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Today, the plane I had, failed the Run up because the magneto test failed. One of the spark plugs fouled out.

Other than making sure the engine is properly leaned out (even then spark plugs in a plane foul out more regularly than a car), what can be done mechanically to the spark plug? Like using non foulers or a spark plug alternative?

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    $\begingroup$ Lean it on the ground too. $\endgroup$
    – Ron Beyer
    Jul 14 at 0:58
  • $\begingroup$ What aircraft? What engine? $\endgroup$ Jul 14 at 1:22
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    $\begingroup$ You mean modifications that haven't already been applied to the design? $\endgroup$
    – GdD
    Jul 14 at 8:10
  • $\begingroup$ I am still looking for a fix or alternative. $\endgroup$ Jul 14 at 23:20
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As Ron notes in the comments

Lean your engine on the ground

Proper leaning during idle and taxi operations is much more important than most pilots understand. It can solve spark plug fouling problems, reduce valve guide wear and valve sticking problems, and prolong engine life.

Mixture distribution is poor at idle. At a rich idle mixture, some fuel doesn’t vaporize and enters the cylinder as a liquid where it partially burns and forms carbon deposits.

In addition, a rich idle mixture causes lead fouling. Since gasoline, tetraethyl lead (the octane enhancer in avgas), and ethylene dibromide (the lead scavenging agent in avgas) all have different boiling points, fractionalization occurs in the induction pipes. Some cylinders get a high dose of lead with no lead scavenger while other cylinders get the scavenger but with little lead.

Or better yet have your mechanic make sure its set up correctly

Rather than manually leaning your engine for ground operations, it’s better to have your mechanic adjust the idle mixture to a properly lean setting. If you lean manually on the ground, you need to understand the risks.

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    $\begingroup$ I agree it's a good practice, but this doesn't answer the question which is about spark plugs. $\endgroup$
    – GdD
    Jul 14 at 7:18
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    $\begingroup$ If you mech adjust the fueling to lean, you lose the full mixture adjust range. I strongly advice against this practise. At full rich setting, your engine is supposed to run rich, not at optimal lean! $\endgroup$
    – Jpe61
    Jul 14 at 9:53
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    $\begingroup$ WRT having your mechanic adjust the idle mixture, how? Airports are at different elevations, so the proper lean mixture is going to be different. If your home field is say Truckee at 5900 ft, and mixture is set for it, you'll be running quite lean if you fly to anywhere along the coast or in the Central Valley. $\endgroup$
    – jamesqf
    Jul 14 at 16:10
  • $\begingroup$ I believe the intention of the note in the article is that your mixture should be adjusted properly so that lean on the adjuster is sufficiently lean and rich is not overly rich (i.e. your range of adjustment is proper) $\endgroup$
    – Dave
    Jul 14 at 17:56
  • $\begingroup$ The mixture is adjusted properly when full lean is too lean, and full rich is too rich 🙃 but yea, i get the point. Correctly adjusted engine will foul pulgs when run full rich on ground, no getting around that. That's how they are. $\endgroup$
    – Jpe61
    Jul 23 at 17:31
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If there were a design change that would prevent fouling, it would be standard by now.

However, the way you operate the plane can prevent or at least reduce fouling, i.e. leaning the mixture properly for taxi and cruise. In most cases, leaning for taxi will clean the plugs enough by the time you get to the run-up area that you’ll pass a mag check.

If not, particularly in flight school planes that spend an abnormally large fraction of their time at a full-rich mixture (for pattern work), you can clean the plugs by leaning the mixture and running the plane at the run-up power setting for a few minutes. Do the mag check again at full rich (or as needed for altitude), and you should pass.

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    $\begingroup$ I have to disagree. Design changes and GA engines just don't go together. There IS a design change that seems to have virtually eliminated plug fouling in automotive engines: EFI and associated automatic mixture controls. In the old carburetor days, it was normal to clean and/or replace plugs at 10-20K miles; nowadays 100K isn't unusual. $\endgroup$
    – jamesqf
    Jul 14 at 21:36
  • $\begingroup$ @jamesqf very good point!!! $\endgroup$ Jul 14 at 23:22
  • $\begingroup$ @jamesqf Some GA piston planes now come with a FADEC that eliminates the mixture and prop controls, but the cost to develop an STC for retrofits would be prohibitive, and most pilots are too cheap to buy it anyway. $\endgroup$
    – StephenS
    Jul 14 at 23:38
  • $\begingroup$ @StephenS: Yes, which is why design changes and GA engines just don't go together. $\endgroup$
    – jamesqf
    Jul 15 at 6:36
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The plug fouling problem that afflicts most of the GA fleet, in good running engines, isn't carbon buildup or other gunk; it's tetraethyl lead in the fuel that sort-of condenses out of the fuel charge into liquid lead droplets, like condensing water in a glass, that hardens into little lead beads sticking to the plugs.

Some of the the tiny lead beads fall into the insulator well of the lower plugs, like dropping rocks into a pit, collecting at the bottom and piling up until they reduce the insulation path down the center electrode and the plug starts to short out (like the guy you threw into the bottom of the pit was able to climb out because of the pile of rocks you made throwing them into the pit at him).

The problem is the combustion temperatures of engines with lower compression ratios aren't high enough to keep all lead in a vapour state through the combustion. 100LL Avgas has a scavenging agent that is supposed to prevent this, but it isn't completely effective if the plug surface temperatures get too cool (Closed throttle idling is the worst time for this - airplanes that spends many minutes idling in long taxiway lineups are most effected. Carbureted engines are also more effected because of the less even fuel distribution and the richest cylinders will foul most).

Beyond the leaning procedures on the ground and in the air and avoiding extended close throttle idle, there isn't a physical mod to deal with lead formation on plugs other than making sure the plug's heat range is correct for the engine (if your engine is approved for the Champion RHM 37BY, a plug specially developed to reduce lead fouling in the Lyc O-235 for the Cessna 152, that could help), but if you are using avgas you can use ALCOR TCP additive to the fuel, which supplements the scavenging effect of the existing scavenging agent.

TCP was developed because the same thing was a big problem on the B-36 bomber using 115/145 octane avgas (which had crazy lead levels to get that octane rating) and lead fouling of the bazillion cylinders in all those engines on the very long cruise flights, at low power settings that that bomber could do, was a big tactical problem.

You have to mix the TCP into the fuel when you refuel, using a graduated syringe (I used to tow gliders in a Cessna L-19, which has a low compression Cont O-470, and the club used it for a while before deciding the hassles were too much with all the mixing and injecting at every refueling, and gave up). A lot of owners can't be bothered with the hassle and expense and just go with elaborate leaning techniques to minimize the problem, and pull the lower plugs to clean them off from time to time.

Interesting aside: I learned a while back that the active ingredient in TCP, tri-cresyl-phosphate, just happens to be the same stuff that Lycoming told owners to put in the oil on the O-320 AD engines that had the cam spalling crisis back in the 80s. In the oil it acts as an anti-scuffing agent, providing initial lubrication when the parts are oil starved at startup. It's also an additive to the Aero Shell semi-synthetic engine oil line.

The other (better) solution is to run the plane on unleaded mogas, which is what I do. My Lycoming O-290, with only about a 7.5:1 compression ratio, does NOT like avgas and fouls the lower plugs in only about 25 hours even with religious leaning. On Mogas it is happy as a clam and I only ever see a light soot coating on the plugs at annual.

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  • $\begingroup$ Downvote because fouling can be caused by many factors, including carbon contrary to your answer. Common types of fouling include lead, carbon, silica, loose debris, and oil. Another common cause of spark plug failure is a cracked ceramic insulator. I have dealt all of these as a mechanic. Lead fouling is one of the most commonly encountered in an otherwise well-running engine, as you highlight. $\endgroup$
    – J Walters
    Jul 14 at 13:27
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    $\begingroup$ I'm assuming an engine is in good condition and discounting mechanical defects because the OP is looking for a solution to the overall GA fleet problem of lead fouling, which is 99% of the issue on engines that aren't pumping oil. When you run on mogas, which makes a lot more soot than avgas, plug fouling on a jug that isn't pumping oil through is almost unheard of. My O-290 has fairly high oil consumption, about a qt in 4-5 hrs, and even at that level I don't have oil fouling issues and on mogas there is only trace carbon buildup. If I run on 100LL, it quickly becomes a mess from lead. $\endgroup$
    – John K
    Jul 14 at 14:49
  • $\begingroup$ @JohnK yes 100 hour inspection etc. $\endgroup$ Jul 15 at 3:16
  • $\begingroup$ @Justintimeforfun I added an edit to mention a fouling resistant plug that Champion makes for the O-235 that is apparently approved for various engines now. Something to look into. $\endgroup$
    – John K
    Jul 15 at 3:36
  • $\begingroup$ @JohnK You're right, in good condition operating in most parts of the US the lead fouling is going to be the biggest issue. I flew an O-320 a little over 1000 hrs and didn't have a single fouled plug. Operated on a combination of 100LL and 91UL. No difference regardless of fuel type. Oil changes once a week (every 30-50 hrs) and carbon was normal. Ran that engine pretty lean. I operated an older O-320 a little and had multiple carbon fouling incidents, not soot, but loose bits of carbon/debris. I used to work on pistons flying in the desert and silica fouling was the biggest problem there. $\endgroup$
    – J Walters
    Jul 17 at 12:09

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