The Dreadded Miss Between 3800 and 4200

Anyone with a Mk111 is probably really keen to read this ! Here's my experience . I thought the miss , hesitation , splutter , stutter at 3800 to 4200 in 4th , was fuel related . Well maybe in many cases it is , so im not saying for a moment that this is the only causation of the problem. Ist I checked the fuel flow rate to the carb , it was good .After Id totally dismantled the carb ( single Mikuni 34 ) , and checked all the fuel / airways , float level then reassembled it it too was good . Then I started checking wiring , the bike has just been rebuilt from the wheels up and all the wiring is brand new loom , switches , battery , everything .
Lastly I figured there was only one thing remaining on the possible cause list , a busted wire inside the insulation somewhere ? ?
So , bit by bit I checked all the circuits for continuity as I wriggled and pulled the wire at the same time , nothing was wrong . SHIT !!

Then as a last resort I took the Kill Switch to bits . This switch has a poorly built normally closed contact inside it . You guessed it , with the Mk111 kill switch the little contact wiper plate zoomed out from in it's little slot and along with it's pressure spring , they shot into orbit . Somewhere in my garage . Shit .

So, using possible trajectory principles , blended with ballistics calculations , I made a bold assumption of where the spring and the wiper plate had gone . Incredibly , they were together about 6" apart on the floor in the opposite direction to where I reckonned they'd be !! ( I didnt allow for several richochets , obviously )

I cleaned the slider and polished the contacts inside the Switch Body and reassembled it all temporarily , without the spring to see what was going on in there . I could get the circuit to make and break without even switching it !! One of the two fixed contacts in the body of the switch was sitting about 2 thou lower than the face where it would otherwise make a good strong contact with the wiper plate . With force , I could get continuity , but it needed alot more than the little spring would provide . After a little scraping and shaving of the material between the two contact points , I was able to remedy the fault.
After I'd fixed the switch I recalled how one day recently , even at idle , I'd noticed a little missing and popping , but I thought nothing of it at the time .

So , even a brand new switch can be faulty.

Now , ask yourself how many hours have you searched for the illusive missing and farting at those revs !

Just a note on the side at this time , IMO the kill switch is SO much cheap crap , I'm even tempted to bridge it totally . God help anyone if they ever get any water inside it , problems forever I reckon .
Good luck , oh did I mention I love my Mk111................................................now.

Supaflee

Interesting info, thanks.
I think....

BTW, have you adjusted your iso's lately, maybe the shakes come into this more than we might think ??

Hi Supaflee.
A latching, clamped relay circuit with momentary activation resolves this issue plus the kill circuit stays killed.
Ta.

Yep needing , I agree with what you say . I've have fixed the problem however, so am reluctant to introduce yet another bit of circuitry .
Thanks
Supaflee

Supaflee said:

Yep needing , I agree with what you say . I've have fixed the problem however, so am reluctant to introduce yet another bit of circuitry .
Thanks
Supaflee

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Goodo.
PM me when the contacts fail next time (hopefully later rather than sooner). Now you know the signs to look for.
Ta.

I agree with your observations regarding the quality of the MKIII kill switch. My approach was to (horrors) wire around it. I hope that never bites me but I never used it on this bike although I have on other bikes,

I'm still keeping an eye out for a piston pin cir-clip that launched a couple of weeks back. I heard a few of the ricochets but still lost track of it. It was on an International Harvester engine and is a very heavy wire size and of course no one had it locally. $5. shipping for a $1. item.

JimNH said:

I'm still keeping an eye out for a piston pin cir-clip that launched a couple of weeks back. I heard a few of the ricochets but still lost track of it. It was on an International Harvester engine and is a very heavy wire size and of course no one had it locally. $5. shipping for a $1. item.

Click to expand...

Somewhere in a very weird other dimension lives little springs, little ball bearings, tiny screws and nuts, one off 'O' rings, clips, little brass Lucas switch parts, knobs, needles, ity bity things you've personally made to replace things lost, and of course,,, socks.

How funny supafee really helps to know sneaky shoe eves visit everyone not just me, ugh=shit. All's I have to say about not having a flick and forget on OFF switch is hope ya never learn why they exist so commonly. Forgetabout the factory one and relay or robust wire through another real kill switch direct. Don't get too smug at recent success as shoe eves may take notice again. Btw way a narrow limit of vibes that resonated in bars implies might want to diddle isos or up tire pressure to calm other conductors.

For those of you who are not familiar with the MK3 killswitch. This is what you can expect when you open it up. And when you remove those two screws, things fly out. A true Rube Goldberg by Lucas.
Jaydee

jaydee75 said:

For those of you who are not familiar with the MK3 killswitch. This is what you can expect when you open it up. And when you remove those two screws, things fly out. A true Rube Goldberg by Lucas.
Jaydee

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That's the easier of the two, the Hi/Lo and Pilot/Head switch is a bear.
Pete

Doesn't the Mark3 have the same Lucas switchgear as the last of the line Meriden T140 elec start jobs?

My mate bought a brand new one in '83, he only had it a little over a year before it got nicked, he caned it horribly, but the only time it let him down was when the bleedin' kill switch played up (and he had to get towed home by a Ural combo)!!!

Eddie,
I think it shares the same part number, but the original on my MKIII has a center run position and two off positions. So it is: OFF-RUN-OFF. The later replacement one is: OFF-RUN.
Pete

I have just come up with a new part number for the Mk3 kill switch
"SH1T"
Supaflee,,,,,,,,,,,,,,and yep , Im gonna bridge the switch in case of future niggles caused by it's design / function faults .

Supaflee,
If you want to bypass the kill switch it might be easier to jump the connection in the head light shell. I believe the solid white wire is the power for starter switch and ignition, white/red is the starter and white/yellow is ignition. The other possibility is to take the power right from the key switch. White wire (switched power) directly to what ever type of ignition you have. I'm working from memory on wire colors. I think if you get the switch clean and lubed it should work fine.
Pete

The oem kill switch works fine if in good condition when cleaned up. Expecting a 50 year old switch to be OK without some attention is a bit much...

I agree that if it's worn out, then that's a different deal.