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Spark Plug question (but hear me out)

H

humptybump

Well Known Member
I've read most of the threads, blog posts, and Savvy articles on aviation spark plugs so my experience confuses me and I could benefit from some experts.

History:

My carbureted O320 runs great but is a bear to start - we're talking that battery draining, often flooding kind of bear.

It actually seems to have gotten worse over the past 3 years and the last 250 hours.

In thinking it all through, I didn't have much trouble when I bought the plane and then I remembered we replaced plugs about three years ago. We pulled the "well used" dual-finewire plugs and install Unison massive electrode plugs.

The confusion:

Today, I pulled the bottom plugs. I measure their resistance. They were all In the 1.2k-1.4K OHM range. I pulled out the mothballed dual-finewire plugs and measured them. They were all in the 4K-5.5K OHM range.

I installed the old dual-finewire plugs into the bottom of the engine. The test run was eye opening.

The engine started almost immediately. Once warmed, the MAG test showed a 60RPM drop on the dual-finewires and 100rpm on the nearly new massive electrodes.

I thought the resistence on the dual-finewires is too high (or right up there) and yet the Bedix MAGs seem to much prefer them.

Is it the resistance? Are dual-finewire plug that much better? Weak MAGs? A total fluke?
 
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Hmmm...

Could be that your engine is pumping a bit of oil, and fouls the lower plugs after a short time. Replacing the new (but fouled) plugs with cleaner plugs (no matter what type) made things better?

Generally, there is no operational difference, if both plug types are in good condition.

Have a good look at the plugs you pulled out - I would suggest the answer is there.

My experience in the big radial world says to run fine wires where there are oil fouling problems (lower cyinders). BTW these type plugs can last 700hrs or so; massives about 350 or so.

Also: fine wire plugs cannot be cleaned by typical aircraft methods (blasting).
How often to clean & gap your plugs? Well, when they get dirty!

If you find lead deposits around the insulator, lean a bit more that you are usually doing.

How to ruin new plugs: lube 'em with the readily available silver slimy stuff in the grey plastic jar. If any of that stuff gets on the electrodes, the plug is going to foul quickly. Please always use aviation spark plug anti-sieze, and shake well before using. Make sure none gets on the electrodes!

Carry on!
Mark
 
Good questions Mark.

My plugs show a tiny bit of carbon at condition inspection time, but no oil and no lead (I run mostly zero-ethanol).

Additionally, as memory serves, the problem is evident even after my condition inspection.

When I put in the old dual-finewires it started after just a few blades. It was SO DIFFERENT from my experience over the past few years, I was pleasantly surprised. I had become accustom to the hard start when cold.

To add to my likely defense against oil. My O320 uses only about 1 QT of oil in 50 hours. (Of course, that could be a red herring.)
 
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Generally speaking

... I'd attribute hard starting to a failure to replace your magnetos with P-mags before next flight. Talk about a difference in starting :)

-Stormy
 
This may apply to aviation plugs but probably not.

Some of our EICommander customers have run into issues with unexplained Timing Divergence Alarms. In talking with them (I'm one of the "them".) it was determined that their auto plugs were well over 100 hours old.

We all replaced our plugs with new ones and the problem went away. OK, that nice but why did the plugs cause a problem?

I sent all eight of my plugs off to a university professor I happen to know who is most interested in sparkplug operations and has the tools and knowledge to test them. (He flies behind a rotary, so he have a vested interest in the research.)

He found that the plug's insulation starts to deteriorate at about 100 hours +. I have his report but the short of it is (no pun intended), replace your plugs every hundred hours +/-.

Again, this may not apply aviation plugs but since a full set of NGK BR8ES's cost less than $20, I don't mess around and I don't spend the money on expensive plugs.
 
Bill, ship your pMags over and I test them out :)

But back on topic, I'm running Bendex MAGs. The only thing I've changed have been the plugs.
 
Bummer. Bad MAG!

I got a call from the A&P who does my condition inspection. He said, "mind if my boss an I stop by? We have a couple things to try." They are the best!

They stopped by with their own "mag proofer". Long story made short. One MAG isn't very healthy.

The bad MAG is likely generating spark once it's above 600 or 700 RPM but is dead at "starter speeds".

So, I'm looking at three options:

  1. overhaul the Bendix (and likely both)
  2. replace the both Bendix with Slick (and replace the wires too) - but we're not sure if change to Slick uses the same impulse coupler and gear
  3. go with something like dual pMags (with new wires and whatever else that entails)

    I'll be posting a WTB over in the classifieds now :(
 
No need for two P-mags at this time. Just buy one now and the other when your remaining mag dies.
 
humptybump said:
I got a call from the A&P who does my condition inspection. He said, "mind if my boss an I stop by? We have a couple things to try." They are the best!

They stopped by with their own "mag proofer". Long story made short. One MAG isn't very healthy.

The bad MAG is likely generating spark once it's above 600 or 700 RPM but is dead at "starter speeds".

So, I'm looking at three options:

  1. overhaul the Bendix (and likely both)
  2. replace the both Bendix with Slick (and replace the wires too) - but we're not sure if change to Slick uses the same impulse coupler and gear
  3. go with something like dual pMags (with new wires and whatever else that entails)

    I'll be posting a WTB over in the classifieds now :(
Click to expand...

Don't Bendix mags have a better reputation than Slicks, particularly from a maintenance perspective? And hasn't Bendix gotten really competitive on pricing new mags? If I was you, I might look into a new Bendix mag or two. (Sez the guy flying behind one Slick and one pMag.)
 
Keep the Bendix!

Magneto ER - I think in Bald Knob AR. Great Bendix service. Have a good look at your wires too,

Carry on!
Mark
 
We have never had a hard start problem on 3 RV's that we have owned. Here is what I attribute that to. We rotate our plugs at every oil change. (50 hrs) We have Slicks on the present plane and had to replace them after about 500 hours. They were beyond rebuilding stage. We also replaced the wires at that time just 'cause we felt the older originals were starting to look shabby. Our new plane will have 2 "p-mags" and we will change the auto plugs at every oil change. Cheap insurance. By the way, even cold the our present plane starts within 2 blades. Also check that the battery spins the starter over at good speed.
 
Thanks "woodman"

We rotate plugs at every condition (after cleaning) and my wires are in good shape.

We definitely have it narrowed to the mag. At higher RPMs it generate plenty of spark but down at the speeds the starter turns the engine, it is too weak to get a consistent spark when everything is cold.
 
Opposite problem...

Mine was a bear to start when it was hot. I put in a rebuilt carb, which helped a bit. Then I took the Slicks off and to the shop. They were in rough shape. I had them rebuilt and now the engine starts easily, hot or cold.
 
500 hr

My .02 ...
All magnetos require 500hr maintenance in order to function reliably. Slick 4300 & 6300 series magnetos are good units, the smallest & lightest of the magnetos, and economical to overhaul, but Continental Motors Inc. (CMI), formerly Bendix, S-20 series (S4 & S6) have a 10 - 15 percent stronger spark due to a larger coil but this comes with a modest size and weight penalty. The CMI 1200 series magnetos are the performance leader with twice the power of a 4300 slick, but are larger and heavier than all the others. None of these will be reliable unless maintenance is performed every 500 hours. Finally all magnetos are inferior, both in power output and required maintenance, compared to modern electronic systems ( EFII, LSE, etc...) also the electronics can vary ignition timing which can save fuel. Russ
 
I forgot to update this thread.

We did a new series of tests which eliminated the magnetos as an issue. It also eliminated the wires.

So we are a loss to explain. All we know is the old plugs (REM38P) work well and the new plugs (UREM40E) don't.

Turbo will be here soon and I want to fly so the old plugs are staying in for now :p
 
I'm not super smart on plugs but I noticed that the heat values are different: 38 on the old vs. 40 on the new. Perhaps if you try massive electrode 38's instead of the 40's you might see a difference.
 
How did you eliminate the 'magnetos as an issue'?
If this was simply by way of a test then it is unlikely to have given you an accurate result especially if the mags are marginal. Tests are not absolute - false positives and false negatives are a reality.
If you replaced the mags in their entirety with known good units and found the same symptoms with the 'new' plugs. - then I agree the problem is very odd.
If you did less than replacing the capacitor and coil then I don't think you can eliminate the mags as an issue just yet. Possibly the lower resistance of the 'new' plugs is loading down the power source (the L & C) within the magneto shortening the spark duration to a point where the cylinder does not fire - especially at low RPM (ie less energy going into the L&C) during start.
 
Try a set of UREM37BY I have always got better starting with the exposed electrode plugs versus the recessed electrode plugs. They are much more forgiving of less than perfect air fuel mixtures also.
 
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