Modern Vespa

Hi all,

My name is GJ from the Netherlands.

After years of reading here from google searches now is the moment to ask a question where i cant find the awnser too.🤙🏻

My mp3 400 blow a piston last season.

I replaced the cil. and piston, sparkplug, injector, oxigen sensor, Cleaned the air intake/throttle housing and idle valve.

Fuel pressure 3Bar as spec.


But after all this.😒

Engine starts, runs nice stationary.
But when i try to drive away it accelerates rough to 3000 rpm and holds back 3000 and above is great.

Sometimes it reacts good/normal to throttle and rockets away.

But most of the times.

Full throttle then sputters to 3000rpm playing with throttle so it gets above 3k and then it rockets to top speed.

Its not the rolllock or seat limitter.

Airfilter en housing is cleaned. Oil vent is disconnected en an catch can is installed.

Used jpdiag...

Tps wont go higher then 85% full throttle...
Butterfly in the throttle body gose 100% throttle cables are well adjusted.

Musnt the tps signal go to 100%?

Did not do a tps reset because i read that it is forbiden. 😉


So most of the times it holds back, but sometimes all is well.

Hope somebody has the holy grail of awnsers.

Clean throttle body

111C engine temp is not normal. Also voltage is too low for engine that is running. Idle rpm should be around 1700rpm

Temp should stay below 105C max, and running voltage at idle should be around 13,5Volt

These bikes don't run right if they're not getting full electric juice

This picture was taken while bleeding the coolant so thats why temp is high.

Scooter was on full battery with charger on it. Thats why its at stationaire voltage and not charging at the moment.

Temp. Sensor has the correct ohm s and numbers now.

Thx for your reply.

But the pic you posted says "Motor status: engine running" along with 1280rpm.

Sub 13 volt is too low to feed the electronics when the engine is running

How to increase idle speed then?

Idle is under full computer control.

How old is the battery and have you load tested it? Battery voltage is a useless statistic. It's all anout the amps

Battery is 4 months old, when parked its under charger optimate 4.

Its a 20Ah battery just because off the known battery probs. Its 300ccA so no shortage of power there.

This screen was a moment pic. Regular voltage is 13V+

I doubt the tps number.
At full open it is 85% can someone relate?

For TPS i think 85 is correct.

It's in degrees not percent and I think it's never reaches 90.

Then i am out of ideas.

Got no vacuum leaks.

Someone?

Only thing I can think of is human error during assembly... That decompression thing on the chain gear on the head, since it works good at high rpm but not low... It supposed to decompress during start so the starter has an easier job turning the motor around.

If it's somehow possible to mount that wrong I would take a second look at that. Maybe its stuck and the centrifugal force isn't enough for it to engage properly at lower revs.
Just speculation on my part, my real answer would be "I don't really know"...

Good morning,

This is a 2010 model.

During piston replacement i havent seen those decomp. Weights.

They are not on the camshaft sprocket.

And i cant find those weights in the 2008 to 2010 tech. Drawings.

Arent those weights only on the later models?

Thx again.

Gr GJ

Morning mate, from your pic above I can see them behind the sprocket, or at least I think it is those. I have a 2008 and I'm sure I've got them, I can hear it when I start the bike, it's not struggling.
Found this: https://www.motorcyclespareparts.eu/en/piaggio-parts/b018492.
As stated above, this is a long shot and might not have to do anything with your problem...

About your injector, it was also new, did you test it before assembly?

I'm no expert but I'd be checking the timing chain is correct. Maybe out of sync by one tooth on the gear on the cam. On my GTS there is a plastic plug near the water pump that can be removed to check a mark for tdc and the cam gear also has a mark.

And of course checks the valve gaps

Did valve clearance when head was off. 15 gauge.

Timings is correct. Did double check.

Also see picture above for camshaft timing.

Hey Petro,

Behind the sprocket is the camshaft directly.
Those counter weights are on the outside of the sprocket in a bowl on the drawings in de workshop manual.

Did 2 injector replacements. Try'd them shooting outside. They worked and sprayed/vapored niceley.

And did fuel pressure test nom. 3 Bar. As spec.

If the timing is off the enige would perform poorly all the time is my guess.

Mine mostly holds back but 1 out of the 5 tries it rockets away.

Everybody thanks in advance.

Have you replaced the fuel filter???

Cheers again.

I found it in the manual and compared to your pic above. Please take into consideration that I've not been in there myself yet.

It seems to me you are right, they are on the outside of the sprocket. However, they should be there, the 250 has them, 400 and 500.
On top of them there's a cover, that lured me believing their on the backside.
Your pic doesn't have the cover, nor the weights or the spring...
Look at the first pic here: https://www.manualslib.com/manual/1259327/Piaggio-Mp3-500-I-E-Sport.html?page=192#manual , that's how it should be,ca cover with the decompression assembly underneath.
The second pic is closer to what you have now I think.

No,

That one is in the gas tank. At the pump.

Thats why i checked fuel pressure to ensure there is no blockage.

Fuel pressure is one thing, but fuel volume is just as important.

Since the injector you pulled was crusty, then dollars to donuts the bike was fed low quality fuel.

Replace the filter, it's a low cost part

IMO soI can't it be neither pressure or volume since the bike runs well att high rpm.

Just to be shure, there is only one in the tank?

There are 2, sorta.

There is the fuel strainer aka fuel sock as well as the main filter element.

Both are part of the fuel pump assembly.

I'd replace the sock and the element at a minimum considering they're about 15 years old

And if the pump looks sludgy or rusty, then you'll need to clean out the tank chemically and replace the entire pump assembly

Fuel pumps tend to be short lived if the tank is regularly run low.

I refuel as soon as the gauge reads 1/3-1/4

I have a new oump... the corolla kind. Waiting in the workshop. Will put it in asap.

If somebody has more ideas to read from jpdiag i would like to hear it.

I dont have any faultcodes.

Again thx for al the help. Hope that something will pop up that will be the golden guess.

Greeting from holland.

Care to elaborate what this means?

Piaggio sells only the complete unit.

But the actual pump only...is the same as a toyota corolla fuel pump. Saves money.

Got one on sale last time and put it on the shelf..

I highly doubt Piaggio would use Toyota fuel pumps.

That is simply not the Italian way

I'm gonna venture to guess your pump is a Chinese Aliexpress knockoff.

Just because it looks the same doesn't mean it is the same. You have to compare spec for spec

Its bosch. Just like the front wheel rpm sensors wich are also used in an opel corsa for example.

Hi guys,

Found the problem.

Pulled the fuel tank and pump..... after al the plastic covers. 😆

The pump was clogged by the filter sock. It was dirty and so floppy that it was sucked in te inlet.....

Replaced the pump because is was there and the filter + sock.

After reinstalling made a short test drive and we got all power again.

Everybody thanks for the support! 🥳🥳

May you all have a great holiday season. 🎄