Gipsy Major that won't stop!

John Rickett · Post by **John Rickett** » Sun Jan 01, 2012 11:32 am

Can any of you engineers out there help me with a baffling problem.
A fault has developed which is stopping the left BTH mag (non-impulse) from grounding. The only way to kill the engine is to turn off the fuel.
I have checked what I think are the obvious things - mag cap making good contact with the central electrode, wiring to the mag switch including the switch itself and everything seems to be ok.
I have had the switch apart and the toggle is making good contact with the poles. If I connect a multimeter (on ohms) from the carbon pick-up in the mag cap to ground and operate the mag switch, the readng goes nicely from infinity to 0.1ohms.
Has anyone any suggestions as to where else could there be a poor connection which is preventing the mag from grounding.
Thanks in anticipation.

G.Dawes · Post by **G.Dawes** » Sun Jan 01, 2012 8:31 pm

Check the other mag, it may be the good one you are checking marked up wrongly, The P lead might be swapped. Just a thought don't mean to make you sound silly but I've come across this type of thing before.
Graham

David Hardaker · Post by **David Hardaker** » Sun Jan 01, 2012 8:40 pm

If the external wiring on the left mag is doing what it should ie the left switch is solidly earthing the left mag P terminal, the fault must be internal to the mag.....

John Rickett · Post by **John Rickett** » Mon Jan 02, 2012 2:23 pm

Thanks for the suggestions.
I'm definitely on the correct mag. I was checking the continuity with a multimeter, which seemed to show that the switch was connecting to ground ok, but then had the bright idea that perhaps the switch could pass a minute current (ie the multimeter) but may not pass a high current, so made up a simple test rig using a 12v battery and a 50w lamp. With the lamp connected to the copper strip in the plug cap and ground, I found that the switch woud switch the 4 amps or so without a problem so the only bit left is the carbon bush which connects the copper strip to the central electrode, but can't see how this could be faulty as it seems to make a good contact. Unfortunately I can't see a way of checking the mag with the cap fitted.....any other suggestions please.

alant · Post by **alant** » Tue Jan 03, 2012 5:42 am

Try a new wire from the cap to a place it can be earthed without being close to the prop. I have just had a problem with a Chipmunk very similar to this and it was caused by a minute sliver of wire that needed a magnifying glass to see it.
Alan Turney
Chiltern Classic Flight

John Rickett · Post by **John Rickett** » Wed Jan 04, 2012 1:30 pm

Thanks for the suggestions.
After doing a bit of internet research I found that BTH mags were used a lot on what are now vintage British motorbikes. One site for Vincent enthusiasts gave me a lead when it said that if the grounding brush was gummed up then the 'kill switch' would have no effect.
I found that Gipsy mags have a grounding brush, its a carbon post on a weak spring and I think earths one side of the primary windings. Its the large brass screw on the side of the mag and was indeed gummed up, so was washed in avgas such that the spring would work. After refittimg and trying the engine twice, I now have a Gipsy which stops when its supposed to!
If anyone else has trouble with what at first appears to be a faulty mag switch, this is one place to look.

Light Aircraft Association

Gipsy Major that won't stop!

Gipsy Major that won't stop!

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