Ignition problem?

My #3 cylinder has always run a little cold
on the right mag only. I fixed what I thought
was an intake leak 20 hours ago and that helped,
or at least seemed like it, but today it became
clear that #3 was not firing at all on the right
mag. To make a very long story a little shorter,
the problem travels with the ignition lead. If I
switch top and bottom leads on #3, it still goes
cold on the right mag, which tells me that it's
not the plug. The harness checks fine on a lead
tester, and when I finally broke down and pulled
the mag I was able to 'spark out' all four leads
against the mag case. I was expecting either the
#3 lead would be bad or it wouldn't spark, but it
looks like that's not the case.

Anybody ever see anything like this? Anyone got
any suggestions?

Note that this is *not* the cylinder I replaced.
On both mags the engine is noticably smoother
than before the new cylinder.

Details:
Lyc O-320D
Slick mags
The mags, harness, and plugs are about 6 months old
Good compression check

Dave '3-2-1-pffffft' Hyde
RV-4 in flight test, EAA tech counselor

My #3 cylinder has always run a little cold
on the right mag only. I fixed what I thought
was an intake leak 20 hours ago and that helped,
or at least seemed like it, but today it became
clear that #3 was not firing at all on the right
mag. To make a very long story a little shorter,
the problem travels with the ignition lead. If I
switch top and bottom leads on #3, it still goes
cold on the right mag, which tells me that it's
not the plug. The harness checks fine on a lead
tester, and when I finally broke down and pulled
the mag I was able to 'spark out' all four leads
against the mag case. I was expecting either the
#3 lead would be bad or it wouldn't spark, but it
looks like that's not the case.

Anybody ever see anything like this? Anyone got
any suggestions?

Note that this is *not* the cylinder I replaced.
On both mags the engine is noticably smoother
than before the new cylinder.

Details:
Lyc O-320D
Slick mags
The mags, harness, and plugs are about 6 months old
Good compression check


Carbon track somewhere?

snipped
I was expecting either the
#3 lead would be bad or it wouldn't spark, but it
looks like that's not the case.

Anybody ever see anything like this? Anyone got
any suggestions?

A friend had much the same problem. Discovered a carbon track inside the
distributor cap of the magneto.

Warren

Whunicut wrote:

snipped
I was expecting either the
#3 lead would be bad or it wouldn't spark, but it
looks like that's not the case.

Anybody ever see anything like this? Anyone got
any suggestions?

A friend had much the same problem. Discovered a carbon track inside the
distributor cap of the magneto.

Warren

Similar problem with a 1939 Continental A-65 was caused by heavy
crud build up in the cigarette plugs that connect the magneto (Bendex?)
to the leads. We came to the conclusion that these plugs were probably
the originals installed when the engine was built....

If your mags use this kind of thing, pull them out and take a look.

Replacements for the Continental were $2 each.


Best of luck,

Richard

Update:

New harness installed today. Same problem,
#3 doesn't fire on the right mag, whether
it's on the top or bottom plug. Runs like a
top on the left mag, top or bottom plug.
I'm going to try to get a new mag ASAP,
but Unison said they're usually all or nothing,
that it's not likely the mag would miss a single
cylinder. I've gotten plenty of local help, and
everyone here thinks it's the mag but not a
common failure, especially in one so new.
If it's *not* the mag, we're all at a loss
for ideas.

So much for flying the time off and taking it to
CA anytime soon.

Any bright ideas?

Dave 'sparky' Hyde

"nauga" wrote in message
news
Any bright ideas?

Loaner mag?

Rich S.

You know the reason, given your tests and replacements.
A magneto part won't hold up to the volt stress.
Maybe the cap carrying the leads, or even a coil.

Brian W

On Wed, 30 Jun 2004 02:39:31 GMT, "nauga" wrote:

Update:

New harness installed today. Same problem,
#3 doesn't fire on the right mag, whether
it's on the top or bottom plug. Runs like a
top on the left mag, top or bottom plug.
I'm going to try to get a new mag ASAP,
but Unison said they're usually all or nothing,
that it's not likely the mag would miss a single
cylinder. I've gotten plenty of local help, and
everyone here thinks it's the mag but not a
common failure, especially in one so new.
If it's *not* the mag, we're all at a loss
for ideas.

So much for flying the time off and taking it to
CA anytime soon.

Any bright ideas?

Dave 'sparky' Hyde

"Rich S." wrote in message ...
"nauga" wrote in message
news
Any bright ideas?

Loaner mag?

Rich S.


Probably a problem with the distributor in the mag. Slick uses
oilite bronze bearings for the distributor spark rotor, and those
bearings will wear and allow the rotor to slop around and increase
clearance between the finger and pickup electrodes. The mag will still
spark when turned by hand and the harness end held near the case, but
under compression pressures the spark is too weak and is snuffed out.
A cracked distributor will do the same thing; as the plug resistance
under compression builds, the spark finds easier paths to ground or to
another plug.
We usually find the distributor parts worn enough for replacement
at 1000 hours. How old is that mag?

Dan

Carbon track in the mag distributor?

--
Cy Galley - Chair, Emergency Aircraft Repair
Safety Programs Editor - TC
EAA Sport Pilot

"nauga" wrote in message
news
Update:

New harness installed today. Same problem,
#3 doesn't fire on the right mag, whether
it's on the top or bottom plug. Runs like a
top on the left mag, top or bottom plug.
I'm going to try to get a new mag ASAP,
but Unison said they're usually all or nothing,
that it's not likely the mag would miss a single
cylinder. I've gotten plenty of local help, and
everyone here thinks it's the mag but not a
common failure, especially in one so new.
If it's *not* the mag, we're all at a loss
for ideas.

So much for flying the time off and taking it to
CA anytime soon.

Any bright ideas?

Dave 'sparky' Hyde

Unison said they're usually all or nothing,

Wow! Having had numerous failed Slick mags over the years in several aircraft I
have never had an 'all or nothing' failure. They've run real bad but still run.

Jim