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Vespa ET4 - Starts at FULL throttle

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hazbaz
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PostPosted: 20:17 - 25 Mar 2023    Post subject: Vespa ET4 - Starts at FULL throttle Reply with quote

Hello,

Vespa ET4 125 Pre-Leader/Mikuni BS Carb

Been chasing some issues with the scoot today.

Have done some work (see below), and now the bike starts at FULL throttle! Throttle cable isn’t sticking. Throttle is closed at carb. Have made sure that the throttle body is properly seated in intake, so pretty sure there’s no air leaks. Idle screw is an issue, as I don’t understand how it works (what it does on the carb), and I’m sure it’s set wrong, but changing it doesn’t seem to effect the throttle. Could something be stuck open in the carb? What is the most likely source of this? TIA.


Drained all the E10 out of it, replaced fuel tap, line and vacuum hose with new. Filled with E5 premium. Should make it happier.

Removed carb, dismantled, checked it all over. Very clean, looks like it was serviced not long before I bought it. Put it back together and then back on scoot. Did a compression test beforehand, 230 PSI. Nice healthy engine.

Changed out fuel contaminated oil for new.
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Nobby the Bastard
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PostPosted: 20:52 - 25 Mar 2023    Post subject: Reply with quote

Air leak.
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hazbaz
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PostPosted: 21:00 - 25 Mar 2023    Post subject: Reply with quote

[quote="Nobby the Bastard"]Air leak.[/quote]

Certainly sounds like one.

Where would be the most likely source?
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Nobby the Bastard
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PostPosted: 21:01 - 25 Mar 2023    Post subject: Reply with quote

Somewhere air shouldn't get in.
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hazbaz
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PostPosted: 21:05 - 25 Mar 2023    Post subject: Reply with quote

[quote="Nobby the Bastard"]Somewhere air shouldn't get in.[/quote]

:lol: :lol: :lol:

Well, obviously.
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to v or not to v
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PostPosted: 22:41 - 25 Mar 2023    Post subject: Reply with quote

can you try spraying something that will make the revs rise around potential leak points when the engine is running.
might be able to narrow it down that way.
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WD Forte
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PostPosted: 22:48 - 25 Mar 2023    Post subject: Reply with quote

"Have done some work and now the bike starts at FULL throttle!"

I think can say with some conviction the root cause of the problem is almost certainly you, so best advice is get a better mechanic to work on it.
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that_impulse_guy
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PostPosted: 22:55 - 25 Mar 2023    Post subject: Re: Vespa ET4 - Starts at FULL throttle Reply with quote

hazbaz wrote:
Have made sure that the throttle body is properly seated in intake,

I need this explained. .. are you saying the round bit of the carb in sitting nicely inside the rubber bit of the airbox/rubber bit of the engine intake?

in which case go check if you have the slide wrong way round.
The slide typically has a groove in it and a corresponding pin, so it can only fit in properly one way. If it is turned around the other way, then that would match your symptoms 100%
https://www.vintagebikebuilder.com/uploads/1/6/9/1/16913922/slide-2_orig.jpg
https://www.vintagebikebuilder.com/uploads/1/6/9/1/16913922/slide-align_orig.jpg
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that_impulse_guy
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PostPosted: 23:05 - 25 Mar 2023    Post subject: Reply with quote

in case its not clear, the "cutaway part", faces the airbox/air intake...the pictures above may not necessarily match your slide, but its a slide with the same grooves and stuff, so representative.
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hazbaz
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PostPosted: 23:23 - 25 Mar 2023    Post subject: Reply with quote

[quote="WD Forte"]I think can say with some conviction the root cause of the problem is almost certainly you, so best advice is get a better mechanic to work on it.[/quote]

Well, yeah. Obviously. That’s why I’m on here for advice, to help narrow down potential avenues for fixing it.

I’m not a mechanic, but I think I have a reasonable set of skills. And can probably figure this one out. Not like I’m stripping the engine down and rebuilding it from scratch is it?

This is possibly one of the most unhelpful comments I’ve ever had on a forum designed for people to get input on a question, to help them to learn.

But thanks. Great work.


Last edited by hazbaz on 01:37 - 26 Mar 2023; edited 2 times in total
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hazbaz
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PostPosted: 01:35 - 26 Mar 2023    Post subject: Reply with quote

[quote="that_impulse_guy"]in case its not clear, the "cutaway part", faces the airbox/air intake...the pictures above may not necessarily match your slide, but its a slide with the same grooves and stuff, so representative.[/quote]

Pretty sure the slide is in the right way. I did not disassemble the membrane, and the slide and needle are fitted to the membrane. The membrane has a notch to ensure it goes back in correctly.

I am going to reseat the TB in the intake tomorrow and take it from there. If I need to disassemble the TB again and check it out I will.

I’ll also check the intake for any splits. At a bit of a loss to explain it really. The carb went back together as it came apart. Refitted properly. Nothing really changed.
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Bhud
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PostPosted: 01:46 - 26 Mar 2023    Post subject: Reply with quote

hazbaz wrote:
Idle screw is an issue, as I don’t understand how it works (what it does on the carb)


Not to be a dick but this is why you should take it to a mechanic.
It's most likely to be an air leak, but whatever it is will be fixed and the mechanic will explain it to you.
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hazbaz
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PostPosted: 02:38 - 26 Mar 2023    Post subject: Reply with quote

[quote="Bhud"] Not to be a dick but this is why you should take it to a mechanic.

It's most likely to be an air leak, but whatever it is will be fixed and the mechanic will explain it to you.[/quote]

Of course. But mechanics charge an hourly rate, and my time is free.

I understand what the idle mix screw does, but adjusting is is not making a difference.

The air/fuel mix screw is tucked away behind the throttle, so I haven’t messed with that.

I have a feeling it’s either that the carb isn’t seated in the inlet properly, the intake has a split, or something didn’t go back together quite right in the carb.

I’m happy to troubleshoot until it works. And if I can’t fix it, then I’ll take it to the garage.
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Pete.
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PostPosted: 06:28 - 26 Mar 2023    Post subject: Re: Vespa ET4 - Starts at FULL throttle Reply with quote

that_impulse_guy wrote:
hazbaz wrote:
Have made sure that the throttle body is properly seated in intake,

I need this explained. .. are you saying the round bit of the carb in sitting nicely inside the rubber bit of the airbox/rubber bit of the engine intake?

in which case go check if you have the slide wrong way round.
The slide typically has a groove in it and a corresponding pin, so it can only fit in properly one way. If it is turned around the other way, then that would match your symptoms 100%
https://www.vintagebikebuilder.com/uploads/1/6/9/1/16913922/slide-2_orig.jpg
https://www.vintagebikebuilder.com/uploads/1/6/9/1/16913922/slide-align_orig.jpg


It's a CV carb, the throttle cable works a butterfly so it doesn't have a slide with an idle cutaway.
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Easy-X
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PostPosted: 12:19 - 26 Mar 2023    Post subject: Reply with quote

hazbaz wrote:
...and my time is free.


When one has free time it's a misconception to say that time has no value. What else could you be doing instead, does it pay >£80? And sometimes you have to draw the line and realise "this is beyond me" Neutral

Anyhoo, wouldn't having the throttle wide open imply too much fuel rather than too much air? This CV carb you have, how does the choke work? If it's a enricher rather than a real choke maybe that's the bit that's "stuck" Thinking
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Nobby the Bastard
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PostPosted: 12:42 - 26 Mar 2023    Post subject: Reply with quote

If the butterfly was stuck open it would give it as much fuel and air as it needed for whatever revs it ended up doing.

If the float was stuck open then it would probably not start in the first place.

Edit: Is the throttle cable far too tight so it's pulling the butterfly open a bit? It wouldn't need to be opened muchfor it to rev out with no load.
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hazbaz
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PostPosted: 13:49 - 26 Mar 2023    Post subject: Reply with quote

Think I figured it out….

Checked for leaks. Reseated carb. Same problem.

Remove carb, opened it, checked it over, reassembled.

The mix screw was turned fully out! Clearly the previous owner had a need for speed. Turned it fully in, then 2.5 turns out.

Scoot fired up and is running beautifully! I even figured out that the idle adjuster just presses slightly on the throttle butterfly, so now I know how that works.

It likes E5.

£80 an hour eat your heart out.
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hazbaz
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PostPosted: 13:52 - 26 Mar 2023    Post subject: Reply with quote

[quote="Easy-X"][quote="hazbaz"]...and my time is free.[/quote]

When one has free time it's a misconception to say that time has no value. What else could you be doing instead, does it pay >£80? And sometimes you have to draw the line and realise "this is beyond me" :|

Anyhoo, wouldn't having the throttle wide open imply too much fuel rather than too much air? This CV carb you have, how does the choke work? If it's a enricher rather than a real choke maybe that's the bit that's "stuck" :think:[/quote]

Meh. It’s a weekend. I could be sitting. Watching TV. At the pub.

I’d rather be tinkering and learning how stuff works. And this tested the skills, but I learned a lot. And the problem is now resolved.

Mix screw was turned all the way out by previous owner. No wonder the thing wanted to go go go. Sorted now.
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hazbaz
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PostPosted: 13:53 - 26 Mar 2023    Post subject: Reply with quote

[quote="Nobby the Bastard"]If the butterfly was stuck open it would give it as much fuel and air as it needed for whatever revs it ended up doing.

If the float was stuck open then it would probably not start in the first place.

Edit: Is the throttle cable far too tight so it's pulling the butterfly open a bit? It wouldn't need to be opened muchfor it to rev out with no load.[/quote]

The throttle cable is tight. But not so tight that it’s holding the butterfly open. And turning the handle bars does not increase revs.
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Nobby the Bastard
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PostPosted: 15:56 - 26 Mar 2023    Post subject: Reply with quote

hazbaz wrote:


The air/fuel mix screw is tucked away behind the throttle, so I haven’t messed with that.


So the thing that you completely discounted from that start was at fault?

Why did you not mention that you had never had the bike running priorto dissembly of the carb?
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hazbaz
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PostPosted: 16:13 - 26 Mar 2023    Post subject: Reply with quote

[quote="Nobby the Bastard"][quote="hazbaz"]

The air/fuel mix screw is tucked away behind the throttle, so I haven’t messed with that.
[/quote]

So the thing that you completely discounted from that start was at fault?

Why did you not mention that you had never had the bike running priorto dissembly of the carb?[/quote]

Erm…. Hang on. I never discounted it. Just stated that I didn’t mess with it when I disassembled the carb. So wind your neck back in Nobby. Wasn’t even your suggestion.

The bike HAD been running previously. Never said it hadn’t. It was running very rich, and also, the fuel tap was hanging open allowing fuel in to wash down into the oil. That issue is also now sorted.

It was previously running on E10. When I switched it to E5 this problem emerged. Which is now fixed.

Yes, I am a relative noob to mechanics. But learn from every success and failure.

What I don’t do is prowl forums looking to be a nobby to people asking for advice, in a WORKSHOP sub-forum. Which I’m sure you’d agree, is the reason it exists.
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Nobby the Bastard
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PostPosted: 16:34 - 26 Mar 2023    Post subject: Reply with quote

Either way, you didn't disclose the full information.How do you expect us to diagnose a problem with partial information?

E10 and E5 would not make any difference to how well it ran or how rich it was before.
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