Suzuki GS650 voltage drops to 8V. Won't start.

[steviewilson87]

Had an intermittent fault with my 1981 Suzuki GS 650 that's now became a bit of an issue..... The f****r won't start anymore!
 It cranks fine & the battery is holding a charge. Spark is pretty weak (swapped plugs for new). Battery drops to 8V when starting, back to 12V when releasing start. Checked coils & getting 4.5ohms & 13kOhm primary/secondary sides. I'm thinking short to earth, but wondering if anyone has any ideas? Cheers!

[R]

I've had a battery that had those exact symptoms.
It wasn't long for this earth after that....

Can you try boosting the battery by jumper leads, say from a car.
This would say for sure if its just a low battery, or if you really do have a problem.
Be ready for a quick disconnect of the hot lead from the boost battery...

btw, you say your battery "returns to 12v".
12 point how many volts, precisely ?
A battery at 12.0 volts only is already rather dead...
hopethishelps.

[Rex]

The battery's had it. Jump lead it from your car to prove.

[steviewilson87]

Sorry, I've already proved its not the battery. I connected my car battery straight to the motorbike battery tails, and it was still immediately dropping to 8V & cranking like crazy.

Its got progressively worse (had the bike a month!). Started fine, then took a bit of effort, now won't start. Battery sits about 12.5V.

[Rex]

Does "cranking like crazy" mean it was turning over at the correct speed on the starter?
Measuring 12.5V doesn't really mean a great deal; take a dead battery out of your car and you can still get 12V on a voltmeter but it's still knackered and won't turn the car over. "Surface charge" and all that...

[steviewilson87]

New to bike mechanics to still trying to figure it all out. It was turning over a lot quicker with the car battery than it normally would. I could borrow a load tester and test it. I'm guessing cranking too quickly would make a difference? Thinking checking the air filter too? New plugs have done maybe 150 miles & they're black at the electrode. Cheers for your help

[R]

Where are you measuring this 8v at then ?
For a good car battery to drop that low, the jumper leads should be glowing red
and smoke would be coming out of something.

Cranking like crazy sounds - well, crazy.
Can you remove the starter for a look at its insides, to see they are intact and good.
Has the bike lost all compression for it to do that ?

This doesn't bode well....

[petrolderek]

could be a engine,starter motor or a battery issue.
You need to isolate the fault.
 If you are confident the battery is good the starter motor could be pulling too much current and pulling the battery down under load.
 First dose the engine run ok when you can get it running if so its unlikely to be an engine problem.
What I would do is get a second battery and wire the ignition from the second battery and just use the bike battery to crank the engine and see what you get. If it starts and runs ok its a battery or starter issue if its the same its an engine issue.good luck.