Kill switch backfire

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Mar 21, 2006
A few days ago when I last wrote the bike it was going really well. Everything felt great. When I started it today it fired ok but I could barely keep it running. It was misfiring and backfiring horrendously. I mused over this for a while and decided to clean and fettle the kill switch. The engine fired and ran absolutely normally - no misfire, no backfire.

The design of these Lucas switches beggars belief. It seems that the switch pad return springs rest against the phosphor bronze contacting leaf, partially fighting against it even before you press it. Over a period of time the leaf will tire and chatter against the contact in the kill postion, thus intermittently cutting the ignition supply volts.

Crikey! What an ill-thought out device! Am I wrong in my assessment here or has anyone else come to the conclusion that these switches are terrible?

Scim77 (850 Commando Mk1)
Spot on assessment, even though mine is still connected, it's only there for emergencies, i use the ignition switch to turn off the power.
I have to agree!

One of the best 'reliability' mods. you can do (in my opinion) is bypass the kill switch.
I threw mine away years ago when I fitted the variable-ratio master cylinder and the Tommaselli twist-grip.

There's always the option of pulling off the plug caps :)

What is actually the intended purpose of a kill switch anyway ? Is it some obscure vehicle regulations thing ? The only time I've ever needed one was when a Benelli two-stroke started dieselling itself to self destruction and of course it didn't help then :oops:
Come on people get a life! This is a relatively minor part on a bike that is over thirty years old! What do you want warranty repairs? A safety recall?
I'd like to see some one put out a product, trying to make a profit, that designed everything to last as long as my kill switches have.
Disconnect it? Great idea! There is a guy that posts to INOAlist who's Norton hung wide open and his gas tank split, apparently from the vibration, as he came through the Hampton Roads tunnel one night. Ask him how hard it was to reach down and find the key as his lap filled with fuel. Suggest he pull the plug wires!
Fix your kill switch and quit crying.
A bit grumpy today Skip :( your fortunate you haven't had a problem, i think the switch design is poor and lends itself to problems, but for the reason you stated mine is still connected, i just don't use it.
What about when your bike cuts out when you really need it to 'go'? Overtaking? Turning at a junction? Having a bike cut out can be just as dangerous.

Fix it?

The internal parts disintegrated long ago!

I have an 850 MkIII, so no 'reaching down' for the ignition switch is necessary!
If you use the kill switch as an earth to switch a relay, rather than as a direct conduit for the coil feed you will find a great improvement in reliability and current to the coil. There was some discussion about this on this forum a couple of weeks ago.
Skip said:
Disconnect it? Great idea! There is a guy that posts to INOAlist who's Norton hung wide open and his gas tank split
Fix your kill switch and quit crying.

If I was that paranoid, I'd wear asbestos underpants and carry a parachute. Come to think of it, anyone know if Martin-Baker make a 12v +ve earth ejector seat ?

All I can say is that in 30 years of motorcycling, I've never needed one. They knock themselves off with tank bags, small children can't leave them alone and they fill up with water on a daily basis. Just another problem looking for an inconvenient moment to arise.

Still, perhaps in the land of compulsory standardised switchgear, left foot gearshifts and safety warnings on everything, it all seems a bit more reasonable.

I could just about argue a case for those wrist-lanyard cut-outs that sidecar racers use for in the event that they separate from the machine. Nothing quite like that feeling of your pride and joy sliding down the road in front of you with the throttles wide open. A kill-switch doesn't help much then :)
Skip didn't use much diplomacy with his comments...but I for one agree with him wholeheartedly.

The old pull the plug wire or use the ignition switch trick, just doesn't cut the mustard in certain circumstances.
I have had to use my kill switch a number of times over the years. I'm not the most balanced of riders, at a a standstill or walking speed, on tip-toe with a couple hundred pounds of machine in delicate balance as I try to get the machine through the doorway to the courtyard, make a u turn or get into a right angle parking space. My boots have been known catch on a cobblestone or two or some nice passenger has been known to decide to stand up and step off the bike before I had my feet planted and was ready for it......add all this together, and the the bike goes for a quick tip to the side which is more than a boy can handle if the the tank is full and the tank bag is full, not to mention being full myself one time. Nice place to be, with a running motor, it still in gear, the clutch lever pulled and all of a sudden, you can't hold it up, you can't get it out of gear, you can't let go of the clutch lever, and the motor runs a bit faster when you are trying for dear life not to drop it and you twist the grip a bit. And, you might not know it, but the motor will continue to run with the bike on its side too...take my word for it. And surprise.......the tank cap isn't leak proof either. One thing's for's going over, you just try to make it as gentle as possible.

Where are your hands? On the plug wires? On the key switch? Nope...they are trying to hold the girl upright, with ALL their strength and at that moment, lads and will be glad to be able to push that kill switch and have it work. Don't tell me different, I won't believe you.

Those of you that wish to ride without a kill switch, be my guest, but don't complain after the fact, when you have made a crispy critter of your machine, and your family jewels have been toasted.

Take Skips' intellegent, if rather poetically put suggestion, and get the kill switch fixed or replace it with a working one, before you need it, not after the fact........ :wink:
kill switch misfire

Thanks for all your interesting input on the kill switch. I think the red mist may have even got to me when I was sorting my switch out and I probably described its inner workings inaccurately. Nonetheless I have tweeked mine and made it functional again for exactly the reasons put forward by Skip and HeWho.

It is unsettling however that a sudden and dramatic misfire (plus hellish backfiring which woke the neighbours early on Sunday morning when I was trying to sneak out for a blat) can be induced by this little switch. I will check it out more routinely in future.
Looks like I'm on my own on this one so I'll shut up.

I would add that I removed mine because I was tidying up the switchgear and controls and just found it superfluous, nothing to do with not being able to make it work. If I was that defeatist, I'd have given up with Nortons years ago :)
Chaps - Just my 5 cents worth: when those Mk1 Amal slides stick at large throttle openings, you'll be glad you had one!
Well their good for plug chops. But I must say that they have caused trouble with my bikes to the point were I have disconnected them in the headlight. There has to be a better way IE relay but just the thought of having this problem come up again gives me fitts. The symtoms will have you looking everywere and it's one of those that will fix itself only to reappear later. Rain is the enemy get caught riding a day in the rain and it will come up sooner or later.
I wouldn't be without a kill switch...saved me twice so far, once when the Mikuni slide hung up when I first got the bike ( rust in tank and hadn't been ridden for a while) and again when an inner set screw vibrated out of the two into one carb manifold and held an inlet valve open. The only way I could start it was with the enrichener open and it immediately went to 7000 and climbing.
Have Hi Temp Locktited the two inner setscrews in manifold and sealed the tank (POR15).
I have had no trouble yet with any switches, but there are aerosol products available that might be of benefit. Electrical Contact clean/lubricant/sealer all in one. This is not your run of the mill CRC/WD40 but specially formulated product normally available from Electricians Suppliers.
Jay in NZ
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