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New spark plugs - spark plugs often fouled.

We are about to change the spark plugs on our Cessna 172sp with an IO-360-L2A engine.
There has always been a tendency that the lower spark plugs foul and that can be annoying.
Is there any advantage to install Iridium fine wire? Is it worth the cost? Will it help the foul problem?
Another solution could be the REM37BY

pmh
ekbr ekbi, Denmark

From my personal experience with an IO-540 on fine wire plugs I wouldn’t bother. I have now changed back to standard plugs top and bottom. Just out of interest are you normal taxi times over say 10mins before takeoff? I have read about people leaning for taxi but always been unsure about this.

Alex
Shoreham (EGKA) White Waltham (EGLM), United Kingdom

I have no clear evidence that fine wire (iridium) plugs do not foul (to the point of shorting out so you get no spark) because they do. However -

  • I have had maybe 2 shorts with the old plugs (2002-2003) and maybe another 2 with the iridium plugs (2003-2015) but those happened in 2003 and 2005
  • I have always found the bottom plugs are the ones which get the lead etc deposits; the top ones are usually clear of deposits
  • On tarmac, I always taxi maximally leaned (the engine cannot be damaged at such low power settings)

I would not use the Champion (RHM38S in my case) anymore due to their rubbish resistors (do a search on
champion resistor
in the search box here) but Tempest do an equivalent; URHM38S in my case which are fine. Champion have reportedly changed their design recently, after spreading marketing BS everywhere previously.

I am not aware of any data showing the iridium plugs produce more power. They do however last much longer. I had a set of the Champions (with the dodgy resistors) for about 500hrs and they showed no visible wear at all. For maybe the last 400hrs I have had the Tempest ones and again no visible wear.

Administrator
Shoreham EGKA, United Kingdom

Alex_ wrote:

I have read about people leaning for taxi but always been unsure about this.

What makes you unsure? It’s a no brainer. A rich mixture gives you dirt on the spark plugs and lots of carbon monoxide in the exhaust. If I don’t lean my engine to peak during taxi, I will have a problem taking off.

Peter wrote:

I had a set of the Champions (with the dodgy resistors) for about 500hrs and they showed no visible wear at all.

The Champion resistor issue was limited to massive plugs, wasn’t it?

I had the Tempest Fine Wire Plugs for a year now and I think the engine runs a bit smoother and starts even better than it already had. This is what they looked at the annual three weeks ago after 150 hours. These are the bottom ones, but the top ones looked exactly the same.

pmh wrote:

We are about to change the spark plugs on our Cessna 172sp with an IO-360-L2A engine.
There has always been a tendency that the lower spark plugs foul and that can be annoying.

As Peter suggested, try leaning aggressively for all ground Ops.

Lycomings tend to pass abit more oil through the rings than Contis and that combined with a rich mixture at idle or very low power settings tend to load up the plugs.

FAA A&P/IA
LFPN

Lycomings tend to pass abit more oil through the rings than Contis

In that case, why do Contis have such poor compressions? Is the gas escaping past the valves? Their acceptable compressions are very low – somewhere around 30/80. Mine (IO540) are still around 78-79/80.

Alexis – those plugs look fine to me. My bottom ones look the same as the top ones from that angle though. The difference is that the bottom ones have the deposits sitting at the bottom of the cavity, so I use the welding nozzle cleaning tool to dig them out.

Administrator
Shoreham EGKA, United Kingdom

Amost no deposits on my plugs. But I lean very aggressively on the groud and almost always fly Lean of Peak.

Last Edited by Flyer59 at 29 Jun 10:50

I always lean to the max point where the engine almost quits. Then just a tad richer – just to get power for taxiing.
The aircraft is most often on grass strips with short taxi times apart from waiting for the engine to heat up during colder months.
One of the problems can be that we are several different pilots and not all follow the same way of leaning.
I have the feeling that the way the aircraft has been taxied in on the last flight have a lot to do with the way it can foul the plugs on the next startup.
The problems has always been resolved with aggressive leaning and higher power setting on run-up. Only one time have I had problems to get it to go away.
Any experience with the REM37BY or a Tempest similar solution?

pmh
ekbr ekbi, Denmark

PMH
Also, make sure to run the engine for 20sec. at 1200 rpm with the engine leaned before you shut down. When the engine turns on low rpm the internal temp. is not high enough, as a result the lead in the fuel accumulates around the plug’s electrodes.

The plugs in the photo seems to be too dirty. I would think that the engine runs rich, also, what is the case with the plug on the left? Is it a wet plug? Seems to have burnt oil deposits, hard to say from the photo.

I always lean (aggressively) during taxi and run the engine before shut down, the plugs were always clean with a light brown sediment on them.

Last Edited by Ben at 29 Jun 12:57
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