My GPZ is a beauty, but I've got starting issues again

[richrock]

Right. After the last debacle of my pride & joy ('99 Kawasaki GPZ500s) not starting, I took it to the shop, got the fuel system cleaned out, carbs done, ended up having new fuel lines put in. Even new plugs, replacing the new ones I'd just put in

Ran like a dream for a while. Until...

It started struggling to start. Sometimes it would fire up first time, others it would take ages. When it did start, it runs fine, ticks over fine. It has got a nasty habit of backfiring a bit on deceleration.

So yesterday I stripped off the bodywork, checked the fuel system, it's all fine there. I went through the electrics next, found the cause of a dodgy indicator (parts to be ordered).

I have the following:

No discernible smell of petrol when cranking over, unless I've been using the choke.
Yellow spark on the plugs. Plugs are clean & dry, look good according to the manual.
Absolutely random chance of it starting first or second time (I'll give her a chance as she's old). One time I had her start 3 times in a row and thought it was solved. 'I'll try it one more time' and failed to start.

My research (extensive goooooogling) reveals that a yellow spark = weak. So I'm considering changing the ignition coil as this may be the cause. Would this be a reasonable assumption?
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I has a motorbikes: Kawasaki GPZ500s (1999)- theory passed 12/06/12, test passed 10/07/12

[P.addy]

Got a different spark plug to check? Grab one, stick it against the head and check for a blue spark, if there is no different then I'd start looking at the ignition circuit/coils.

[stinkwheel]

Seperate ignition coils on that bike so unlikely both would have gone bad at the same time.

A very common thing on them is for there to be a corroded wire on the main loom. It's the one that carries the main power to the whole bike.

Look for the starter solenoid, the main fuse is on top of it. Pull the cover off and check where the fuse attaches for corrosion. Also check either end of the fat white wire running away from this for corrosion. I've had three kawasaki 500 twins and this has happened on all three of them.

I've also had three ignition switches go bad. They make and break several connections when you turn them on. They can fail in such a way as the ignition lights come on and the starter turns but the CDI doesn't power-up.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[richrock]

I'll check the big fat wire, thanks. The fuses are fine, I replaced the 10a and 30a to be safe.

When I start the ignition, I get a single click in a little box with fans (rectifier?) and it starts to crank. Is that an indicator of it powering?

Sorry if I'm sounding dull, but I'll be taking another look tonight, so anything to look for is well appreciated
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I has a motorbikes: Kawasaki GPZ500s (1999)- theory passed 12/06/12, test passed 10/07/12

[richrock]

Stinkwheel, you are a genius! I cleaned off a couple of the contacts you mentioned, the lead was fine. I put the key in, wiggled it, and started first time.

Then I did it again, started each time after.

I did try starting before I did anything, and it just cranked over.

Looks like I'm ordering an ignition switch now...
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I has a motorbikes: Kawasaki GPZ500s (1999)- theory passed 12/06/12, test passed 10/07/12

[P.addy]

Man knows his GPZ's.