Modern Vespa

Hello, I think what's happening is there is a vacuum line which powers your fuel pump and there is a crack in the vacuum line which opens up a little off the center stand. The crack when the engine is running still allows enough vacuum for the fuel pump to work, but when you are just cranking the engine to start it there isn't enough vacuum and hence it wont start due to no fuel being pumped. What you could do is take a long pick, and with the engine running try flexing some of the vacuum lines that are connected directly or indirectly to the fuel pump. Start with the one that comes off the intake manifold as that's where the vacuum originates and the intake manifold is flexing relative to the fuel pump when you take it off the stand since the whole motor moves relative to the frame in this action and the fuel pump is screwed to the frame. When you flex a vacuum line and the motor starts to stutter or stall you've found your crack in the line. Replace your cracked vacuum line then you'll be good. I could be wrong but I think its vacuum related. The old vacuum lines can crack from age especially ones that are exposed to heat and flexing like the one off the intake. Good luck.

I can take a look at this. But would it explain how the bike runs fine off the centre stand? Again the issue is just related to starting

If it runs fine off the stand, but won't start or the starter motor seems 'feeble' then I bet it's a loose connection on the starter motor wire from the relay - the one under that cover.

There's a vacuum operated fuel petcock, no? No vacuum no fuel no start. Check your vacuum lines. A weak starter in my experience is usually a weak battery which wouldnt change on or off center stand or a problem with the starter itself which would still be there on or off the stand. As far as wiring to the starter thats either connected and the starter works or its not connected and it doesnt. Spark plug wire flexing also doesnt make sense because if that was the problem it wouldnt just be with starting, it would stick around when running.

Which cover?

I secured the boot that goes over the starter and it was initially loose which I was hoping was my silver bullet

I couldnt quite figure out the other end of the wire that leads to the boot

Once the bike is running youre generating so much vacuum even if theres a small crack in the vacuum line theres still enough vacuum to keep the fuel petcock open. As time progresses and the crack gets worse, then it wont run at all because the vacuum leak will get worse. This is assuming my theory of a cracked vacuum line is correct. If Im right, you just need to find the vacuum leak and replace the hose. Maybe Im wrong. I had a problem with my own scooter once where it would start on the center stand and the second I took it off it would stall unless I revved it a lot. But it took a while to get to that point, for a while it just ran funny like feeling like it would stall at random times at idle. But mine had a vacuum petcock and vacuum pump, yours it seems just has a vacuum petcock not sure what if any pump you have. Anyway, i might be completely wrong, but I have a hunch its vacuum related.

How old is your battery? Do you have a good battery? Have you ever replaced your starter relay, how old is it? Starter relays last maybe 7 years or so in my experience, but they usually just completely stop working. What about at the terminals of your battery, what does your multimeter say when its running?

But did you tighten the connection under that boot - cover? It might make a better connection when on the stand, but is loose enough it's a high resistance off the stand.

And one thing want to emphasize: when it's off centre stand and it doesn't start… it kinna wants to start. Like the starter will spin once and quit or it will click or whine or whatever. So it's getting some juice but not enough?

I only clarify this because if we were to blame the other two wires that go to the relay (from starter button and from kill switch) would they allow a partial current? Seems to me they would be more so binary ie on or off

So youre saying off stand where you normally have trouble the trouble goes away with battery direct to starter. But when you try to start it normally with your starter button off the stand now its a problem. So that tells you the switch that connects the battery to the starter when you press the starter button (the starter relay) is somehow not working well off the stand but works ok on the stand. Maybe you need a new starter relay they are cheap and easy to install but check any wires and its connection to the harness. Also, not sure if this could be an issue but usually the bike will not start unless your brake light is working like you have to have a working brake light circuit for the starter to work. Make sure your brake light comes on off the stand vs on the stand.

you can have current flow issues due to dirty or corroded connections, especially with high draw circuits at a switch. which is exactly what a relay is.

have you tried another relay to rule that out? they do go bad from time to time. arcing on the connections or they just burn up inside. it's real exciting when they fail in the "on" position.

from there, I'd check the wiring from the battery to the relay. then suss out the starter switch, brake switches (both) as well as the brake light and all of the associated wiring.

a test light and multimeter should get you there for those tests.

hope that helps!

Any difference when handlebars are turned?

K another update.

When I connect a multimeter to the negative termanal and the post on the starter. It has continuity. I'll include a picture which doesn't tell much.

Does this make sense? The post has a rubber boot on it and should be grounded. I can't see why the post would be grounded. It's not physically touching and the little rubber but between the two nuts looks fine. But obviously I can't totally confirm until I pull it out.

I just want someone to tell me if I'm crazy or if no, that post shouldn't be grounded.

What is weird is I did the on the centre stand, where, for the most part I am not have of problems

The starter motor is very low resistance - unless you have a very good multimeter, you'd find it hard to read it, it'll look like ground at the positive post. That's why you need a very low resistance path through the starter relay and also through the engine's grounding to the negative post of the battery. Any measurable resistance there and the volts available to start the starter motor turning just won't be enough.

Ao basically i shouldn't read to much into the continuity between ground and that post?

Ao basically i shouldn't read to much into the continuity between ground and that post?

Does the resistance change when off the stand? It might be a grounding issue.

Did you have the transmission cover off and re-seat the bendix? If not that could have fallen out of touch with the starter splines.

That's what I'm thinking. I just have to borrow a clutched remover tool 😤

Just get a u-bolt from the hardware store:

Ok got it off. Not sure how to tell if it looks bad. Silly question. Am I allowed to start it?