes are less likely because they are not going to be less or more when running on one mag or the other, and they dont occur overnight.
A general check of ignition systems is if the engine starts right away when cranked. The other end is when running at max rpm if it doesnt misfire, which is harder.

Again, Thanks for the responses. All suggestions sound good, but must restate. If all your suggestions for misfire, missing, backfire are caused by worn cam lobe, improper valve clearance and other issues related to individual cylinders, would the symptoms not be noticed when selecting the right mag as well. Remember, right mag operation is normal.

When you replaced the switch did you get the p lead connected to the wrong mag?
You may have replaced the wrong mag?

P-Leads in correct mags.