Modern Vespa

Cranks and turns over fine sometimes. Then, randomly, starter will crank and crank but won't start. Will try for days and nothing. Then, randomly, will.

I've done the following to no avail: checked batt voltage under load (good), wriggled every wire I can find, wriggled the kill switch, key, and starter button (nothing), and replaced the fuel tap, fuel line, and spark plug.

Am told it could be the CDI. Thoughts on that? Other suggestions?

Any help very appreciated.

Check the kill switch. I had something similar happen. I would lose power momentarily while riding. Turned out to be a problem with the kill switch.

I recently watched one of Robot's videos in which he diagnosed a similar issue. Believe it or not it was a faulty brake light switch that would not allow the scooter to start. I'm no expert so take this with a grain of salt, but it shouldn't be too hard to check. Turn the ignition switch on and squeeze the brake handles one at a time. If the brake light doesn't come on, it could be that.

Go back to the basics. You need spark, fuel and compression. It could be a loose plug or bad plug. Could be low compression. The valves could need adjusting. If its the brake switch it won't crank. The kill switch it won't crank. I can't remember if this has the kill switch relay that will let it crank but no start. Check for spake and fuel and let us know.

Thanks. Fuel is fine. Pulled line from carb. Fuel flows when cranking. Plug has spark. Doubt it's a compression issue since when it does run, it runs fine. Also, doesn't die after running. This is a 'either it starts or it doesn't' kind of issue.

But does the plug have spark when fitted and under compression? A bad plug can show a spark when held against the outside of the engine, but fail (usually broken resistor in the insulator) under compression.

Try a new plug - this will pretty definitely eliminate that. Not totally mind, new != tested and working.

Jinx, thank you for responding. Yes, replaced plug w new. No difference.

Am still wondering about the CDI. Does anyone have experience with how they fail?

Is this a 2T or 4T LX50?

You explained there is fuel and there is spark and the engine is cranking. If it is cranking things like the kill switch or the side stand sensor aso do not cause your problem. Then the starter will not work.

You say it is a 'either it starts or it doesn't' kind of issue. When it starts, how does it start? Just immediately as you are used to or does it need some longer cranking than usual before it starts? And once started: does the engine run stable immediately?

Connectors on CDI's do go bad and can cause this kind of grief. Check that the wires are all seated in the CDI. When running give the connector a wiggle. Also check your ground cables they could be dodgy as well.

I've had a CDI fail on a 2007 LX50, but in my case it just didn't want to start at all anymore. Changed the CDI and all was fine.

So I did not have the intermittent thing you have.

Many carb'd model's kill-switch circuitry will happily allow the engine to be turned over without allowing it to start.