Which plugs/wires are fed by left mag?

by Al C in 912 / 914 Technical Questions 12 years ago

If a Rotax 912UL is getting excessive RPM drop when the right mag is switched off on runup,
which four plugs (and their wiring) need to be investigated?

We have had in the past a bit more roughness and a bit more drop when turning off the right mag than when turning off the left mag on runup in the past. All within specifications. But not symetrical.

Put in new plugs today. Old plugs had 200 hours on them.
(They looked text-book perfect -- really probably din`t NEED to be changed)

Install per Rotax specs, small amount of heat-transfer compound on upper threads, used torque wrench to tighten exactly.
Starts and runs fine...
BUT: After new plugs installed on run-up when turning off right mag we now get an even greater (and out of spec) RPM drop. Like a 500 rpm drop now.

Theory-1: Is I wonder if maybe I didn`t get one of the plug leads/caps on really snug (although I double checked)
or somehow got something on one plug`s electrode?
Or got one plug`s lead/cap loosened off its wire?
Theory-2: Is that whatever was going on before I changed the plugs was excerbated during the change. Like maybe I damaged and already defective plug lead during handling while changing the plugs.

In any case now need to investigate what plugs/leads are fed by the left mag.
Don`t know which those are.

Anyone know specifically which plugs/leads are fed by the left mag?

All help appreciated as knowing which four to investigate could cut down my trouble-shooting and wire-checking time by half :unsure: .

Al

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Re: Which plugs/wires are fed by left mag?

by lloyd english » 12 years ago

lloyd english

2 4 0

check the wires on the cdi have found some to be broken and they will work intermittently, usualy work fine untill disturbed note it won't look broken as the insulation is in one piece its the wires inside that break.

Thank you said by: Al C

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Re: Which plugs/wires are fed by left mag?

by Roger Lee » 12 years ago

Roger Lee

125 5.11k 4.52k

Hi Al,

This doesn't quite sound like a single plug problem. The 500 rpm drop is a little too much. It could also be a combination of 1-3 small issues.
There are a couple of things to check. First I never and most people I know don't ever leave plugs in for 200 hrs. I know what the manual says, but it's a poor practice. 75-100 hrs and toss them. They're too cheap to mess with. Make sure the gap is .023 to .027. The wider gap is for the warmer OAT that are coming up now. An excessive gap in either direction can affect the mag drop, but not usually this much.
Have the carbs been synced in the last few hours? Has the carb needle clip been changed at all? Double check your ground wires. They are considered hand tight, but wrench loose until proven otherwise. Could a plug wire have been accidentally swapped. Just to be safe double check them. You may have a bad connection on the plug cap where it threads onto the plug wire. You can unscrew each cap on the affected side and trim the wire back 1/4".
Usually these types of issues are simple corrections, you just have to find it.
I have attached the ignition diagram.

IgnitionSchematic.pdf (You do not have access to download this file.)

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Al C

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Re: Which plugs/wires are fed by left mag?

by Al C » 12 years ago

Al C

63 138 106

Thanks for the diagram, Roger.
What I CAN'T tell on it is this:
Obviously ignition circuit A and B are the left and right dual systems.

But which is which?
My mag switches are of course marked "left" and "right" not "A" and "B"?
Can you or anyone tell me if "A" is left, or right?

Without knowing THAT knowing that circuit-A supplies
1 - 2 top and 3 -4 bottom
doesn't tell me if it's the left mag or right mag feeding those plugs.

---------------------
As for your suggestions, all worth checking since at the moment I'm stymied.

The gaps are right. Both on the old plugs and the new.

The carbs have NOT been synced in the last few hours but this bigger drop has happened suddenly and specifically exactly after I changed the plugs. Before the change it was running virtually perfectly.
Only a very very slight roughness when running on left-mag-only on runup testing.
Seems unlikely the carbs would suddenly get badly out of sync during the half hour I was changing plugs.

Am absolutely 110percent sure no plug wires were swapped. Am complusive, careful and meticulous about that.

Which ground wires are you talking about checking, Rodger? And where are they?

Your suggestion of bad connection on the plug cap sounds, so far, the most likely, as I did of course handle thoses during the plug replacement.

Al

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Re: Which plugs/wires are fed by left mag?

by Al C » 12 years ago

Al C

63 138 106

Thanks Lloyd,

The intermittent nature of the earlier milder form of this misbehavior is somewhat consistant with your suggestion.

Excuse the newbie dumb question but when you say the CDI are you talking about the ignition module in our case mounted on the firewall. Need to know where to look.

Al

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Re: Which plugs/wires are fed by left mag?

by Al C » 12 years ago

Al C

63 138 106

Well,

I'm both happy and very embarrassed to report that my problem was what in my business (computer assistance) we call
"Error type UFB."
The UFB stands for "User Brain Failure." :blush:

And in this case simple to remedy. :)

Some how I failed to put the ignition lead onto one of the lower plugs.
(I know _how_ I overlooked it now, but it's an embarrassing lesson on
"you can't recheck your work too many times"... Perhaps especially when it's a simply thing that I believe I couldn't POSSIBLY do wrong.)

Of course as soon as I snapped on the plug lead the problem was resolved.

The other lesson in it is a reminder/demonstration of what the purpose of the mag check is -- to reveal issues you otherwise might not be aware of.
Without the mag check we would have flown for many many hours with one cylinder running one one plug and never noticed it until the next time we pulled the cowl off.

Al

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Which plugs/wires are fed by left mag?

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