Modern Vespa

Hi, long time lurker but first time poster!

Today, while heading home on my 2020 Primavera 125 (3,500 miles on the clock), I began to feel a lack of power as I cruised in slow traffic. The bike then cut out and I rolled to a stop. I thought it was a bit strange and the bike fired straight back up but, at the next junction, it cut out again. Nothing happened when pressing the starter switch, so I pushed the bike to the side of the road and began investigating.

Some Googling at this point showed me lots of tales of fuel pumps dying and then refusing to prime again. However, I turned the ignition on and off multiple times during the next ten minutes or so, leaving a minute or so between each try. Sometimes, the fuel pump would prime and then the bike would fire up again, only to die after a short while of idling, or much sooner if I revved the throttle. Most of the time, unfortunately, the fuel pump wouldn't prime at all, and the starter button wouldn't do anything (as to be expected).

I pushed the bike a mile or so back to my workplace and, at multiple times during the walk, I switched the ignition on to see what would happen. On the majority of occasions, nothing happened at all but, a few times, the fuel pump primed and I was able to get the bike started and running for 10-20 seconds.

Once back at work I checked all of the fuses in the glovebox, the battery terminals and the two fuses next to the battery - everything was ok. It's worth noting that the battery is only a few months old as well, and the bike is used regularly. It's been on a number of long rides recently (including the ride into work this morning) so the battery should be fully charged.

I also checked the killswitch on the side stand for any loose connections, in case it was kicking in and cutting the engine sporadically, as if I'd just put the side stand down. However, some rudimentary checks appeared to show that the switch was working correctly, although I'll need to remove the floorboard to investigate the rest of the wiring properly with a multimeter.

My main thoughts are that the fuel pump relay, fuse and/or associated wiring are loose and might have a loose connection somewhere. I managed to get the bike to idle for a few minutes and bounced it up and down, rocked it side to side etc but none of this seemed to make it cut out. I left it on its own for a few minutes longer and then it proceeded to cut out as expected.

I struggled to find a concrete answer as to where the fuel pump relay and fuse were located, so any help with this would be greatly appreciated. Any other general advice would be hugely appreciated as I'm keen to try and get to the bottom of this!

Thank you very much.

A brief update: while the fuel gauge was showing at least half a tank of fuel, I thought I'd manually check it myself!

I filled a jerry can with 5 litres of E5 Vpower and poured away - I got about 4 litres in there before it was brimmed, so it certainly wasn't low on fuel and the fuel gauge was pretty accurate.

While the bike is used regularly (even throughout winter), I never ride it until it's super low on fuel, so I was wondering if some of the fuel at the bottom of the tank was in fact really quite old. I wouldn't have thought this would explain the intermittent fuel pump priming, but it might explain the cutting out and rough running etc.

The bike started up first time after pouring in the 4 litres of E5, although it did seem to be idling quite rough. I left it alone for a few minutes and gave it a few blips of the throttle; it didn't cut out so I gave it a few runs up and down the road, using full throttle where possible to ensure that the demand on the fuel system was as high as possible. The bike felt perfectly fine and continued to idle away just fine when I parked up.

It is a little bit colder today compared to the day it broke down (18 degrees C vs 22) - I thought I'd mention this as I see a lot of comments regarding fuel pumps and hot weather (although these temperatures are nothing compared to more central and southern parts of Europe!).

Really quite confused about this and I've unfortunately temporarily lost faith in the bike as a reliable means of getting about, but I'll keep an eye on it and keep doing journeys closer to home for the time being.

I know this sounds crazy, but have you checked the tire pressures? If your pressure is low, your bike will exhibit similar symptoms.

I finally got chance to check this at the weekend - the front tyre was at 20psi and the rear was at 24psi, so both substantially under inflated. I inflated them both back up to spec (before riding the bike, so the tyres were still cold).

I rode the bike for 10 miles afterwards and it was perfectly fine, albeit in quite cool weather. I'll see how it goes in much warmer temperatures (as it was when the issue occurred).

I didn't think that the Primavera had a traction control system but I'm guessing the ABS sensor on the front wheel might have been affected? I've seen posts of low tyre pressures affecting the GTS traction control system, which is why I mention it.

Thanks again!

This sounds more like a spark issue. Have you checked that the spark plug wire is screwed tight into the spark plug cap and that the cap is tight on the plug?

As far as I'm aware, the fuel pump issues are related to the GTS and not the Primavera / Sprint.

I doubt the age of the fuel is an issue if you're running the Vespa through winter, but running some E5 is good to do and at least rules that out.

Spark plug is very cheap, so worth checking as mentioned above. If you do change it, take a few pictures of the end and post them on here. Someone will then be able to advise if the engine has been fuel starved / running lean.

When it doesn't want to start, does it chug over with lots of power - just not fire? Or does the start button do nothing at all?

I've not yet - I'll take a look and report back! That's what I thought when reading through various posts. It was just odd that my symptoms were so similar. Will check the spark plug ASAP. Regarding the issue:

Sometimes the fuel pump wouldn't prime when turning the key, in which case the start button wouldn't do anything.

Sometimes the fuel pump would prime when turning the key; the bike would start as normal but would then die within a few seconds (much quicker if you revved it, i.e. using more fuel). To me it seemed that the fuel pump was priming (giving the bike just enough fuel to start and run for a few seconds) but then wasn't running afterwards, starving the engine of fuel and then cutting out.

There seemed to be no logical reason as to why sometimes the fuel pump would prime and the start button work, and then other times when turning the key the fuel pump wouldn't prime and the start button wouldn't work. This is why I assumed a loose connection/relay fault at first.

Strange!

When you say prime, you mean the noise when the key is turned?

I'll double check, but I thought my 125cc didn't prime every time the key was turned.

I did have an issue where my starter switch had corroded a bit from being left outside all the time. It would take a few times of pushing the button before the Vespa would actually try and fire up. It never cut out while riding though, and there was no power difference when it was running. My switch was cleaned up with some contact cleaner and then worked every time.

If you have both keys, does the other key cause the same issue?

I starting to think you need to focus on the electrical system starting by having your battery checked first. Just because it is not that old, doesn't mean it's good. So, if possible, take it out and get it load tested. Then I would check the ignition switch as you mentioned that sometimes it does not start at all. Brake light switch is possibility but that's an easy by seeing if the rear brake light goes on when you pull either lever. Which also leads to the starter relay next - another check needing done. Most of these tests could be done by you at home with a fairly cheap multi-meter. As you work down this line with the meter it will take the guess work out. Report back with your findings so we can figure it out.