Mag Drop Question

[DennisRhodes]

Didn t by chance allow one of the springs in the spark plug lead excape while you had the plugs out did you.? Slick parts usually have springs in the spark plug cap to contact the plug top electrictly.

[Bavafa]

Do you have CHT/EGT prob and if so, do you see if one cylinder is different than the others. If you have not touched anything else but the sparkplugs, then I would suspect the sparkplug wires since the plugs have been changed with no effect. The plug wire might not be all broken but enough to effect this.

[mahlon_r]

Trying leaning the engine slightly and rechecking. It could be a bit of an mixture issue. Nothing wrong, just if its a bit rich, one mag will commonly have a slightly larger drop that the other. If you suspect ignition do a mag check at full power. If there is a problem, it will show up more at higher power then when checking at just mag check RPM.
Good Luck,
Mahlon

[kamikaze]

Originally Posted by mahlon_r

Trying leaning the engine slightly and rechecking. It could be a bit of an mixture issue. Nothing wrong, just if its a bit rich, one mag will commonly have a slightly larger drop that the other. If you suspect ignition do a mag check at full power. If there is a problem, it will show up more at higher power then when checking at just mag check RPM.
Good Luck,
Mahlon

Mixture issue? How so? Could you elaborate? Which does being a little rich, affect the two mags differently?

Thx

[mahlon_r]

One mag fires the top plug and one fires the bottom. Excessively Rich or lean mixture in the combustion chamber burns differently when lit from the top or bottom thus causing the discrepancy.
Good Luck,
Mahlon

[kamikaze]

Originally Posted by mahlon_r

One mag fires the top plug and one fires the bottom. Excessively Rich or lean mixture in the combustion chamber burns differently when lit from the top or bottom thus causing the discrepancy.
Good Luck,
Mahlon

Alright, good to know ... but this could happen on specific cylinder(s) only? I ask because in my a/c at least, one mag lights the tops on one side, and the bottoms on the other ...

[Robski]

Originally Posted by kamikaze

... in my a/c at least, one mag lights the tops on one side, and the bottoms on the other ...

R mag: top plugs R side, bottom plugs L side.
L mag: top plugs L side, bottom plugs R side.

Standard Lycoming practice, AFAIK.

A question for Figs: injected or carburetted engine?

My money is on a duff lead or plug cap (spring or insulator), seeing as they've been disturbed. Had a set of leads go duff after maintenance - two leads at the same time - couple of years ago. Leads were less than 350 hours / 6 years old.

[BobTurner]

re-read post #2. check the timing.

[jrs14855]

Originally Posted by BobTurner

re-read post #2. check the timing.

Plus one on this. Why do so many ignore outstanding advice as well as the details of the problem described in the op and the follow up by the op.
The odds are at least 1000 to 1 that a plug lead problem CANNOT produce a SMOOTH mag drop.
The 0 235 L in the Cessna 152 and its near identical counterpart in the Tomahawk are notorious for fouled spark plugs. This always produced a very rough running engine on at least one mag during mag check. A four cylinder engine running on three cylinders cannot possibly be smooth.
All of the IO Lycomings are notorious for fouling plugs due to rich mixture.
this and the 0 235 issue can be largely eliminated by aggressive leaning for all ground operations. Some IO operators go so far as to leave the mixture leaned for descent, approach and landing.

[kamikaze]

How/why would any mag related timing shift by that much, all of a sudden, when left undisturbed?

A fouled plug doesn't fire hence the roughness ... right?

Though it may not necessarily be the root cause here, i surmise a dirty lead, slightly damaged spring, poor contact, etc. can still cause firing, though with less energy, possibly leading to the symptoms described?

Anyways, at least check the sync, the buzz box test takes 5 minutes ...