Modern Vespa

Having trouble starting my mp3 500 cold. After crank it for a long time and give it just a little bit of throttle. Then it will finally start.
Just did a very extensive throttle body cleaning. It does idle better but still has the cold start problem
Wondering what tells the ECU to have the injector fire every stroke in cold mode.
Could it be a bad temperature sensor? If so which one's for the ECU?

Maybe check spark plug if spark plug is old, and gap

try turning the key on/off a few times and cycling the fuel pump.

Spark plugs are fairly new.

Had tried that before the throttle body service, just went out and tried it again. Cycled it three times and didn't want to start.
It almost seems like the ECU isn't telling it that it's in cold mode, so it's not firing the injector on every stroke at first. Which from what I understand is the for better terms choke mode for the injection.

I don't understand the bit about the injector firing on every stroke. Why would it do that? Wouldn't every stroke except the intake stroke be wasted? How could fuel injection on any stroke except intake help the engine start?

Maybe I'm completely on the wrong track, I dunno.

It is my understanding like on most bikes never to give throttle as Ecu attemps to adjust, and only crank twice, if no success stop and crank twice again until it starts, weak battery also will attempt to crank but not have enough to totally do the job

Hi! When was the last time you looked at valve clearance?
I had a similar problem and resolved it by adjusting them.
Previously I'd gone through the throttle cleaning and it didn't totally sort the problem of starting.

For it to be valve adjustment affecting a cold start, it would have to be a very, very loose intake valve. My valve train is very quiet.
And since intakes wear looser, and exhaust wear tighter, it'd be very uncommon for it to be an exhaust valve being too tight causing a cold start issue. When it starts perfectly well warm.

The way the Rich mixture setting works on these motors, is to fire the injector every stroke. There will be excess gas in the intake, but it gets into the cylinder and gets fired when the injector fires for the intake stroke. So you're getting a rich richer mixture. It's like closing the choke on a carburetor. Just getting more fuel in there.
If you listen to the mp3 when it starts you can hear when that actually goes away once the engine is running.

Fixed, I think I wasn't patient enough after the throttle body cleaning. Got it started and let idle for about 5 minutes last night. Then took it for a good ride today. Now 6 hours later it fires right up. I think the ECU just had to relearn after the throttle body service.

Yep like I said above Ecu needs to adjust, had this loads of times on my bikes, problem of today is the damn Euro laws making bikes run leaner is a pain in the rear.

Still having the cold start problem.

So, seriously, when was your last valve adjust?

Pull the injector out and check it actually is working when you crank I have had dud injectors before this. and faulty pressure regulators.

Injectors definitely firing, it tries starting. Then it just acts like it's too lean. It's just not getting enough gas. That's why I think it might be a temp sensor. Not telling the motor to enrich for cold start up.

Hi again! I like the way you're thinking! I plugged in a new temp sensor(the one next to the injector in the head) and unfortunately it made no difference to starting. I'm curious if you took a moment to observe the throttle body stepper doing its thing after you'd cleaned it up.
I plugged it in before re connecting after cleaning and checked the rod for opening and closing by turning the key on and off. It works but roughly, and is far from smooth in its action. There have been a few owners since 2008 so who knows what it's been cleaned with in the past!
I had the feeling maybe it was staying shut and not allowing enough air for starting.

All I did for throttle body service is my normal yearly spraying throttle body cleaner into the idle port.
What are you talking about stepper motor and taking stuff apart? That sounds like what Cheeky Thomas did and I'd love to see how to do that.

Hi again! I remove the complete throttle body for cleaning and before bolting it back temporarily plug it in and turn the key on and off to observe the rod opening and closing.
Nothing more than that, you can't as far as I'm aware do any more.
I was surprised how much muck was hidden inside the throttle body and cannot be seen by just cleaning with the body attached.

Guess I am pulling the throttle body next. Thanks for the info.

Are you cleaning behind the side cover?
Or are you cleaning just the back side of the throttle body? I don't see the rod you're talking about that I'm supposed to see if it moves.

Hi! (08 model UK) The dark rectangle above the butterfly valve is where I can see the rod moving on mine, but yours looks slightly different from memory. My throttle body is No 78510 yours 78511.
Maybe some difference?

I did see a rod in that rectangle area, sprayed a bunch of throttle body cleaner up in there. Seems to be working, will find out in the morning. But 99% sure it's fixed this time. Thanks.

It is definitely fixed, fired right up this morning like it was new.
Just needed a much better cleaning than what I was doing. Which obviously involved removing the throttle body. Guess I have to do this every 80,000 mi or so. 😋