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GTS 300 super (US v.)
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Having starting problems on my Vespa 300gts (2014, 16k km). Serviced regularly, by myself, driven shorter distances most of the days.
The scooter usually starts on a first start, but after a first drive (and a quick stop) there is trouble cranking. Sometimes it just thud and nothing, sometimes it is like a jarring clunk after I press start. I wait – sometimes for a few minutes, try again, and it starts. Sometimes I have to wait for like 5 minutes.
I spent hours diagnosing and jumping through hoops:
- Lights are up on key turn, break lights operate, on/off switch is ok. Fuel pump primes, dash lights up.
- Battery less than 1y old. Holds current 12.7, goes down first then up to 13.0 on start, on revs - 13.6. Honestly the same symptoms were present on the old battery.
- Checked for all the electrical issues (as per robots video). All normal readings. Battery charges ok. (did not check regulator/ too deep).
- Starter relay replaced to a new one. Again current to relay equals to that of the battery.
- Continuity checked battery->relay, relay-> starter, starter->battery, all ok.
- Starter removed and checked on external battery – it spins, with a large spark though. -
Cant tell if anything is abnormal. Put back into the scooter – symptoms are the same.
- Checked for parasitic current draw (pulling fuses, Amp reading) – nothing unusual detected.
- There is a short somewhere in the steering wheel cabling (clock resets occasionally when turned to the left, with a buzzing sound). It has been like that for years tho.

The only thing I seem to have left – sprag clutch? weak starter?

starting video here:
Thanks for your help
xS
⚠️ Last edited by keltis99 on UTC; edited 1 time
@old_as_dirt avatar
UTC

Veni, Vidi, Posti
2007 GTS
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UTC quote
sounds like the valves might be tight.
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old as dirt wrote:
sounds like the valves might be tight.
valves adjusted 5 months ago (intake = ok; exhaust both slightly tight = 1/4 turn to loosen both)
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Veni, Vidi, Posti
2007 GTS
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UTC quote
keltis99 wrote:
valves adjusted 5 months ago (intake = ok; exhaust both slightly tight = 1/4 turn to loosen both)
the lock nuts could have backed off , seen this before.
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ECU/throttle body??? Next time it happens, give it the ole Throttle Body Tap and see if it starts.

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Here is another video showcasing the starting problem of my 300gts (stuck on the 2nd try at 0.40sec - which is the usual scenario of my reoccurring problems). Got stuck in the city today after a few short runs…jumping the battery helped to start it again but just barely...

Can it really be the battery just after 1 year? I have yuasa YTX12-BS 12V/10AH

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Do you occasionally fully charge the battery with a charger or maintainer? Unless the battery can get a decent bulk charge at 14.4V or thereabouts once in a while it will gradually lose capacity and sulphate.

I have seen several batteries die within a year, usually because of lots of short journeys without them ever getting back to full charge.
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I use CX trickle charger once a month and now more often....so yes it gets a boost certainly
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keltis99 wrote:
I use CX trickle charger once a month and now more often....so yes it gets a boost certainly
Can you give more details on this charger? Google reveals nothing. I do know that some so-called trickle chargers just don't do anywhere near a decent job of fully charging a battery. They're fine for keeping a fully charged battery happy, not for fully charging it in the first place.
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sorry its is ctek mxs 5.0 trickle charger. quite a popular choice - had it for years and used successfully on many other bikes/vehicles.
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my own impression of the problem is that:
a) the battery is not good (does not have enough cranking power) - buy new battery?
b) or something with the sprag clutch (mechanical damage?)/starter motor (needs cleaning?) - open of the water pump side/new starter motor etc.
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keltis99 wrote:
sorry its is ctek mxs 5.0 trickle charger. quite a popular choice - had it for years and used successfully on many other bikes/vehicles.
That's fine.
keltis99 wrote:
my own impression of the problem is that:
a) the battery is not good (does not have enough cranking power) - buy new battery?
Cheapest and easiest test and solution.
Quote:
b) or something with the sprag clutch (mechanical damage?)/starter motor (needs cleaning?) - open of the water pump side/new starter motor etc.
The sprag clutch playing up would probably make much more of a grinding noise (I've had two go on me!) so a last resort IMHO.
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Thanks Jimc. So after thinking it over I decided to open the crank case side and inspect the (sprag) clutch mechanism. Main reason: after being parked on the side stand my vespa had many more starting issues than when the central stand was used and I suspect a mechanical cause – a clutch wheel not sliding back into correct position etc.

After opening I noticed that both the smaller and larger clutch wheels had quite a bit of play (I would say ~5mm forth and back on the shaft). Again – sometimes vespa started, sometimes not (with some grinding noise). Battery was always at 12.7v and I am quite confident its not electrical related. Any suggestion if they are (smaller or larger wheel – parts nr. 4 & 5) on the way out based on my videos?

https://youtube.com/shorts/mdH5u5EkKBE?feature=share

https://youtube.com/shorts/3JCrzbSOYcg?feature=share
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It still sounds like a bad battery
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Did you give the throttle body/ECU a light tap (see video above)? That's what mine sounded like until I replaced the throttle body/ECU assembly. It sounded like it was turning over (and going to start), but it just never had the "oomph" to do so.
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UTC quote
yes i have - many times. does not help.

After yesterday's venture into the crankcase now the Vespa does not start. only the whirling sound from the starter (which I also rebuilt and cleaned). Symptoms are the same as in this post: 300GTS Not starting (Page 2) Maybe the clutch did not like my WD40 action?

So pretty much i made peace with the sprag clutch-proper mechanic required situation. Will keep you posted.
theschuman wrote:
Did you give the throttle body/ECU a light tap (see video above)? That's what mine sounded like until I replaced the throttle body/ECU assembly. It sounded like it was turning over (and going to start), but it just never had the "oomph" to do so.
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Harry brenner wrote:
It still sounds like a bad battery
tested it with a different and more powerful battery - same issue.
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so the end of the story - it was a problem in the flywheel/rotor assembly (sprag?). the mechanic fixed it replacing the whole part (nr: 1A016720)
he explained that the circular ring attached to the rotor/flywheel had these pins that need to eject to get traction from the starter wheel - one of the pins was jammed inside its placeholder (circled). Since the rotor/flywheel comes in one piece the whole thing had to be replaced (at a hefty price.)...he said the part was updated in the newer Vespas and apparently rebuilding should be more of an option from now on. Mechanic could not give me a definitive answer why the part failed (not a very common failure).
so yes, anything with a starter clutch is expensive (just shy of 400e)...at least i have a fine running Vespa now
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I'm glad you have a successful outcome. It sounds like you have a great mechanic to be able to diagnose and remedy the fault. It might be an idea to amend your post title by adding 'SOLVED', like I did with my recent post from yesterday.
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