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-   -   Mag Drop Question (https://vansairforce.net/community/showthread.php?t=139235)

lr172 06-27-2016 08:51 AM

Quote:

Originally Posted by kamikaze (Post 1090732)

A fouled plug doesn't fire hence the roughness ... right?

Though it may not necessarily be the root cause here, i surmise a dirty lead, slightly damaged spring, poor contact, etc. can still cause firing, though with less energy, possibly leading to the symptoms described?

FYI...

Anything that cause additional resistance in path to the plug electrodes, including the plug itself, can/will reduce the energy and quality/size of spark. However, the ignition of the fuel/air mixture is binary. It either lights or it doesn't. The issue is range of conditions under which the spark will ignite the mixture. A weak spark is less tolerant of non-optimum mixture and other conditions which make the mix harder to fire. On the fringes, this often manifests as intermittent firing.

Larry

uk_figs 06-29-2016 02:24 PM

Check EGT?
 
I plan on more troubleshooting this weekend, I have a full set of EGT probes so what specifically am I looking for on the EGT test? Should they all rise the same?

I plan on the buzz box test also to see if the timing has shifted.

Not sure how I would test the leads apart from making sure they are connected properly.

Appreciate all the inputs.
Figs

Caveman 06-29-2016 04:58 PM

BAD mag, plug, lead
 
Hi Figs,

I sent you a PM.

Take a look at this:

http://www.aircraftmagnetoservice.ne...ng-guide/c15r3

You may need something like this to check your leads.

http://www.aircraftspruce.com/catalo...abletester.php

uk_figs 07-02-2016 02:23 PM

It was a timing issue
 
Checked the timing today and the right mag was off by about three teeth on the starter ring (3.5 degrees?). Fixed the timing and both mag drops are now the same (about 40 rpm), static full power run up is 2250 with the Catto which is about right and all EGT's rose about the same on the mag tests.

Still puzzled as to the root cause as the mage were cinched down and I did not notice a gradual change in the mag drop on the right.

All HT cables, springs etc seem fine.:confused:

Figs

lr172 07-02-2016 03:20 PM

Quote:

Originally Posted by uk_figs (Post 1092162)
Checked the timing today and the right mag was off by about three teeth on the starter ring (3.5 degrees?). Fixed the timing and both mag drops are now the same (about 40 rpm), static full power run up is 2250 with the Catto which is about right and all EGT's rose about the same on the mag tests.

Still puzzled as to the root cause as the mage were cinched down and I did not notice a gradual change in the mag drop on the right.

All HT cables, springs etc seem fine.:confused:

Figs

Glad you figured it out. On a 144 tooth flywheel, each tooth would be about 2.5 degrees

Caveman 07-02-2016 06:14 PM

Quote:

Originally Posted by uk_figs (Post 1092162)
Still puzzled as to the root cause as the mage were cinched down and I did not notice a gradual change in the mag drop on the right.
Figs

Glad you figured it out, Dave. I'd keep an eye on that mag for possible further drift. You may have accelerated cam wear going on. Did you notice higher egt's in cruise? Was it any harder to start? From the document I sent earlier:

Quote:

A magneto drop that exceeds the allowable limit, but is smooth, with no roughness is in most cases, late engine timing. Cam follower wear makes point opening late.
?Check magneto to engine timing.
The magneto designers have gone to great lengths to have the cam follower, or cam in the case of Slick, wear at the same rate as the point faces. This design feature keeps the magnetos internal timing, ?e-gap?, at the correct angular opening point for long periods of time.
?Inadequate lubrication of cam and cam follower will accelerate wear and result in late engine timing.
?A late spark reduces the sustainable RPM at the mag check. All the fuel is not burned and returned as energy to the piston, but is burned in the exhaust system. Higher than normal EGT will result with late ignition timing.
?On impulse coupled magnetos a broken impulse coupling spring will severely retard engine timing and result in a very large, slowly occurring RPM drop. In some cases the engine will slow down sufficiently to quit. With single impulse coupled installations hard starting will occur.


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