750GSL Jump starts, stops running when I pull jumper cables

[Dignan]

So I'm a big fan or Robinjo's 750 cafe racer. I want to try a few of the trick things he did to that bike on my '78. Sunday I decided to do a little investigating; moved all the wiring/battery/etc out of the center triangle just to see if I could hide it all under the seat without splicing wires and all that mess (90% sure it's possible). I labeled everything, and put it all back as I found it.

Now the bike doesn't start.
Before, I could kick start it every time. Electric start never worked, but I figured the battery was just too dead to do the cranking. Got a new battery today, and still... no dice.

The bike does start & run as long as I have jumper cables on it, hooked up to my car (it even starts using the electric start). The second I pull them, the bike dies. All the lights still work when I turn the key to the "run" position, but the oil pressure + neutral light don't.

Maybe there's a ground I'm not seeing? Main fuse (red wire to +) is still good.
Thanks for any info.

[Steve]

First of all, check the really obvious...you "got a new battery today". Is it a wet-cell (flooded) battery? Did you fill it properly with acid? Did you take the time to charge it properly? (Hint: if you get a new battery, then charge it properly, you won't be using the bike for at least 24 hours.)

Next, take a look at exactly where you have your jumper cables connected. Since the bike works with the cables, but not without them, there is something wrong between where the jumper cables are connected and the bike's battery. If they are clamped to the wires that are attached to the battery, check the cable connections to the battery or even where the connectors are crimped to the cable itself. If they are clamped at the starter relay, you might suspect the entire cable.


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[Dignan]

New battery, charged 24ish hours on a 1.5 amp slow charger (which reads "full charge"). Wet-cell, filled at the local bike parts shop. Jumper cables connected to the wire attached to the battery, at the crimped connector on (+) side, frame on (-) side (doesn't start when I hook cable to cable on (-)). All looks good, but still no idiot lights when I switch the key to "start", and it still doesn't kick over. Neutral light brigtens when I kick, but the bike is otherwise dead (starter button does nothing.. not even click). TS/headlight/brakelight still work like it's cool.

I'm going to go through the wiring diagram vs. my bike with a fine-toothed comb tomorrow to see what I may have screwed up.
Any ideas offhand from a similar screw-up?

Thanks again, in advance.

[Dignan]

Checked the wiring, and it all looks good. Lost my test light and my multimeter stopped working, so I couldn't test anything.

Odd thing:
When I was poking around in the dash/headlight/TS wiring, I moved the turn signals (actual hounsings) and the idiot lights came on. Kicked it over and it fired up just fine. Previous owner installed aftermarket turn signals and the wires were spliced pretty sloppily. I fixed all that, cleaned grounds and tightened it all down and previous problems came back.
Checked all the wiring in that area very well, and it all looks solid.

Also, it only starts when I ground the neg. jumper cable onto the frame. There has to be a bad/disconnected ground hiding somewhere. Right?

[Billyboy]

Right. Or you got screwed on the battery.