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Ash (?) deposits on spark plugs using auto gas?

pjc

Well Known Member
Seeking knowledge ...

I have an RV-6 with a 150hp (7:1 compression) O-320 E2D. Dual slick mags, L is 120 since new, R is 160 since overhaul. Champion REM40E plugs. Absolutely stock in configuration as far as I know. Engine is 980 SMOH (many years ago). Compressions were good at last annual 10 months and 60 hrs ago. Starts, idles, runs smoothly historically. Uses 1-2 quarts between oil changes every 40 hrs.

I have recently (about 30 hrs ago) began running 89 octane ethanol free auto gas for which the fuel system and engine are designed/approved. (Please don’t hate, it’s experimentation). At the last annual I switched from Aeroshell 100W to Phillips XC 20-50. I have not observed any operational changes with the fuel or oil switches. CHTs routinely 300-360F (depending on flight regime). Never seen any rapid CHT transients that might suggest detonation.

This weekend I got a poor mag drop (~250 rpm and roughness) on the R mag. Typical drop is ~75 rpm from both sides. L is still ~75 rpm. Engine was heat soaked on startup (had been sitting a hour or so) so vaporizing fuel was a thought/concern but doesn’t correlate well to one bad mag drop. Start up and run up on Both mags was unremarkable.

Investigation begun with plugs, which appear Ash (?) fouled (photo attached). The deposit is chalky (soft) and degrades to a fine powder when removed. All 8 were similar in appearance. Some were a little worse than the example in the photo. I have not experienced deposits such as these previously. Plugs cleaned re-gapped and re-installed. I have not re-confirmed static timing (25 BTDC) but intend to before a ground run.

What I was able to find using search suggests:
A). Possibly over-lean condition - this would be concerning (and unexpected)
B). Possibly ash deposits in the fuel (presumably the newly introduced auto gas)
C). Possibly ash deposits from burning oil (but oil consumption has been low)
D). Possibly ash deposits from oil additive package (although this experience seems to be from auto engines using auto oils).

Any thoughts about these theories, or suggestions about what is going on, diagnostics or maintenance to be performed, or operational tests or changes to be implemented?

Thx in advance,
Peter
 

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Obligatory try 4oz of MMO to 10 gals...

Those electrode gaps look a bit wide and the Mags may be due an inspection/ service- possibly a stack up of wear asking for some love if they are not also 160 or less SOH.

If the mags are good and gaps about .018- a plug cleaning may be all it needs. I am a 90 octane no ethanol 50/50 with 100LL user. I have not seen ash but that does not look like oil, to me.

Do your tailpipes look the same?

I rotate upper and lower plugs to even the electrode footballing and always ohm plugs at a cleaning. If they go high, they stress the mag.
 
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Those deposits look typical for the **** they call auto gas today. The cars don't have the same qty of deposits, as the plugs are much hotter than the aviation plugs. UREM37BY plugs run hotter and therefore less deposit accumulation. What you are seeing is not carbon.

I also observe that your gaps look much larger than .018" Running large gaps will cause an early death for the coils in your mags.

Larry
 
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Those deposits look typical for the **** they call auto gas today. The cars don't have the same qty of deposits, as the plugs are much hotter than the aviation plugs. UREM37BY plugs run hotter and therefore less deposit accumulation. What you are seeing is not carbon.

I also observe that your gaps look much larger than .018" Running large gaps will cause an early death for the coils in your mags.

Larry

Your and Rich’s observations on the gaps are valid. All were worn to well over 0.018 (typically 0.025-0.028) and were corrected back to 0.018 after cleaning. I have not historically rotated plugs top to bottom (on the same mag) but I understand this to be a practice worth implementing.

Other thoughts/observations/ideas welcome.

Peter
 
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Lead

Looks like lead deposits. Probably residual combustion chamber deposits from when it was burning Avgas.
 
I run similar fuel in my RV7. 91 octane no alcohol. I run mostly LOP ( IO360-M1B) so my deposits are some what less. My deposits can be easily brushed of with a brass bristle brush.

So far,on my RV7, I have never had a fouled plug, but I have had a plug completely stop working. I have also had coil failures on my electronic ignition and a ignition wire failure.
 
I run similar fuel in my RV7. 91 octane no alcohol. I run mostly LOP ( IO360-M1B) so my deposits are some what less. My deposits can be easily brushed of with a brass bristle brush.

So far,on my RV7, I have never had a fouled plug, but I have had a plug completely stop working. I have also had coil failures on my electronic ignition and a ignition wire failure.
 
Update

Cleaning and regapping the plugs has helped. Ground and airborne mag checks are equal and around 100 rpm.

I did get a stumble on initial acceleration on takeoff, bit perhaps I pushed a little enthusiastically. I’ll have a thorough look at the carb and fuel system at annual in December to see if the auto gas has lead to any deterioration or contamination.

Have not yet recreated the heat soak scenario to further test the mags, but plan to shortly.

Finally, I’ve found an alternate source of ethanol free auto gas. 92 octane in this case and will be trying that. (I am assuming, without hard data, that it is from a different jobber/refinery.). It seems to have less of the lacquer thinner smell and yellowish tint of the previous 89 octane stuff.

More to come,
Peter
 
Agree with others, gaps look too large. Also football shape is getting close to 2:1 (width to height) at which point plugs are trash. As the width of the center electrode narrows the thermal conductivity decreases, the tip runs hotter. If it gets too hot you can get pre-ignition.
Edit: if you properly rotate the plugs you’ll double their lifetime.
 
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More cleaning

I trust you cleaned out the deposits/buildup between the electrode and the two grounds. Sometimes it's difficult to see the buildup. I created a wire tool/brush just for this purpose & made it part of the annual conditional inspection.

If anyone knows of ethanol free mogas near Tucson, please advise.
 
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