Gapping spark plugs

Gapping spark plugs

<HTML>Pulled the REM40E's (a dozen) out of the O-300D to clean them.  The motor was running smooth, purring like a kitten, but I did an oil change so I clean/antiseize/torque the plugs when I do the oil change.

This time I checked the gaps and noticed they were way past .020
So with some help, we gapped them to .016

Now the engine is rough at idle.  Sure, it maintains idle with carb heat on and one mag, but it's not smooth anymore.

It's rougher at idle on either one of the mags, so I think it's the gapping, not just one plug messed up.  Me and my friend double checked every plug and they were all .016 with the feeler.

Of course, everything else seems normal, and at 1000 RPM or above, there is no roughness.

Does the smaller gap make more spark and change the timing?  Does it run hotter?  Is the smaller gap needed only at high altitudes?  When would you gap at the high end of the range (.018) vs. the low end (.016)? 

I'm gonna spring for $240 worth of new plugs from somewhere anyway (2 sets is nice, then you can just have a nice clean set lying around), since plugs are a pain to "ungap" (widen the gap).

What plugs and gaps do other O-300 engine owners use in their smooth running steeds?

Mark</HTML>

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Re: Gapping spark plugs

<HTML>mark, I gap plugs at .016 all the time and never have a problem.   You must have a bug somewhere. jack</HTML>

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Re: Gapping spark plugs

<HTML>I have worked on cars and other internal combustion engines quite a bit over the years, but not aircraft engines - so my advise may not be directly comparable -

That said, My experience has been that smaller gap plugs generally fired easier and ran smoother - under certain operating parameters - However the smaller gap was easier to foul out, thereby possibly creating a misfire easier.

Larger gap plugs fired through more fuel, higher oil consumption problems, rich fuel mixtures, and whatever, but ran rougher - and thats assuming, among other things, that you have the high enough voltage to fire through them, Which most mags probably do not have, unlike automotive coils where you can have several times the spark (voltage).

If your (smaller gapped) plugs begin to look fouled pretty easily then the smaller gap probably probably exagerated this. A REAL BIG HOWEVER, although a larger gap may have made the engine seem smoother - Your mags were not designed to operate (voltage - sustained voltage - and voltage when you load up the engine on power and fuel settings, especially on a hot day ) with plugs gapped wider than spec so be carefull of a short term apparent fix.


Suggest You look deeper into the underlying causes that may not be readily apparent - Have a good mechanic check out the Mags - their respective voltage and or firing, and wiring harness, or whatever his best suggestion is . preferable on a hot, humid day when ignition is under a lot of stress and problems or potential problems are exagerated.

Checking it out could show an impending mag problem - or ? 

I hadn't seen the need to, but a trusted mechanic suggested that I have the carb rebuilt on My 172K, which was costly, but made a considerable improvement in smoothness and fuel burn, etc.

Something that I would have not questioned because the engine seemed to be running pretty good - but again the improvement was very noticeable

Good Luck

Ken Wanagas</HTML>

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Re: Gapping spark plugs

<HTML>Are you certain that you didn't drop one of the plugs?  If you drop one, then throw it away. 
Are you certain you got the mag leads onto the correct plugs  and didn't get one or more crossed?  Did you contaminate a "cigarette"? (the insulated lead that inserts into the top of the plug.)  Grease, oil, etc will short out that connection and cause rough running.  Did you break a lead?  Use a high-tension lead tester to determine whether handling the leads broke or shorted one.
.016 is the correct gap.</HTML>

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Re: Gapping spark plugs

<HTML>Hmmm...

  We didn't drop any.  As far as crossing the leads, the leads are on little lead holders (little u clamps) and have different lengths, so they go to top or bottom and each cylinder pretty unambiguously, I think/hope.

  But, does it matter if they cross over each other?  I think they avoid this on cars to avoid inductive coupling or something like that... 

  As far as the other end (cigarette), this is something I will check, because if we dirtied one, we may have dirtied several. 

  As far as breaking or shorting a lead, I'll need to test those too.  Again I can see how this might happen to more than one plug lead if they are all handled the same abusive way.

  So we didn't DROP any (used an egg carton and were very careful) but we DID use one of those vibrating etcher things to clean the lead out of the plugs.  We did that to all the plugs, so I could also see how that might mess up more than one.  That may have broken several of the plugs.

  I can't imagine it's a magneto itself, since we didn't do anything to the mags.  We did something to mess up the plugs or leads, since that's all we changed (or even touched).

  Anyway, I'm gonna order a new set and a box of copper washers and swap 'em out before I run the engine again.  I can't imagine it's just one plug because it happens on either mag equally.  I also can't imagine it's something else wrong because we didn't change anything else.  (every time I fix something and then find another problem, it's usually because of the last thing I "fixed", i.e. fixed the fuel sending unit and then got a fuel leak and water in the sumps...hmm...a chafing fuel line?  a loose fuel cap?  nope, we broke the sending unit gasket when we "repaired" the sending unit!  Bingo!  New seals, no problem).

  Well, fortunately no hurry, since I've got other planes to fly until I get her back up smile

Thanks for the input!

Mark</HTML>

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Re: Gapping spark plugs

<HTML>I have a suspicion that youve inadvertantly cracked an insulator.This crack may not be easily visible,but it will compromise the insulation capabilities of the plug,and could cause the problem.</HTML>

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Re: Gapping spark plugs

<HTML>Hmm...everyone keeps talking about AN insulator or A plug.

Given that both mags were running smoothly before the cleaning, how is it possible or even remotely conceivable that there is AN insulator or A plug misfiring which causes roughness when both the left mag or right mag is selected exclusively?

Isn't that the whole reason we have two mags?  So that any single failure of a lead or a plug would be utterly invisible if the other (non-associated) mag is selected?

Am I off my rocker to completely assume that it absolutely cannot be a sole plug or lead that is causing roughness on these two independent systems?

If I am wrong about this, please let me know.  I had thought this was one of the fundamentals of how engines work.  If it's not, I've got a big understanding problem about how mags work...

If it isn't, I can only think we've cracked several insulators, contaminated several cigarettes, crossed several wires, or that the gapping has caused a timing change somehow in several of the plugs.  (Or the remaining 100LL has fouled several plugs quickly now that the gaps have gone from .020 to .016).

Am I wrong to assume at least two plugs/wires are involved, given there was no roughness before and there is roughness now on both mags?

Mark

P.S.  Alan and George, do you guys ever use those little vibrating etchers?  I'm getting a little nervous that I shouldn't use that to clean the lead out of the plugs anymore...that could have cracked several of the plugs...I dunno...(but they sure looked mighty clean afterwards)</HTML>

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Re: Gapping spark plugs

<HTML>Oh, forgot to mention we tested each of the plugs in one of those vaccuum chamber spark things before we put them in and they worked (and we got sparks out of both sides!  That's good, right?).  I don't have a lead field tester though...gotta find one of those to check the leads...

Oh, also forgot that I did do a grounding check to make sure both mags do ground correctly.  One thing I hadn't considered is that maybe both the left and right mags on the switch really only ground one of the two mags.  If it was ever misinstalled, I suppose when you check the left and then the right, you'd really be checking just the one mag both times.  The grounding check would still work, and this mis-installation would be otherwise invisible.

Hmmm...whenever I check left and right they DO have identical mag drops.  Then this would explain how one failed plug could cause drops when either the left or right mag was selected (since this switch wouldn't actually select the left or the right mag).

OK, that's easy to test, just hook up one of these lead testers to one of the bottoms, and select one mag at a time and see if they ground and unground correctly when you turn the prop, right?  Then do the same with one of the tops.

Has anyone ever heard of this kind of miswired mag switch?</HTML>

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Re: Gapping spark plugs

<HTML>You did change the gap which meant that at least for a while you changed the resistance the mags fire through.

I re-read your note on carb heat and maintaing idle but I didn't understand it thoroughly from the description, but carb heat directly affects fuel ratio, and a change in fuel ratio also affects resistance directly.

I believe these are the links or symptoms to whats happening and it would seem that changing or regapping the plugs is just enough to get something else to show up.

As you describe - sometimes something fails when something else if fixed - usually it was close to failure or ? anyway and it just takes disturbing the system
a little to actually push it over the edge.
Thats why so many starters and alternators fail when the weather turns - They were operating fine before, but unknown to the operator, they were becoming marginal - then the weather turned - not the cause, but the catalyst.   

Good Luck

Ken Wanagas</HTML>

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Re: Gapping spark plugs

<HTML>An after thought -

It does seem that if its a plug or two, that it would be a miss and not " roughness ".

On the mixture/idle etc. - if its running rough and you lean it out a little - even at idle - does it get better or worse?

Ken Wanagas</HTML>

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<HTML>I had a problem where the engine would run ruff on both mags.  It turned out to be a spark plug.  The engine was losing compression through a plug.  I found the problem when I removed the ignition wire from the bad plug and it was damp with fuel.</HTML>

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Re: Gapping spark plugs

<HTML>Mark,
I have never used(and dont intend to) the vibrating device to clean my plugs.I have a Bead Blaster at work,and generally keep one complete set of plugs clean and ready to go whilst the other set is in operation.
Typically I would think that any type of"intrusive" blade type device could potentially damage the insulator.This may or may not be visible to the naked eye,and provides a good area for immediate carbon build-up,and subsequent arcing to the ground area of the plug.AND while it IS in effect still firing,its not in the atomized stream of the fuel/air mixture,and doesent burn as efficiently.
I also seem to remember something someone asked about one mag firing one set of plugs being"normal combustion operation" this is in fact NOT the case.The reason that BOTH mags are firing at the same time is for more even fuel combustion,and smoother running(this is why youll typically have some"mag drop" when youre doing youre typical run-up.)
Agreed that the engine will run on one magneto,and it IS designed to be a safety feature,but the compressed fuel/air mixture in the cylinders will burn much more completely being ingnited by both plugs at the same time.

Im relatively sure George(whom I havent heard from in awhile(Grin) will agree.</HTML>

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Re: Gapping spark plugs

<HTML>Hi, Alan!
  I do agree that the "normal" combustion process includes the consideration for both mags firing,...
  I just don't agree that they must fire at the same time.  In fact, many engines have specified different timing for the mags.  This is most common in setups where one mag fires the lower plugs, and the other fires the upper ones.  (Lower plugs typically are timed to fire slightly ahead of upper plugs in normally aspirated engines.)  The flame propagation patterns are studied and allowed for by the manufacturer when they set those timing specifications.  Also, some magneto systems take into consideration starting conditions, i.e., retard breakers/"shower of sparks", left mag only, and/or impulse couplings.
  But the bottom line is still the same, timing of each mag should follow factory specifications.
  Vibrating cleaners are OK as long as they're used correctly and the plugs are properly tested prior to re-installation.
  (I'm still of the opinion that two or more plugs have been damaged, and/or the reinstallation included an error either in wiring or high-tension lead damage.)
  While it's possible that a magneto is in trouble, it's not high on my list.  But to be truthful, cleaning and regapping plugs does indeed change impedance feed-back to the mag, and in rare circumstances can bring to surface a marginal magneto's shortcomings that may have been camoflaged in the previous dirty-sparkplug condition.</HTML>

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Re: Gapping spark plugs

<HTML>One thing that's not been mentioned yet is:
  Look at the lower vent-plugs of the magnetos.  Is there a drop of oil hanging from it?  If so, your magneto oil-seal is leaking and allowing oil to enter your magneto.  This can play havoc with idling engines, and can also appear as a transitory roughness in flight.  It'll be there one time, and disappear the next, then reappear....driving you nuts.  Until you finally discover it and make the repair.</HTML>

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Re: Gapping spark plugs

<HTML>Tickeled to hear youre still out there.I agree about the mag timing issue,and do remember something about the difference in mag firing(be it all so slight).
Take care my firend.</HTML>

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Re: Gapping spark plugs

<HTML>Cool,

  Thanks for all the input.  Several of you said you have a second set of plugs, so I think I'm gonna shell for a second set and then install them and run up three plugs at a time.  That way I can figure out if it was a coupla plugs, or crossed wires, or if the problem gets slightly better with each new three, we may have gapped 'em too tight.

  I'll also try the oil-seal leak check and see if I can do a bit of testing.  I am absolutely not gonna change the timing though.  That I'll leave to a professional if the tests show problems...

  Hmmm...what's a lower vent-plug?  I'll have to look this one up :-]

Mark


P.S.  Went to TVL and TRK.  On the way back we figured out that when the FSS says "expect clear above and 1000 SCT at WVI" they are really hedging a little and mean "3-5 miles in haze, layer tops 7000 and 2000, inversion in between, and overcast 800".
Slam dunk from 7000, and then a scary call when I heard a plane I know call "no-gyro" in actual at MRY.  I hope they're ok...</HTML>

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