Modern Vespa

Hi first time posting long time reader. I have a 2016 MP3 300 Yourban. I didn't use it for couple of weeks. I decided to start it as it had been sat. It started fine a ran fine for about 5 mins. Then there was a noticeable engine sound change the rpm dropped and it ran rough then the check engine light comes on. If you turn the bike off and then start it again it runs fine again but if you leave it ticking over it starts to run rough after a few mins. If you ride it it's fine the whole time. Stop for 5 mins ticking over and it runs rough again and light comes on. Any ideas cos I'm thinking injector or Lambada sensor?
Cheers nick

How old is the battery? The battery is usually the first victim after the bike sits, and has a useful life of about 2-3 years.

Have the battery and charging systems tested first.

So I tested the battery and it's float voltage ie bike not started was 12.82v. bike started voltage jumps up to 14v so in my opinion the battery and voltage regulator are fine.

Voltage is useless, it's all about the amps (Soh, soc, age, temp, etc).

Have the battery load tested and see if it passes mustard.

as a test unplug the lambda and try it. normally this will cause an engine light but still run (and idle higher than usual).

I've got a proper battery tester coming so when it shows up I'll probably test the battery. I unplugged the lambda sensor and it didn't make any difference to the way the engine ran at all. One thing I did notice is there is a lot of crank case pressure when you remove the dipstick with it running. I've ordered a compression tester to check if the rings are worn and causing this. Would this cause rough running and engine light on??

Right so far I've tested the battery and it's perfect tested with a proper load tester. I've checked the voltage and idle it sits at 14.3 volts and rises to a steady 14.5 volts. I unplugged the lambda sensor and it does nothing to the way the engine runs. I've removed the injector and powered it up and sprayed intake cleaner through it. It's seems fine.
I've even changed the oil and filter and cleaned the air filter. Removed the throttle body and cleaned that to. Still it runs fine for 5 mins and then starts to run noticeably rough and then the light kicks on. Turn the bike off and then it runs fine for 5 mins and does the same again. I've got a new HT coil coming to change that incase it's that. It's had a new plug but I did notice the old plug was dark so possibly running rich.

Did you measure the battery voltage when it starts to run rough? A long shot, to be sure, but batteries act as voltage regulators for quite a while after a largish-current discharge (the starting current in this case).

Yes checked the voltage during rough running and it stays the same.

I'm starting to suspect fuel pump. Did the bike ever sit for a extended amount of time?

2 weeks at the most. Would fuel pump turn on eml light cos I didn't think so

Does the fuel pump prime when turning the ignition without making odd noises?

You should see some fuel being sloshed in the tank when the pump primes.

A fuel pump is pretty reliable. The only 2 things that are the kiss of death is running the tank low on fuel or fuel contamination

I'd focus on 3 items. Fuel pump, ignition and O2 sensor

Coils can go intermittent with heat - trouble is, you'll have to build a test rig to prove it without just replacing it - but it sounds as thought you'd be able to do that.

So update. I checked the valves and the intake were lose and the exhaust valves were tight so redone them to spec. I've changed the HV coil and I took it for a long ride good 30 mins. At no point did the EML come on. Got home left it ticking over for 5 mins boom rough running and then EML comes on. So it's not valves and it's not HV coil. Bike rides absolutely fine when EML light is not on. I did notice when it ticking over it sometimes idles a little higher and then drops back down. Not loads just maybe 100rpm.

Fluctuating idle usually is indicative of false air being sucked in aka vacuum leak, and causing the AF ratio to be thrown off which can cause the CE light to come on.

So I replaced the intake manifold a while back as it had holes in it where the rubber was old and it had worn. I've checked everything else regarding air intake so it's not that.

So I'm posting an update. I replaced my front steering bearing and I've now been riding it for 2 weeks. This seems to have now solved the issue of the EML coming on cos it hasn't done it since I've been riding it for 2 hours or so every day. Obviously I don't mean replacing the steering bearings has solved the EML just riding it seems to have solved it. Tbh tho I can't wait to get rid of it as it's been nothing but issues since I got it. Bloody thing!!! No idea if there all the same but my god Ive had to fix and replace so much on this bike and it's only done 30k