Had our 1st engine failure with the MP3
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Member
Gilera Fuoco
Joined: 13 Aug 2020
Posts: 20
Location: Russia
Thu Sep 10, 2020 12:08 am quote
Squeazel wrote:
Most of the people with the broken piston problem didn't see any loss of power- the airbox just had more and more oil in it over time. Maybe 10 ml, then 50 ml, then 100 ml over a couple of years. Until the broken piston got a hole in it- then there was lots of noise and lost power. But then it's too late.
Wow! Big thanks for this info.
Enthusiast
mp3 500 2016
Joined: 30 Dec 2011
Posts: 83
Location: singapore
Sat Sep 12, 2020 10:28 pm quote
Can compression test check on the broken piston ring problems?
Member
Gilera Fuoco
Joined: 13 Aug 2020
Posts: 20
Location: Russia
Sun Sep 13, 2020 2:15 am quote
Yes, this is the natural and best indicator of piston damage or rings wear.

Now in my Fuoco the compression is 10.5 kgs / cm2.
Kgs / cm2 is a unit adopted in our country, and I do not know how to translate it into the American system of measures.

There is one indicator in the scooter's passport: "geometric compression ratio". This is not fact compression and is the ratio of the cylinder volume at the lowest point of the piston to the volume at its highest point.

The compression ratio from the moped's passport tells us only this indicator - the geometric compression ratio. According to the passport, the compression ratio of the Master 500 is "10.5".

But the compression ratio is geometric and has nothing to do with actual compression. From my experience with Japanese engines, I know that the actual compression is always 10% ... 20% higher than geometric, so I expected my engine to have a compression of about 12.

Due to the fact that the actual compression in my motor is less, and also due to the high oil consumption, I come to the conclusion that my piston or rings are destroyed.

_screenshot 2020-09-13 в 15.13.48.JPG

Hooked
50cc Beo, BigBeo
Joined: 30 May 2015
Posts: 266
Location: Chelyabinsk, Russia
Sun Sep 13, 2020 3:37 am quote
Please do not forget about decompressor. It causes significant measurement errors.
Member
Gilera Fuoco
Joined: 13 Aug 2020
Posts: 20
Location: Russia
Sun Sep 13, 2020 6:45 am quote
Of course. In engines with a manual decompression mechanism, it must be turned off; with an automatic mechanism I do not know what to do.

I have seen a mention of decompression in the workshop manual, but I assume it is a translation error (see screenshot).
I don't understand what this page is about. If you direct my thoughts in the right direction, I will be grateful to you.

_screenshot 2020-09-13 в 19.39.24.JPG

Hooked
50cc Beo, BigBeo
Joined: 30 May 2015
Posts: 266
Location: Chelyabinsk, Russia
Sun Sep 13, 2020 11:03 am quote
Indeed these are not related to engine That's keyhole, seat lock and an actuator which opens it
Moderaptor
The Hornet (GT200, aka Love Bug) and 'Dimples' - a GTS 300
Joined: 26 Aug 2007
Posts: 39015
Location: Pleasant Hill, CA
Sun Sep 13, 2020 11:55 am quote
You want page 34 of the parts list:

https://manuals.wotmeworry.org.uk/Piaggio/Piaggio%20MP3/MP3%20500/

Fuoco decompressor.jpg

Member
Gilera Fuoco
Joined: 13 Aug 2020
Posts: 20
Location: Russia
Sun Sep 13, 2020 12:16 pm quote
Haha, "counter weight" then everything is logical, thanks, jimc!

In fact, I had another suspicion, but I hadn't wrote about it, because I didn't know what it meant.
When I installed the compression gauge and turned on the starter motor, I was confused by the uniform increase in compression ratio. It started at about 6 and gradually increased by ≈1 kgs.
That's right: if the engine has a decompressor mechanism, the increase in the compression meter reading should be exactly that. And I thought, that this indicates the destruction of the piston or rings

So, i must to answer to echris108:
echris108 wrote:
Can compression test check on the broken piston ring problems?
No, in this engine, a compression test is useless.
Hooked
50cc Beo, BigBeo
Joined: 30 May 2015
Posts: 266
Location: Chelyabinsk, Russia
Sun Sep 13, 2020 9:14 pm quote
Exactly.
Hooked
Piaggio MP3 500
Joined: 21 Nov 2017
Posts: 288
Location: Austin, TX
Mon Sep 14, 2020 5:58 am quote
Yes, the best way to measure compression integrity of these bikes is by the leakdown method:

With a known and stable pressure reservoir, put pressure through a calibrated diameter orifice into the cylinder and measure the pressure before and after the orifice. The leakdown is the percentage of the pressure in the cylinder- if you have 100 psi before a .040 orifice and 97 psi in the cylinder, the measurement is 97%. Anything greater than 95 is good for this diameter piston. Less than 95, you can hear the hissing of air where the leak is located- intake valve leak will hiss from the throttle throat, exhaust leak will hiss from the exhaust and rings will hiss from the crankcase (oil filler).

Before the rebuild, I had a broken piston land, and the leakdown was 85%. After the rebuild it was 98%. Performance and everything seemed the same before and after, but my airbox is now dry.
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