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Hi guys,

I've been reading through the forum for the past few days, tried some things, but all to no avail.

My scooter has been filling air filter for the past few 1000 kms and stalled from time to time (rarely) when coming to a stop. But now it turned much worse. It's filling the air filter much faster. If I install a new air filter (oiled and then squeezed out all excess oil) it starts dripping out of the airbox after like 100 kms. If I open the airbox a lot of it comes out at the back (you can imagine the front pipe and the rear bulb being full at that time).

It starts and if it doesn't stall I can drive it normally. But if it stalls it takes much longer to start it again. While driving I really can't feel any difference from before - I think it runs normally. Maybe it has a bit less power, but I'm not sure. And from time to time it feels like it looses a bit of power for a second when driving like 50 km/s (so with throttle open just a bit). When I come to a stop there's a high chance it will stall again. And then it's pretty hard to start again (have to try a few times).

I remember it idling very stable at 1500 rpm in the past, but now it almost never reaches that rpm and acts pretty erratic.

https://youtube.com/shorts/O6FXiAyGZvk

I also started to hear decel popping when approaching 0 km/h, especially if I brake hard - I imagine that's when the clutch disengages and the idle loop should take over.

When dealing with that issue I noticed that it starts much easier if the crankcase vent hose is disconnected from the back of the airbox. I imagine that's because it has quiet a decent amount of blow-by and if the hose is disconnected it breaths much easier. I don't really know how much of a blow-by is normal for this engine.

https://youtube.com/shorts/ofYxBprwZAA

So, what I tried... replacing the air filter, removing throttle body and cleaning the butterfly and the walls around it. I also tried to spray some throttle body cleaner through the idle hole in the throttle body. Also tried to spray cleaner into the idle hole while running at idle, but it kills it quickly since it gets all the air through that hole when idling. I replaced the spark plug.
I checked for air leaks but I can't find any. Did the compression test and it shows something very low - like 78 psi, but it's not relevant because it has a release-valve or something... that's what I read on the forum.

At this point I'm out of ideas. And considering the blow-by volume, I think the piston is cracked. I would be glad to hear some of your ideas. I know there are hundreds of topics regarding issues just like that, but I have a feeling I'm missing something... I would like to avoid tearing it apart and finding that it's not the piston.

Maybe it's just a failing coil or something... I also read that someone had idle-stalling issues and solved it by replacing the coil. I don't know.

The scooter has 31k kms on it.
I'm grateful for any suggestion.
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Molto Verboso
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"oil in airbox" is a classic symptom of being overfilled.

Piaggio lists 1,7Liters as the refill capacity but the actual is 1,5.

If I refill using the prescribed qty of 1,7 liters then I also get oil in the airbox and the plastic tube fills up with oil.

Basically, do not go above the middle on the safe zone stamped on the oil dipstick.
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First of all thanks for your answer sbaert. I don't think it's that. It's throwing oil even though the dipstick shows 2mm above minimum. After a 15 km ride I have a teaspoon of oil in the tube, that's connected at the front of the airbox. It's really sucking that oil from the crankcase. Any other ideas?
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Crankcase pressure too high or reed valve defective???
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sbaert wrote:
reed valve defective???
Are you sure it has one?

(it does, I checked the parts list)
⚠️ Last edited by SteelBytes on UTC; edited 1 time
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A 400 is still a Master engine, so I venture to guess yes
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Can anyone tell me what is the purpose of this reed valve? If I understand this correctly it should allow the air to pass from the crankcase into the airbox, but not the other way around. Why not? How does the crankcase "breathe" then when the piston moves up? There must be something I'm missing here...
However - a few days ago I disconnected this vent hose off the airbox, cleaned it and tried to blow into the hose. I remember it passed some air, but pretty slowly. I could feel the resistance rising after a few seconds because the pressure went up in the crankcase. Just to be sure I unscrewed the oil dipstick while doing that - in that case there was no more resistance after a few seconds of blowing, because the air could escape through the dipstick hole.
Am I correct in that I should not be able to blow any air into crankcase breather hole if the reed valve was working? Not even slowly?

I also tried to suck the air out... all this sucking and blowing doesn't sound right, but yeah... I could suck some air out slowly, but if tried to suck harder (LOOOL ROFL emoticon) I could hear the reed valve vibrate or something (it sounded like some kind of a trumpet). That's probably the correct behavior since it sounds like it's opening and closing. It would probably stop doing that if blew harder (I'm out of appropriate emojis at this point).
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and it never did get the software update i think 2010/11 there was one then for the stalling issue
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I decided to take the motor out so I can open it up and inspect the reed valve and then the piston itself. It's just blowing too much oil and I believe the volume of blow-by is way too high.

I'll try to follow the instructions from the workshop manual on how to remove the engine, but I was wondering if there is a video available on the internet somewhere on how to remove the engine. It would help me a lot!
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There's always the possibility of the dreaded cracked piston ring, although that will usually foul the spark plugs and thus easy to verify.

Either way, open heart surgery (wrenchfest) is required
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sbaert wrote:
There's always the possibility of the dreaded cracked piston ring, although that will usually foul the spark plugs and thus easy to verify.
That's a very good comment to check before going deeper, not sure if you (GrPo) do spark plug check?
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not common on a 400 a cracked ring
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This is how the plug looks like. Any thoughts?
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That plug looks to be alright outside of the slight soot buildup.

Are you using top tier premium gasoline? And when is the last time you used a fuel system cleaner additive such as LiquiMoly?
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I never used any additive whatsoever. Always just a a pure gasoline 95 octane (which equals to 91 octane in the US as far as I understand). I am willing to try any additive in order to decrease the volume of blow-by, but I don't see how would a fuel additive do that?
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Where are you located?

If you are living in Europe or the US there is with Very few exceptions no such thing as pure gasoline.

If you are living in Europe you should run 98 octane to minimize the bio content that is added to gasoline
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By saying "pure" I ment "only" gasoline - meaning no additives. We have 95 and 100 octane gasoline available at the gas stations. I understand that a bad batch of gasoline could be reponsible for bad idling, stalling and hard starting conditions, but I can't see how would it cause huge amounts of oil being pulled from the crankcase into the airbox... This is what really concerns me.
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GrPo wrote:
By saying "pure" I ment "only" gasoline - meaning no additives. We have 95 and 100 octane gasoline available at the gas stations. I understand that a bad batch of gasoline could be reponsible for bad idling, stalling and hard starting conditions, but I can't see how would it cause huge amounts of oil being pulled from the crankcase into the airbox... This is what really concerns me.
Agree whit you, the oil sounds as excessive and I doubt the gas quality is the problem for that
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I think I'm running out of options... so an open-heart surgery (as sbaert said) is probably unavoidable. Has anyone seen a video on youtube on how to remove the engine from the mp3 400? I found the workshop manual online, but video would noticeably speed up the process for me I think.
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You remove the engine and drivetrain as a set from the rear. The hardest part is removing the exhaust strap. Once you have the exhaust muffler removed, then it is just a matter of removing the rear shocks and the big bolt holding the swing axle.

The biggest time eater will be the "onion peeling" process aka removing the body panels to disconnect connectors and linkages.

The peeling starts from the rear and you work forward.
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So today I had a few hours and I think I got through the onion peeling process (thanks to videos by Mr MicBergsma on YouTube).
According to the workshop manuals that's just the very first step. But yeah.. at least I started.
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Hey guys, today I took it apart. I did expect to find something like that... maybe not that bad, but yeah... it's not very pretty Facepalm emoticon . I hope I can reuse the head even though it got beaten by the ring pieces... Maybe I should check whether the valve seats are ok since these ring bits were flying through them at some point... any recommendations?
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I don't expect be that bad, so sorry.

As you have all parts apart check valves only to be sure all is ok and the plane of valves assembly
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Ouch, reading this thread it seemed like it would be something bad, but oh boy oh boy did that escalate quickly.

As an aside, I wonder if investing in an endoscope or inspection camera might have helped speed the diagnosis. There are a few on Amazon with heads small enough to fit through the spark-plug hole. The end result could have been the same but the diagnosis could have been quicker.

I have this one for which there are many clones. Its not super great but its useful to check inside holes, pipes, or tight spaces to see where a wire goes.

https://www.amazon.com/gp/product/B01MYTHWK4/
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That damage appears to have been caused by engine knock/pinging (pre-ignition timing).

Are you sure you used only high quality big brand 98RON grade gasoline?
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Since I have endoscope I could have checked it earlier and maybe ordered the parts sooner, but I simply didn't expect this kind of damage. Next time I'll do that for sure, cause it only takes a few minutes.

Regarding the gasoline - I've always been using 95 octane gasoline, not 98. As I said we don't even have 98 available here... I remember we had it in the past but they "renamed" it to 100 like 20 years ago. So we have only 95 and 100. I believe these engines were designed for 95 octane fuel. Correct me if I'm wrong. I highly doubt this was the culprit for the cracked ring, but I really can't say for sure, cause that's not one of my expertise.

I made a list of parts I suppose I need:

1A003631R Stud bolt (4x)
830276 Gasket - cylinder base (0,6 mm)
B018504 Cylinder kit (400 ccm)
847928 Gasket - chain tensioner
844957 Cylinder head gasket (400 ccm)
1-002-164-591 Exhaust gasket
830820 Gasket - head cover
830249 Damper drift (6x)
848140 Thermostat

I'm not sure about replacing the reed valve... I'm not sure if replacing it makes any sense since I had a huge blow-by, which was probably the cause for so much oil in the airbox. It's quite a bit of additional work and gaskets...
The part number is: 82778R Reeds bracket

I also have a question regarding coolant - there was red/pink/orange-ish coolant in the motor. I couldn't find in the manual which type of coolant should be used.
Maybe it's this one... it's of the same color at least: MOTUL MOTOCOOL FACTORY LINE -35°C

And now for the head - I rotated it upside down, placed it onto a flat desk and covered the valves with gasoline. After a few minutes the level of gasoline remained the same and the paper I placed under the head was dry. So I guess the head is OK. Some say I should flatten its mating surface but some say that it's not necessary if the engine wasn't overheated... any thoughts?
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Trying to help in some questions and comments:

- Your test for the valves is fine and if opens an close fine, in my opinion you are ok
- About the coolant, the motul factory line is perfect, you need a organic one (don't use the yellow from motul, is not the correct one). The Piaggio manual talk about Ethylene glycol antifreeze liquid with organic. ASTM D 3306 - ASTM D 4656 - ASTM D 4985 inhibition additives. Red, ready to use. - CUNA NC 956-16 but this is Castrol. Any organic goes fine, and the Motul you say is perfect if is available in your country.
- In your part list, not sure if you really need the stud bolts if yours are ok. Piaggio ones are not the stretchable ones.
- Maybe (and is only a opinion) you change your cylinder from 400 to 493 (the one in the mp3 500), more torque and the other pieces are the same (if I'm wrong please correct me)
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fit a 500 cylinder its also cheaper the a 400 or 460cc
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The factory coolant is G12 spec. Easiest replacement source is your nearest VW dealer.
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Ok, I'm erasing stud bolts from the list (thanks dariusz).

G12 spec coolant is of the same color indeed, but I wasn't sure if it's actually the correct one... It's available everywhere.

Regarding the cylinder kit - now that you mentioned I checked and it really is MUCH cheaper. 455€ versus 170€. Can't see why though. They look a bit different, so I'm not sure if it will fit...

https://oem-bike-parts.com/en/parts/piaggio/mp3-500/2009/mp3-500-2007-2009-nafta/engine-cylinder-piston-wrist-pin-unit/p/1a001833-piaggiogroup-1a001833-cylinder-kit
https://oem-bike-parts.com/en/products/p/b018504-piaggiogroup-cylinder-kit-400-ccm

Oil and coolant passages look a bit different (bigger)... in that case I would probably need to replace the head. I guess the connecting rod is probably bigger on 500 in order to withstand additional forces. And maybe I would need to remap the ECU to give it enough fuel. It might be, that I'm looking into a wrong cylinder set... There are many models... I mean, I like the idea of installing the bigger cylinder set, but I guess it would be far from plug&play Has anyone tried that?
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dariusz wrote:
The Piaggio manual talk about: Ethylene glycol antifreeze liquid with organic. ASTM D 3306 - ASTM D 4656 - ASTM D 4985 inhibition additives. Red, ready to use. - CUNA NC 956-16 but this is Castrol.
This info is from the Piaggio manual, so any coolant that meet this works for you

About the studs not sure what to answer, but searching for the studs I find that the 2009 400 have a Bore x Stroke of 85.8 x 69 mm and the 500 is 94 x 71 mm, so i think is not a direct conversion as the connecting rod is different in long, and I think you don't like to go this path...

About maps and fueling is very possible that is ok to support the change seeing the injectors specs between both.
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Since I can't find any in-depth case of installing a 500 ccm kit onto the 400 cmm engine, I guess I'll have to go with the expensive 400 ccm kit.

I'm not sure which cylinder base gasket I have to buy. I found three sizes:
0.4 mm
0.6 mm
0.8 mm.

Since I had 0.6 mm gasket installed I assume the head was already resurfaced once, so it's probably a good idea to take the 0.6 mm one?
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I have a question for those who already took a look into their engine...
The manual says that there should be the following parts mounted over the timing wheel:
- counterweight mass
- and a valve lifter mass stop bell

I dont have any of that! The timing wheel was secured directly by one single central bolt and that's it. Is my engine missing some parts or were these parts added in newer versions of engines?
My engine.
My engine.
parts from the manual
parts from the manual
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FYI: I just found that drawing on oem-bike-parts.ro
Seems like I'm not missing parts. HUH, that's good
Maybe the engineers added this counterweight on the later models to solve some vibration issues or something... I don't know.
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the 400 doesn't have a counterweight Razz emoticon
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Maksor wrote:
the 400 doesn't have a counterweight Razz emoticon
Interesting - how does it achieve lowering compression for starting - or is that not necessary for the 400?
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I didn't know the counterweight is used for lowering compression. But now when I think about it... the only way to lower compression is by adjusting the valve opening/closing time. Since mine does not have that I guess my compression test result was valid (80 psi). I'll redo that test after I assemble the engine with new parts and it should go way up (I hope).

One more thing I found yesterday... eventhough I checked the valves for leaks by turning the head upside down and pouring in gasoline I decided to take it all apart just to see how it looks like inside. I found damages on one of the intake valve seats. Since I'm still waiting for the parts I decided to polish the seats using the valve polishing paste and the stick with suction cups. But before I do that - has anyone tried that? Any thoughts? I certainly don't want to do more damage.
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Play the safe card, and get a brand new complete head, jug and piston vs. trying to patch up the old one OR buy a good used complete donor engine & drivetrain

Given your 2 choices on gasoline, always run the higher octane especially with these little engines that do not have knock sensors.
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sbaert wrote:
Play the safe card, and get a brand new complete head, jug and piston vs. trying to patch up the old one OR buy a good used complete donor engine & drivetrain

Given your 2 choices on gasoline, always run the higher octane especially with these little engines that do not have knock sensors.
Huh? 95 RON is what Piaggio specify. Why use 100?
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GrPo wrote:
One more thing I found yesterday... eventhough I checked the valves for leaks by turning the head upside down and pouring in gasoline I decided to take it all apart just to see how it looks like inside. I found damages on one of the intake valve seats. Since I'm still waiting for the parts I decided to polish the seats using the valve polishing paste and the stick with suction cups. But before I do that - has anyone tried that? Any thoughts? I certainly don't want to do more damage.
You can do a valve seating process as with any engine, and it is not a bad idea since you have everything disassembled. The marks are too small to think about new pieces and with the polish and the normal carbon accumulation from combustion will disappear
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