Modern Vespa

Hi Guys,

Searched high and low in the threads and on the internet for a solution to this one. Backstory....

Long story short... a painter decided to do some unsolicited service to my Vespa. I told him not to, but he hooked up a massive 4ft tall Industrial battery charge to my battery to boost it... (i heard a pop) and then he also opened up the carb and then didn't put it back together properly.

I finally got the carb seated properly (at least I think it is) and now the Vespa won't go faster than 60 km/h. (~25 mph)

I've checked the variator/rollers/belt and anything else it could be... there is just no power and it's super sluggish maxing out at 50-60 KM/h.

Any idea whether this is just that Carb issue where you think it's seated but it's not... or if there is something more serious at work with the ECU being fried from the charger?

Vespa is very low mileage and has me stumped on what else it could be.

Hoping for some help from the legends on here!

Just to follow up here.

I've read numerous posts, noting greasy125 's feedback about making sure the diaphragm is 1000% seated.

It seems like it is... but I've seen that even if you think it is... it's not.

I'm going to manually remove it and check the operation with compressed air.... but before I do that, would love to verify that the ECU affects the advance timing in someway and perhaps that's why I'm getting sluggish performance and/or a complete lack of power. A motorcycle mechanic I have a lot of trust and experience with suggested this might be the case.

Also worth noting that the engine sat with old gas in it for a while. I put in some Seafoam and ran it through a full tank of fresh gas. Tried a different spring. Cleaned the carb. Checked the plugs...

I'm really at a loss here... Is it common for the ECU to pop when being charged by a heavy charging device? Or is this most likely still a carb issue?

Assuming the huge charging device was a 12V one it should have been no problem - it's the voltage (and polarity!) that's important, NOT the current delivery capability.

Thanks for that. I always assumed it would draw the current it needs... regardless of how much it can supply.

I can't be 100% certain but the unit he used looked something like this,
Which does have a 24V mode. I did hear a pop, but that could have just been the battery arcing as it connected.

Set the diaphragm with silicone grease and be sure it is 100% in place before you do anything else.

That's the next step for sure.

WOW!

It totally worked! The power is back. I did notice that I do have some slight backfiring now... usually after I lay off the gas.
I also noticed a white film/residue on the diaphragm. Cleaned that off too and it seemed better.

I was really laying into the accelerator to test the power and speed, and when I let off I could hear a slight pop coming from the exhaust.

I wonder if I might have left a bit too much silicone grease out of the groove.

Thank you so much for the tip. Saved me a ton of effort!

The backfire is usually an exhaust leak. Put the scooter on the center stand, start it up and plug the end of the exhaust with a towel. If it doesn't die, you have an exhaust leak.

Will do. Thank You!
Trying that in the next couple of days.
This is a new issue and I'd never had it before any of these carb issues occured. I'd be suprised if it ends up being an exhaust leak... but surprises ARE common it seems for me and this Vespa Journey!

Plugged the exhaust with a towel and it stopped. Investigated the exhaust and all looks brand new... the backfire seems to have stopped now and I feel like the culprit might be the idle jet.

All the symptoms seem to point towards that. Slight hesitation on takeoff. Engine will idle low or shut off when running within the first 5 mins, or when it gets super warm (running for 30+ mins).

Going to clean out the jets and see if that works. Otherwise it's driving lovely... Debating adjusting the idle screw but I think that's just masking the issue.

Still love my S-150 a bit more than the LXV as it seems to handle a bit more nimbly.

But they are both amazing machines.

Adjusting your throttle cable might help with some of the hesitation on takeoff.

Rule out the idle jet and give the scoot the cheapest little upgrade possible by replacing the stock idle jet, size 35, with a more generous one, size 38. Lots written here over the years about the benefits of upjetting like this - only drawback is a slight decrease in mpg.

https://scooterpartsco.com/carburetors-c-751_754/idle-jet-for-keihin-carburetors-p-1696.html

Well it seems like just riding it seems to have resolved most of the issues.
No more hesitation on Takeoff. Plenty of power. I adjusted the idle screw a tad bit and that really seemed to help but now there seems to be another issue (could be resolved by upgrading the idle jet as mentioned in here).

When I start and run it, it's perfect.

But when it's hot, or the weather outside is hot, or it's been sitting in the sun... the idle becomes so low that it wants to cut out... and does.

Wonder if this would be solved by the idle jet upgrade.

I thought about doing this but it seems the idle screw seems to have sorted most of this out.

The idle jet upgrade might help, but if the scooter is stalling on idle, goose the idle speed jet a little to compensate. That's what it's for, and it isn't unusual to adjust it a bit seasonally.

Assuming you're aware of the difference between the idle SPEED screw (readily and easily adjustable) and the idle MIXTURE screw (which should probably not need adjustment, and if it does, you must be careful)….?

Indeed.

It's bizarre because it's so across the board.
Some days driving, the idle is so high that I can feel it pulling the vespa forward intermittently... it's fine... then tugs forward... revs back down again, then tugs forward.

Then after 40 mins of driving or sitting in the hot sun, I drive and it idles SUPER LOW.... like barely staying on...

This happens in the same day. I am aware of the mixture screw... didn't think I needed to adjust that but I'll see.

Just adjusted the idle screw again today. I'll see how it holds up tomorrow.

Thanks again for your feedback here! I really appreciate it.

Idle speed fluctuating with engine temperature sounds to me like an air leak somewhere - clogged air filter, air box not sealed properly, aged/cracked rubber manifold after the carb, etc.

It could also be a misadjusted idle mixture screw - MAYBE. But seriously, tweak that very carefully. It is easy to end up with that set too lean and your engine will burn itself up at idle.