Modern Vespa

Occasionally, when decelerating, when I've rolled off the throttle and the clutch dis-engages (effectively free wheeling) the engine wants to stall and sometimes does. The slightest turn of the throttle stops this, and the engine idle at tick over sounds normal.

There is very slight play in the throttle handle, would a little tightening of the throttle cable stop this?

Last edited by Touring300 on Tue, 21 Jun 2022 16:28:01 +0000; edited 1 time

Doesn't sound normal to me.

I can't see where any mechanical adjustment at the throttle would matter.

Is your engine at full working temperature ie: you've ridden it at least 5 miles or more?

If it is at normal working temperature it shouldn't stall but occasionally the engine management system (automatic idle control) can be slow to do it's job, allowing the engine revs to drop a bit more than usual before suddenly normalising. That is kinda normal as long as it's not stalling. Just about all the GTS bikes from 2014 onwards seem to do this. My GTS300 engine idle revs drop dramatically sometimes when I'm in traffic usually when the engine is very hot, and the engine goes to idle after the clutch disengages, but this normally only happens when the engine cooling fan is on in traffic. This loads the engine more than usual, but mine has never stalled.

Even the dirty air filter can cause the engine to shut down at idle ...

Thanks Ian, your explanation makes a lot of sense. The symptom only happens when the engine is hot. When I shut off the throttle and the clutch disengages, I feel the revs drop dramatically and a slight pause before the automatic idle control kicks in. Only on the odd occasion does this lead to a stall.

I have only noticed this symptom fairly recently and I wondered if it had something to do with the stretching of the throttle cable. Although the scoot is 5 years old, I have been slow to put miles on (only 3000 Klm's since new). If I were to tighten the cable by a couple of mm, then when I shut off the throttle, there would be less 'slack' in the cable to induce the low revs/automatic idle scenario?

From my experience I can say that the minimum must be kept by the throttle body settings of the injection lock (once with the carburetor there was a screw) even if the throttle cable was slow. Sorry if I disagree ...
True that by pulling the cable you can raise the minimum but (assuming it is the injection model) it is sufficient to go to the dealer who, using a computer, will reprogram the ECU with the correct minimum.

Hopefully he'll just upload the most recent mapping into the ECU - there's nearly always an upgrade available.

BUT I do hope that dealer WILL NOT reset the throttle position sensor (TPS). Please read this quote from Cheeky Thomas (a very experienced UK scooter mechanic):
(It's worth reading the whole thing - https://cheekythoma3.wixsite.com/itsme/idle )

This might be a symptom of dirty (oily) throttle position sensor, which, in turn, might be a symptom of lots of oil in intake/airbox or too much smoke from crankcase.

Andrew, yeah as jim is saying, don't adjust the TPS or alter the throttle cable settings. Those actually will make matters worse. It's highly likely an ecu update will cure your occasional stall. Again, as jim says there have been a few since your bike was built and if you haven't had those, then there's the issue I'd say. Good luck and let us know how it goes if you get it done.

Possible stale fuel? 5k in 5 years isn't much really.

So finally, 3 years later, I have resolved my 'stalling when clutch disengages' issue. What had started out in 2019 as an 'occasional stall' had slowly over time, manifested itself into 'stalling on most occasions' when free-wheeling to a stop and the clutch disengaging from the engine.

My scooter has always run perfectly except for this very specific issue of the engine stalling out when rolling to a stop, just as the clutch disengages from the engine. Since then I have followed similar stalling issue threads on MV and observed the many members suggestions as possible remedies. As this summer arrived, I decided to change my decision from accepting and living with the fault (especially as some owners' reactions to my symptoms were, "well they all do that") to pro-actively attempting to getting to the bottom of it. Here is a list of actions I took in my search to find the possible cause:

The first step I took was to book my scooter in with my nearest Vespa/Piaggio dealer in the hope that the P.A.D.S diagnostics would offer-up an ECU update and all would be well. Sadly there was no diagnostic update to be had for my GTS and after the mechanic took it out for a test ride, managing in the process to induce the stall, he returned telling me he could not suggest a remedy and advised I could spend many hours and many ££££, and still not find the root cause.

Returning home somewhat dejected, I embarked on a fault finding mission. Here is the list of things I checked:

Health of the battery, checked the 'kill switch', added fuel/injector cleaner into the petrol tank, cleaned the air filter, replaced the spark plug and checked the spark plug cable, adjusted the 'play' in the throttle cable, cleaned the throttle body using specific throttle body cleaner, dropped the engine and checked the valve clearances, removed the fuel injector and sent it away to be professionally cleaned (I was not confident that I would be able to clean it thoroughly enough and the cleaning service only cost £25.00 which included return postage and a printed graph showing pre/post test results).

I was considering replacing the fuel pump, but I did not need to as the culprit was found. A discounted and brand new ECU/Throttle Body for my year of GTS was sourced which, when installed, instantly cured the stall. Since the install I have been out on a couple of long rides during which I deliberately coasted to a stop many times with the throttle rolled off, and the engine has not stalled once. I am a very happy chappie

That all makes me wonder if some misguided mechanic re-set the Throttle Position Sensor at some point - which is specifically pronounced as a NO-NO in the workshop manual (unless a brand-new throttle body is installed).

Thanks for posting your process and the resolution! I got a new wrinkle in my brain.

or worn out like mine was in my supertech

True - of course Piaggio would say that it should be replaced, but resetting the TPS at least enables the worn-out one to keep going for a while.

I've suffered exactly the same cutting out issues on my 2016 GTS 300, tried all the mentioned remedies including just replacing the HT cable and plug cap (why did they bury the coil inside the frame?) still stalling intermittently so my question is where can I source a 'new ECU/Throttle body' in the U.K. for a 'reasonable price'? Many thanks.

My 2007 GTS250 stalls occasionally at idle, either hot or cold. I think it's because of the SuperCorsa chip I installed messing with the EFI. When I think about it, I let the engine idle for a little while at startup, which seems to help the EFI sellf-calibrate. When I jump on and immediately rev up and start off is when I notice it will stall at my first few stops.

My 2016 gts 300 ie would stall frequently when new. The dealer ecu remap helped a lot, and the problem finally disappeared after I cleaned the throttle body/injector with fuel additive & carb cleaner.

There is a lot to this and would require a long explanation that most of if not all would have no interest in hearing.
Here are two shorter answers that might solve stalling when coming to a stop.
If the engine has higher mileage, clean the throttle body. Crankcase vapors are no longer vented to atmosphere so they are routed to be burned in the engine. Because of the routing, air box, they gum up the throttle body and throttle plate as they pass through. A clean can correct.
Other is octane, if you are using what is recommended drop a grade lower, not higher. Do not go over the recommended octane rating, no benefit but can definitely cause issues on a high heat day. This is one of those just try it, when you need gas add a half gallon or so of a step down rating and see if it helps (using 91, try 89).

Thanks for those suggestions my cutting out began early in its life (owned from new) now at 15.5k miles it's still there having been main dealer data checked (no faults shown) changed HT cable and plug I'm currently awaiting a part (tick over valve/sensor? Advise from local scooter shop) should this correct things I'll be posting results to this not unusual annoying headache! Oh, I will try your 'air box clean' suggestion while I await the part at this stage I'm not going to let it beat me!