Well in that case I'd suspect some other wiring fault. Handlebar switch, side stand switch, clutch safety switch, etc. Bad wiring, worn insulation shorting something, disconnected thing, etc.

Just have to debug.

I've done that just want to know what could cause no power to the ignition. Could it be the ecu. I had problems with that a while ago and sorted it. Well thought I had. 

There's no ECU on a GS500. Get a wiring diagram, they're all over the internet, and trace it, you unfortunately have to basically chase down every wire. Start with the wiring going to the starter solenoid. When the start button is pressed, is it hot? No? Move to the front side of the start switch. Is that wire hot? No? Move to the next component, the run/start switch. Is the wire feeding that hot? Etc. You just have to chase the entire wiring of the bike. No easier way, no shortcuts.

But it's likely the side stand relay or side stand circuit, clutch switch or circuit feeding that, etc.

If there are no LIGHTS (not just no-start) then something is bad between the main fuse and the ignition switch. Might even e a bad ignition switch, or the wiring from the ignition switch to elsewhere.

Yep you're right the ignition switch is my next test. Really hope I sort it soon me n the old girl have been through a lot. Dirt quake and lots of other kinds of other crazy races. It is getting to though.
I've raced a lot of bikes over the years and never had this much hassle. When I said ecu I meant cd I!,

+1 on it most likely being the side stand switch or clutch switch.

I by past both of them when I first had the problem no joy am afraid.

The problem most likely is not the actual side stand switch or the actual clutch switch. It's more likely the WIRING to the side stand switch, the side stand relay, or the clutch switch wiring.

Bypassing the parts locally doesn't rule out the wiring being at fault. Wiring, corroded connectors, wire insulation worn and shorting against the frame, etc., all very common.

New loom then could be there for months in that case. On the positive side of things I put a by pass switch in between the starter selinoid and if I press the original starter but nothing happens but if I keep my finger on it and just flick the bypass the starter stays on. Means that the original starter switch still has power. I'll be able to get the multimeter in the new year when the work shop opens. So I can properly test it. Also I've been looking at the wiring diagram and the cdi unit does connect to the starter switch and stop button.

Mmmm it's gone quiet maybe the cdi is ecu ?  Definitely looks like one when you look at the wiring diagram.please can someone that knows help.

No the CDI is NOT an ECU.

It's a CDI. "Capacitive Discharge Ignition" module. It is not the cause of your problem, well most likely not. Are you getting a spark? If so, the CDI is working.

Seriously. You are going to HAVE TO CHASE THE WIRING and follow the wiring diagram to do so. There's no parts-swapping shortcut.

Just found out that if I short the starter selanoid it runs but the starter stays engaged..if you then take off the starter wire it runs with out but only with the lights on. Think it's trip to a auto Election.....
Because I've been through the wiring over n over again. I read another thread on here that some one had replaced the loom and was having the same problem. But just my luck he never finished the thread saying what the problem was.  And just for the record I've looked at the wiring diagram over and over again

Still no joy!

Put a new loom on it nothing

Bummer--I am sure there are people on this forum that can help you out. As for me, I worked on old Brit bikes in the '60s. The electrical systems were not as reliable, but they were easier to fix. Best wishes.

Cheers matey well I left her in garage for a while am back trying to sort it still no luck.  I really can't think what it can be. Is there any way fo wiring it to another switch may be.?

Problem solved it was the starter relay. I put a brand new one off eBay way back and it didn't sort it.  I tried everything once more so I put a second hand starter relay (selanoid) on and bingo the shed is up and running happy days.  Thanks for all the replies 😄😄😄😄😄