IO-540 excessive RPM drop

I currently have 21 hours in my RV-10, brand new engine and prop, and I've been following the break-in advice by running it at 75% power as much as possible at near full rich. Lately when doing my magneto checks at 1800RPM I'm noticing that the RPM drop has ratcheted up to 180RPM per magneto (it started out at around 130RPM).

I had a similar problem on one magneto when the engine was brand-brand new and I was able to track it down to a timing issue. I haven't re-checked the timing yet, but the RPM drop is near identical for both magnetos.

Also, when doing the magneto check I can feel a slight shudder when I switch off the right one, which smooths out again right away. I feel no such shudder with the other one.

I haven't seen a major temperature drop in one of the cylinders to indicate a misfire, but I also don't generally run it long enough on one magneto to let the temps stablize. Once both magnetos are on the engine runs smoothly.

I'm a little concerned I'm fouling my plugs. I had an instructor once show me a little trick where he would lean out the mixture while running on one magneto to elevate the temperature enough to burn off any plug fouling. I tried that, and I saw the CHT on one cylinder rocket up 50 degrees in a matter of a few seconds. I immediately shut that little exercise down.

So - is it plug fouling or something else? Is this sort of stuff normal?

Agree - you fouled the plugs.

The biggest mistake I?ve seen is not aggressively leaning when on the ground (like the engine almost stops) even with a new engine.

Clean the plugs and fly on.

Carl

Double check the idle mixture adjustment per the fuel controller manufacturers instructions.

If the idle mixture is excessively rich, it can foul plugs quickly.

Try leaning to max rpm when doing your runup, if it's overly rich the mag drop will be excessive. And yes check the idle mixture adjustment.

Check your timing. New gaskets often compress when they first get hot on new engines. If you didn't tighten the mag hold down bolts after the first couple of runs the mags could have rotated a bit. Increased timing advance translates into larger RPM drops

While there is a benefit to running higher power levels during break in, there is no benefit to running filthy rich after the first couple of hours of break in. I understand if you don't want to go LOP, but no point going richer than max power (80-100 ROP)

Larry

Walt said:

Try leaning to max rpm when doing your runup, if it's overly rich the mag drop will be excessive. And yes check the idle mixture adjustment.

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+1 - I noticed this last week in the higher temps. Leaned for sunup, and right back to normal.

lr172 said:

While there is a benefit to running higher power levels during break in, there is no benefit to running filthy rich after the first couple of hours of break in. I understand if you don't want to go LOP, but no point going richer than max power (80-100 ROP)

Larry

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Unless, of course, you need to run rich to control CHTs.

BobTurner said:

Unless, of course, you need to run rich to control CHTs.

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Of course. I should have mentioned that.