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T/O power and leaning mixture on the ground to clean plugs?

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T/O power and leaning mixture on the ground to clean plugs?

Old 4th Mar 2020, 16:51
  #1 (permalink)  
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Join Date: Sep 2011
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T/O power and leaning mixture on the ground to clean plugs?

With a Cessna 152 suffering from rough running on one mag during runup, the advice of a (highly experienced) FI was to set T/O power and lean the mixture to max EGT for about two minutes on the ground (as soon as oil temp is in the green and paying attention to what is behind you WRT propwash, of course). While I can see how this might get the job done, is this not too harsh on the engine? Or am I just too timid and shy?
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Old 4th Mar 2020, 18:18
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Leaning to MAX EGT will raise the Temperature in the cylinders and is likely to damage the valves and if continued for too long the cylinder heads.

A far better course of action would be to take the advice in the Lycoming service instruction on the subject. This publication advises the following action for the PREVENTION of spark plug fouling.

Shut the engine down in the following way , Idle the engine at 1200 RPM and check the Mag drop on both Mags, increase the RPM to 1800 RPM and hold it for 60 seconds, return the engine to 1200 RPM and immediately set the Mixture to ICO.

Using the shut down procedure in accordance with the Lycoming SI virtually eliminated spark plug problems with my Cessna 152 fleet when introduced by the operators who leased my aircraft.
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Old 5th Mar 2020, 01:37
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I second A and C. For several reasons, I would not run full power on the ground for any longer than necessary. Cooling is very poor. and potential for prop damage are two of the reasons. The EGT's will not be balanced, so you risk cooling one cylinder, while accomplishing not much on another.

If you're using the Lycoming technique which A and C offers, have a care as to what the prop is over, even 1800 RPM for a minute over gravel or loose asphalt can trash a prop. The prop on my 150, which I have owned for 33 years, has never needed to be dressed, because I am very careful where I power up.
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Old 5th Mar 2020, 03:38
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Join Date: Mar 2005
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Leaning on the ground is perfectly OK to clear fouled plugs, it's how you go about it, and you don't do it at full throttle - unless you want to pay some big, big bills.

John Deakin articles on the subject.



On the big radials during cruise it was normal to carry out what was called a "high power burn out" to ensure the plugs were kept clean.
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Old 6th Mar 2020, 22:06
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Join Date: Dec 2005
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There is a special spark plug for this application...


This whole heat-range equation can be summed up by looking at a special spark plug created for the Lycoming O-235 L2C engine developed for the Cessna 152. When first introduced, this engine kept technicians very busy cleaning spark plugs. The engine had the unprecedented characteristic of extracting what seemed like all of the tet-raethyl lead in the fuel before depositing it on the firing end of the recommended RHM 40E spark plug. Champion quickly developed the RHM 37BY, a colder plug with an unusual-appearing extended tip. This plug was designed specifically to deal with the lead-fouling problem in that particular engine. It has done that job so well that it's now approved for installation in many low-compression engines that need help preventing lead fouling.
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Old 7th Mar 2020, 02:27
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Runup RPM, lean until the engine starts to stumble, wait 15 seconds then go full rich and do a mag check. If you are still getting too much drop or rough running it is time to go back to the ramp and get the aircraft looked at.
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Old 7th Mar 2020, 07:44
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If the aircraft has a chronic fouling issue then get the primer checked for fuel leakage. I've had a Piper Archer with a three year fouling issue that could only be cleared by leaning for up to a minute or more before take off. The primer was overhauled twice but that doesn't allow for fuel seeping around the plunger seal.
All four cylinders were scored so were replaced, both mags overhauled, new carburettor fitted and still the problem persisted. In the end someone suggested we change the primer and lo and behold all symptoms cleared. I also fitted Iridium tipped spark plugs for good measure.

Leaning on the ground is no problem and as mentioned above is now recommended by Lycoming however a constant issue warrants mechanical investigation.

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Old 7th Mar 2020, 13:13
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Thanks @all for the input and suggestions; the spark plug and primer "options" were new to me.
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Old 7th Mar 2020, 21:10
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The procedure mentioned by A and C is an excellent way to prevent plug fouling but it's no use what so ever if you already have a fouled plug.

I've used the leaning at high power method many many times over the years with great success and no issues. In the worst cases it has taken full power to clear the fouling.

In my experience the fouling is generally a result of extended low power running on the previous flight, I might add usually a flight I hadn't undertaken. Things like a long low power descent, something I would chastise other pilots for doing, a glide approach at the end of a flight, or in the case of a twin a simulated single engine instrument approach immediately followed by a landing. If these types of scenarios can be avoided then the incidence of plug fouling will be significantly reduced or even eliminated, else carry out the procedure outlined by A and C.

You might not pilots flying aircraft with the likes of the MP14 radial (Yak 52, Wilga) carry out a similar procedure to the one outlined by A and C.
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Old 8th Mar 2020, 00:29
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What Lycoming have to say on the issue. IO-540 Operators Manual,
A proper magneto check is important. Additional factors, other than the ignition system, affect magneto drop-off. They are load-power output, propeller pitch and mixture strength. The important thing is that the engine runs smoothly because magneto drop-off is affected by the variables listed above. Make the magneto check in accordance with the following procedures.

(I) (Controllable Pitch Propeller.) With propeller in minimum pitch angle, set the engine to produce 50-65% power as indicated by the manifold pressure gage unless otherwise specified in the aircraft manufacturer's manual. Set the mixture control to the full rich position. At these settings, the ignition system and spark plugs must work harder because of the greater pressure within the cylinders. Under these conditions ignition problems, if they exist, will occur. Mag checks at low power settings will only indicate fuel-air distribution quality.

NOTE]Aircraft that are equipped with fixed pitch propellers‘, or not equipped with manifold pressure gage. may check magneto drop-of with engine operating at approximately 2100 - 2200 RPM.

Switch from both magnetos to one and note drop-off’, return to both until engine regains speed and switch to the other magneto and note drop-off, then return to both. Drop-off should not exceed 175 RPM and should not exceed 50 RPM between magnetos. A smooth drop-off past the normal specification of 175 RPM is usually a sign of a too lean or too rich mixture.

If the RPM drop exceeds 175 RPM, slowly lean the mixture until the RPM peaks. Then retard the throttle to the RPM specified in step (l} for the magneto check and repeat the check. lf the drop-off does not exceed 175 RPM. the difference between the magnetos does not exceed 50 RPM. and the engine is running smoothly, then the ignition system is operating properly. Return the mixture to full rich.

Do not operate on a single magneto For too long a period, a few seconds is usually sufficient to cheek drop-off and will minimize plug fouling
From the Lycoming produced "Flyer" series of articles, note para 4 at bottom of first column.

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