Modern Vespa

So here's the deal.

I hadn't ridden it in almost a month, we were going to have a nice sunny day around 60 degrees (F), great for early December around here.

I gave the (2020) GTS300 HPE a quicky bucket wash to get the dust off. Oil was showing a little low in the sight window so squirted a some into the crankcase to bring the level up to, and a tiny bit above center of the glass.

It started right up and idled perfectly.

I geared up and rode it 2.5 miles to a gas station to top up the tank in anticipation of the 100 mile round trip to Deadwood that I was planning to do.

It ran to the station...sorta ok, but had a few minor episodes of 'throttle 'bog' as I took it up to 50-55 mph.

Put one gallon of 91 octane non-ethanol in it to fill it, then proceeded out onto the 55mph highway that starts my trip.

The GTS would accelerate normally up until around 45 mph or so, then, with any more throttle, it would, as they say, 'bog down' and refuse to run any faster.

After a half mile of this, I pulled over, turned around and hobbled back home.

By 'bog down', I mean that it would runn normally, then with increased throttle to have it run up to highway speed, it would act and feel about what a twin cylinder bike would if one of it's cylinders stopped firing. It could maintain 45 mph that way, but no more than that.

It didn't run rough, but just felt like it had lost half of it's normal power.

Does this make any sense?

It's not a vacuum issue with the gas cap, at the gas station I deliberately left it on the loose side to see if it made any difference, but it didn't.

By the way it runs...or doesn't run, It does seem like a fuel starvation issue, but could use some more specific thoughts on where to go with that.

It's been doing this for quite some time, but only with a momentary hesitation on the application of full throttle for hills, and full-on acceleration at mid-to-higher road speeds.

I did search the wiki for 'bogging Vespa engines', and there were definitely a few, but nothing quite seemed to fit my situation.

So....fuel filter clogged? ASR problem? Vacuum leak somewhere?

I do hope that it's not a deeply invasive mechanical thing, I can do some necessary maintenance work on the bike, but I'm not a rocket scientist by any means, and the nearest servicing dealership is about 400 miles away.

check tires pressures and then calib ASR (see manual for how).

I checked the tire pressure before heading to the gas station, so I know that's not part of the problem, but that ASR calibration is what I should try next. Thanks!

310. Tell Trixie MV were unanimous in their advice except that one Aussie fella.

I was riding Ghost, a 2023 Vespa GTS, in north New Mexico during the Cannonball last year when it started doing exactly what you describe.

I had run over a nail and the rear tire was leaking air.

I didn't know what it was and kept riding as it cleared up, then started again so I pulled over, found the pressure very low and plugged it.

Rode almost 200 miles to Guymon, OK where I removed the wheel, took it to a local shop where they changed it for the spare I brought in the support truck.

I can't help but think your issue is due to the ASR.

With your tire pressures checked and confirmed, it may be a sensor.

Bill

Can you plug in an OBD reader and see if there are any codes? Not sure if thats doable but if it is going into limp-home mode there must be something not happy.

This or brake dragging

Our weather has turned cold, windy, snowy, and rainy, so I haven't attempted to do a reset on the ASR system yet, but should get a chance next Saturday when the temps will at least hit 50 F and be dry.

Might need a valve adjustment.
A too tight exhaust valve can cause those symptoms too.
How many miles are on it?

About 9,000, with a valve clearance check done at 6000.

Probably not the valves then.
Dirty fuel injectors?
Old gas?
Fuel pump not keeping up?
Got to be a reason!

FWIW, the recommended octane is 95.
I use 94 in mine, (the highest available around here, not many gas stations even sell that)
In Italy 95 is considered regular gas, but here it's not.
I'm not sure, is there a fuel filter on these, if so, it could need changing or cleaning.

The highest octane gasoline in my area is 91 non-ethanol. I use it in all of our bikes, and the Miata MX-5. The Ford F-150 and Subaru Crosstrek get 87 10% ethanol gas.

Everything runs fine on what we're putting in them.

Don't know what to suggest then.


Perplexing isn't it?
It does sound like a fuel starvation problem though.
If it runs OK at lower speeds but not higher speeds, you would assume it's fuel starved.
I guess it could be something more serious, but lets' hope not.
Have you tried putting some Seafoam in the tank? Great stuff!

Good luck with it.
When you do solve this, please let us all know what the problem was, for our collective problem solving knowledge.
I'm really curious now.

FYI, 95 octane in Europe is equivalent to 91 US. Different calculation methods. RON in Europe, AKI ([RON+MON]/2; average) in the US.