Runup RPM drop

[gevertex]

Hey All,

   Looking for some advice. My 92 M20J MSE has an overhauled engine and dual mag. I have noticed a few times on run up that the right mag drops a lot more than the left. ~70rpm more. It dropped today from 2030 to 1830 which is obviously higher than what is on my checklist. It runs smooth on both left and right. 
 

I do use pretty aggressive ground leaning.  In the few cases I had an excessive rpm drop, I went to full power leaned out and stayed there for a minute or so before trying the runup again. Everything alway checks out during the second runup. Plug fowling is the obvious first possibility, but I thought I had eliminated that with aggressive ground leaning. Recently I have leaned so much that I had to richen slightly to make more than 1200rpm when pulling out of my spot. 
 

Are plugs just fowling or is there something else going on I should be thinking of?

Thanks all

[toto]

Sounds like plug fouling to me. I had ongoing problems with plug fouling for a long time despite very aggressive ground leaning, and I ended up investing in a set of fine wire plugs - I’ve had zero problems since then. I was skeptical of the fine wires, but they turned a frequent occurrence into a non-event. 

[ArtVandelay]

What does the engine monitor show you?

[EricJ]

I put fine wires in the bottom plug spots and haven't had that trouble since.

[A64Pilot]

I have the same at times, it’s plugs. Anything else wouldn’t recover.

How long since you cleaned them?

[gevertex]

3 hours ago, ArtVandelay said:

What does the engine monitor show you?

Good point. I'll pull the cards and look at the data next time I go out to the airplane.

[gevertex]

1 hour ago, A64Pilot said:

I have the same at times, it’s plugs. Anything else wouldn’t recover.

How long since you cleaned them?

According to the logs, they were cleaned and gapped at annual about 20 hours ago.

[EricJ]

13 hours ago, gevertex said:

According to the logs, they were cleaned and gapped at annual about 20 hours ago.

It only takes one long or too rich idle to foul the bottom plugs.

[kortopates]

Don't guess, download your engine monitor data, yet an inflight lean Mag check provides much better data than an ground mag check, the ground check will show any fouled plugs; and specifically which plug(s).

[PT20J]

Spark plugs can be fouled by lead or oil. Lead fouling became prevalent when 80 octane was discontinued and engines originally designed for it were forced to use 100LL which has a higher TEL content. One fix was to lean on the ground which increases combustion temperature which better activates the lead scavenger in the fuel. However, ground leaning may not produce enough heat to prevent oil fouling. The oil is usually getting past the rings which don’t seal as well at the lower cylinder pressures at near idle power. This is why an engine with low oil consumption can still foul plugs during prolonged ground operation.

Some find that fine wire plugs help because the electrodes are smaller and more exposed to combustion temperatures. A cheaper alternative is BY plugs which have extended electrodes. I believe these plugs were developed for the O-235 engine in C-152s which was especially prone to fouling spark plugs.

Skip

[Adverseyaw0317]

I have been having the same exact issue but with the left mag. Put some fine wire plugs on order... hopefully that fixes the issue.

[A64Pilot]

I just ignore it, by that I mean I’ll go to higher power very lean for a short time and that clears it.

I do usually taxi VERY lean as in I can’t get past  1500 without it cutting out, that helps but isn’t a cure.

I may go to fine wires when the massives wear out, I’ve bought 24 fine wires on two different airplanes, but honestly mine starts so well and runs smoothly way leaner than needed I don’t think I have anything that needs fixing.

On my other airplanes, I’ve not owned the Mooney long enough to say, but when they got to where the occasional mag check failure started happening more frequently the plugs needed cleaning. Running one at high RPM and lean only cleans the very tip of the electrodes off, it doesn’t do much cleaning at all.

Edited July 30, 2023 by A64Pilot

[M20F]

On 7/29/2023 at 4:45 PM, gevertex said:

Hey All,

   Looking for some advice. My 92 M20J MSE has an overhauled engine and dual mag. I have noticed a few times on run up that the right mag drops a lot more than the left. ~70rpm more. It dropped today from 2030 to 1830 which is obviously higher than what is on my checklist. It runs smooth on both left and right. 
 

I do use pretty aggressive ground leaning.  In the few cases I had an excessive rpm drop, I went to full power leaned out and stayed there for a minute or so before trying the runup again. Everything alway checks out during the second runup. Plug fowling is the obvious first possibility, but I thought I had eliminated that with aggressive ground leaning. Recently I have leaned so much that I had to richen slightly to make more than 1200rpm when pulling out of my spot. 
 

Are plugs just fowling or is there something else going on I should be thinking of?

Thanks all

So your right mag drops 30 and your left drops 100?

[Ragsf15e]

As long as it recovers, it’s the plugs, but if it’s slowly getting a greater rpm drop/spread and not coming back up again, the timing could be slipping as the mag wears.    Just throwing that out as an alternative if it’s not the plug (which is pretty easy to id with the engine monitor.

[PeteMc]

Gad, DO NOT do anything yet!  Excessive leaning is a great thing to do on the ground to stop plugs from fouling.  But checking the Mags while you're running lean will give you EXACTLY what you saw. 

Next time run up the engine close to your normal RPMs and go Full Rich, set your RPM and do the Mag Check.  If it is still off, check with your mechanic.  But odds are when you're rich you'll get a normal Mag check.  If you do get a normal Mag check, then start to get a feel for what your engine does when you have leaned for ground ops.  Then you'll know what's normal for your engine. 

 

[gevertex]

9 hours ago, M20F said:

So your right mag drops 30 and your left drops 100?

Left mag dropped  130rpm

Right mag dropped 200rpm

limits are 175 drop and 50 difference between both. 

[gevertex]

6 hours ago, PeteMc said:

Gad, DO NOT do anything yet!  Excessive leaning is a great thing to do on the ground to stop plugs from fouling.  But checking the Mags while you're running lean will give you EXACTLY what you saw. 

Next time run up the engine close to your normal RPMs and go Full Rich, set your RPM and do the Mag Check.  If it is still off, check with your mechanic.  But odds are when you're rich you'll get a normal Mag check.  If you do get a normal Mag check, then start to get a feel for what your engine does when you have leaned for ground ops.  Then you'll know what's normal for your engine. 

 

Yes, I always run full rich for mag checks. I only leaned for ground ops and to clear a suspected fowled plug. Subsequent runup was at full rich and checked ok. 
 

When you say checking mags while lean will give me exactly what I saw, do you mean the excessive drop or fixing the excessive drop?

[ArtVandelay]

Hmm, I think it’s a better stress test to do the mag test with the mixture leaned but not severely so. I have to enrich the mixture to get rpms up, but not fully. Full mixture only on low altitude takeoffs. I have an engine monitor, I ignore rpm dropping, and focus on cylinder EGTs to make sure all cylinders are firing.

[Ragsf15e]

4 hours ago, ArtVandelay said:

Hmm, I think it’s a better stress test to do the mag test with the mixture leaned but not severely so. I have to enrich the mixture to get rpms up, but not fully. Full mixture only on low altitude takeoffs. I have an engine monitor, I ignore rpm dropping, and focus on cylinder EGTs to make sure all cylinders are firing.

I’m mostly with you on just looking for all the cylinders firing, but it’s probably worth looking at the drops too.  You can start to see the timing change over lots of flights.   Last annual i had my surefly dropping about 50rpm and its very stable over time.  The mag though was only dropping ~20.  It was actually hard to see a drop and I was worried that my switch was bad or something.  Weird.  I told my mechanic I thought it had advanced somehow.  He checked the timing and it was somewhere around 3-5 degrees advanced.  It didn’t take much to be noticed.  Don’t know how/why, maybe just as it wears?

I do lean just like you said, very lean on the ground, about halfway to rich for the mag check.

[PeteMc]

7 hours ago, gevertex said:

When you say checking mags while lean will give me exactly what I saw, do you mean the excessive drop or fixing the excessive drop?

I meant excessive drop.   That's why I thought you were doing the Mag check while still leaned.