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MG Midget and Sprite Technical - Its as long as your (rotor) arm !

The old midget's been a bit sluggish starting in the last couple of weeks,taking 3 or 4 secs cranking to fire. Then the other morning,it wouldn't start at all.
So after the usual diagnostic checks, I swapped the rotor arm with a slightly used spare and off she went.So thinking that there was a conductive path in the rotor arm I applied 12V between the brass and plastic with an ammeter that went down to 0,5 milliamp on full scale deflection but couldn't detect any leakage.So next up was to measure the wear from the center to the edge of the brass. The duff one measured 21mm,another partly used one( a chap can't have too many spare rotors) measured 22mm. Aha I thought, there's the problem; then I measured a red rotor purchased today and that only measures 20mm.
Anyway we're up and running ok but I am a little surprised by these dimensional variances especially considering we're setting the plugs and points to +/- a couple of thou.
M J Chapman

I guess it depends on where the carbon bush makes contact and not so much where the center of the rotor is measured from

any toughts as to how the carbon bush was making contact to the rotor

Prop and the Blackhole Midget

id be courious to see what would happen if you reinstalled the old rotor agian but left it sitting proud by say a 1/8th inch from being seated firmly

Prop and the Blackhole Midget

Can't measure high voltage stuff with 12V.

A spark that will only fire .030 in a combustion chamber - gas, heat, pressure - will jump near an inch in ambient air, so air gaps in mm make no difference whatsoever. The problem comes when there is a conductor (aka MY FINGER!) too close to the air gap - the spark will then go to the conductor rather than the other side of the designed air gap. The problem with bad rotors is that they provide a short air or carbon track path to the distributor shaft, instead of HV insulation.

Many spark plugs have air gaps built in, so you get open circuit readings with an ohmmeter, but the air gap causes voltages to rise high enough to jump even a dodgy plug gap.

FR Millmore

FRM - of course - its switching HV not LV - doh!
M J Chapman

This thread was discussed between 15/02/2013 and 16/02/2013

MG Midget and Sprite Technical index

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