In a message dated 00-05-11 11:51:24 EDT, you write:

>  I'm not talking about a normal kind of roughness you get when you
>  run a mag check too long.  
That's not good.  The engine should run smoothly for an indefinite length
of 
time on one mag.  If you don't believe this then what are you going to do 
when you have a mag failure at night over the mountains?  


>  I have noticed an odd vibration from time to time
>  when flying at reduced power.  Does anybody know what will make one cut
out
>  at low speed like that?
A single plug missfiring will get your attention.  You might think not, 
sisnce you still have all cylinders, but not so.  The effect is more 
pronounced depending on how dark it is, how high the mountains are, how
far 
the trees extend (or water) , etc.

Bottom plugs often foul from oil in the cylinders.  What's your oil 
consumption rate?  Are you burning 100LL?  Using TCP? Once a plug fouls,
it's 
a slippery down hill ride as the deposits tend to build faster on a
missing 
plug.  The plug itself could be bad.  Simple test is to clean and gap it
and 
reinstall it in the top, putting the (good) top plug in the bottom.  If
the 
top plug fouls, replace it.  If the bottom one fouls, look at the lead and

the cylinder.  At higher RPM, the mag is generating a hotter spark, and
the 
cylinder is running hotter which helps remove the deposits.  This is
probably 
why it fires more reliably at higher RPM.

Good luck
John
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