Odd electrical prob zxr250a

[tomtyer-drake]

alright, trying to get my flatmates zxr250a sorted...

it had a bit of a journey sideways the other day due to overheating and fluid pissing on the rear tyre.

anyway its not happy to idle at below 4000rpm, and even though this could be carb related, there is an electrical issue which is still being difficult to diagnose.

with the headlights on its pretty much useless, no power, stalls etc.

so we got stuck into it with a voltmeter today to try work out what is causing it....

heres what we got...

ignition off - battery @ 12.74v
ignition on - battery @12.70v
bike running - battery @ 13.8v

ignition on, headlight on - 12.5v dropping slowly
bike running@4k rpm, indicator switched on - v drops from 13.8 down to about 8v before returning to aroudn 12v and dropping again..
bike running@4k rpm, headlight switched on - drops almost instantly to 6ish volts and then the bike stalls.

so its dropping faster than the headlight would drain the battery by itself, and is triggered by an increase in current across the entire electrical system...

but thinking of what makes a battery voltage drop, its basically gotta be a decent short.

if its only triggered by an increase in current, what changes when current changes?
is it that the alternator gets more resistance across it? and therefor could be shorting across itself?

I'm out of ideas...

any ideas much appreciated???

Tom

[NinjaNanna]

dodgy earth. Probably the main earth from the frame to the battery, little current small voltage drop, lots of current big voltage drop.

[tomtyer-drake]

Thanks ninjananna!

well a fairly successful day yesterday...

started off by re-earthing the battery, the earth that was there was split in two, with a larger earth going to the starter and a smaller wire doing the rest of the bike...

the smaller wire had a join in in which was pretty corroded, so we cut it out, stripped both bits of wire, twisted them together, then scraped the paint off a bit of the frame and taped the wire against that, suddenly the voltage wasnt instantly dropping, but was slowly decreasing.

so we kept looking at the wiring, and found an amazing bit of melted, corroded, semi working, bodged connector block... which was attaching the reg/rec. fixed that and tried to get the bike started, it didnt want to start (not a new thing) but with some coaxing on the choke it finaly got going, but wouldnt idle off the choke, until we wacked the throttle fully open at the same time as closing the choke... success! it actually revved and sounded a bit like a zxr250 should sound!

next jobs are getting the carbs setup (like, vaguely balanced) the plugs cleaned and regapped, and the exhaust hole wielded, then it might just be a happy bike...

if not, it will need valve clearances...

its gonna be a busy summer!

Tom