Battery Voltage Under Load

I'm no Electra expert but that seems a very small capacity battery for an electric start bike. The voltage will drop quite a bit when starting but not as far as 9 volts. Most bikes I've tested with correct and healthy battery stay above 10 volts while cranking. 

It also sounds like your ground wire is too thin to carry the current which will make the voltage drop worse. 

Ian

13V is normal for a well charged battery. Is the 9V reading between battery poles or between battery and frame earth?

The battery on my little 300 Ninja twin is at 8Ah.

A voltage drop of nearly 4 volts is too much. It sometimes indicates a failing battery, They can read 12-13volts but just not deliver the amps. An auto electrical establishment or most garages can test your battery to see if it's up to spec.

I think there must be some useful amps in yours though, if it can smoke the leads.

Heat is the result of resistance, too thin a cable will cause resistance and heat up. I would recommend a 16mmsq cross section cable for both starter leads.

The other source of resistance is the connections. These must be clean, bright and tight where they contact, and have the maximum area of contact available. The earth is as important as the live. The end fittings should be tightly crimped or soldered... Take no prisoners!

Regarding the Amp hours of your battery, this does not mean it will only deliver 6 amps, but rather that it will deliver 1 amp for 6 hours. You need to look at the battery's CCA rating; that is cold cranking amps.

If you are going to replace the battery I would recommend a Motobatt, which is a gel type. Get the largest one that will fit. You will find that Motobatts often exceed the amp hours of an equivalent sized conventional battery and often have very respectable CCA  figures too.

You're going to be limited by where the battery fits but look for the biggest Ah capacity that will physically fit on your bike. I have a couple of Motobatt batteries and they give pretty high cold cranking amps for their size. 

Hi

you should have a thick earth wire from the battery to someware near the starter motor if you don't you will burn your earth wires out in your loom should be the same size or bigger than the feed to the starter motor