Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
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?
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.
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.
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.
