How to 'Kill' a Lucas KVF Magneto?

[passenger0_0]

I went for a nice ride on my Rapide today and found that pressing the recently fitted low-tension magneto cut-out caused the engine to drop down to one cylinder but kept idling.

Previously I had used the decompressor but as we know this is not a good thing so fitted the kill button.

I know the idle is a little fast(not a plonking steam engine) but nothing out of the ordinary.

The irony here is that usually these magnetos fail to spark. This Lucas KVF magneto was rebuilt 7,000 miles ago and failed at around 3,000 miles with loss of spark on one cylinder. This was diagnosed as the cam ring not being properly timed to the magnetic poles.

Does anyone have any suggestions as to what could be causing this mag to keep sparking on one cylinder when the LT is earthed?

 

[david bowen]

Go to Bunnings they sell boxes of sparks at half price special sale, also left handed screw drivers

 

[greg brillus]

Some of those aftermarket end caps with a "Kill wire" facility are a bit ordinary, the small brass holder for the carbon contact piece tends to move around.........Perhaps the mag still has another hidden issue that is being highlighted by the end cap............Check the earth brush at the other end is in good contact with the armature as well. Cheers.................Greg.

 

[Chris Launders]

The only parts involved with the cut-out are the end cap and centre bolt, as Greg says the reproduction ones can be quite poor plus your centre bolt could be slightly bent or the whole points plate slightly off centre, none of these will effect the spark in use but could when using the cut out.

 

[greg brillus]

As an example of how odd electrics can be, the other day I was running an experiment on a twin to check each cylinder......... I had shorted out the earth electrode on a spark plug and tried it alternating from the front and rear cylinders, whilst the other plug gap was normal. After firing up several times and running the engine up to about 2000 RPM the cylinder with the shorted out plug was still trying to fire.........More missing than a constant even beat, but I could not see how that could happen. It did the same when I swapped the two plugs over and ran it up again, same result. Go figure..............

 

[nkt267]

I went for a nice ride on my Rapide today and found that pressing the recently fitted low-tension magneto cut-out caused the engine to drop down to one cylinder but kept idling.

Previously I had used the decompressor but as we know this is not a good thing so fitted the kill button.

I know the idle is a little fast(not a plonking steam engine) but nothing out of the ordinary.

The irony here is that usually these magnetos fail to spark. This Lucas KVF magneto was rebuilt 7,000 miles ago and failed at around 3,000 miles with loss of spark on one cylinder. This was diagnosed as the cam ring not being properly timed to the magnetic poles.

Does anyone have any suggestions as to what could be causing this mag to keep sparking on one cylinder when the LT is earthed?

I wonder if you are getting a high resistance path to earth from the kill button instead of a dead short and this kills the 'weak' spark but not the other?..john

 

[david bowen]

A friend in perth was racing his Vincent it had a kill button same effect take button off and try

 

[passenger0_0]

I wonder if you are getting a high resistance path to earth from the kill button instead of a dead short and this kills the 'weak' spark but not the other?..john

Thanks for your replies so far.

I've checked the continuity of the kill button and it definitely has no resistance as I had wired in a separate earth wire from the switch to a shared ground to get over the painted handle bar.

Interesting point about the 'cheap' magneto end cap potentially being the problem as I was very unhappy with the quality of the rebuild previously done.

Please keep making suggestions as I don't want to use the valve lifter more than I have to.

 

[vibrac]

Another good reason for a head decompressor

 

[greg brillus]

Are you sure the carbon contact on the end cap is held up against the center screw.........The other thing to try is run it with the end cap off, and using a piece of insulated wire earthed to the engine, and touch the other end of the wire against the spinning center screw and see if it kills both cylinders. If it still fires on one, then the issue is further within the mag, possibly a dirty slip ring.........Some of the pickups have very poor carbon brushes which leaves a trail of particles around the slip ring, this can cause issues of misfiring or jumping to the safety gap screws. The better brushes have a small groove along their length, these seem to give little trouble. Another point is the after market points assemblies (as opposed to the genuine brass bodied ones) give trouble, the ones with the double sided earth contact, I have seen many where the earth contact comes loose, like a loose rivet.......and this causes all sorts of poor running issues. Cheers and good luck.....................Greg.