Modern Vespa

The engine in my 500 is very hard to start and then when it does, feels kind of sluggish to rev and only idles for less than a minute then dies. Spark plugs are new, injector is new, changed the gas in the tank, disconnected the fuel line to the injector and there seems to be plenty of fuel being pumped out. My next step is to check the valve clearances.

Anyone have any other ideas of what might be going on?

Also, anyone know what these connectors with the blue wires are for?

Cleaned the air bypass valve yet?

Is that located next to the throttle body and accessible by removing the airbox?

The air bypass valve is part of the throttle body

If the bike still has a hard time starting, after cleaning the air bypass valve, then next job is adjustment of the intake & exhaust valves

Got it. Thanks.

Still trying to figure out what those blue wires are for.

Does your bike have 1 or 2 ignition coils? Maybe a coil is missing or unplugged, hence the wires, hence the hard starting??? Strictly a guess, since I don't know the early gen 1&2 bikes very well

The wires look factory, and not aftermarket junk

Not sure how many coils it has, but I know that both spark plugs are firing because I tested them.

Does your bike have a cigarette lighter? On mine (a early gen3 mp3) the factory cig lighter sits just above the engine next to the battery.

On later bikes the cigarette lighter was removed as a cost cutting measure

Does one of the wires have constant +12V under all conditions including key off?

if you trace them backwards do they go into the main harness bundle?

Yes, there is a 12V socket under the seat. Don't know if it has power though because I haven't tested it. The underseat light doesn't turn on when the seat is opened. There doesn't seem to be a switch that I can see that would turn on that light though. Anyone know where the switch is located? Maybe those wires are for one of those. I'll have to check.

So I took the air filter box off to take a look at the air bypass valve. I assume it's that small hole next to the throttle plate. Makes sense now. That's how the idle circuit gets its air when the throttle is closed.
Interesting thing, the engine doesn't seem to want to start if I don't twist the throttle a little bit (thereby allowing air to get in). But, once it starts, it will sit and idle forever with the throttle closed. I haven't sprayed cleaner into the air bypass yet. This is puzzling because if it is blocked, it shouldn't be able to idle.

Maybe the valve lash needs to be adjusted. Sigh. I don't really want to remove all the tupperware around the engine so I'll just have to see if I can make the adjustments using only the provided access holes.

Do not adjust the throttle plate under any circumstances. The plate is never 100% closed

Clean out the bypass valve first using carb cleaner or throttle body cleaner, followed by adjusting the intake & exhaust valves.

Then report back

The cig lighter socket is easy to test. It is hot at all times, so plug in something and see if it powers up.

The light is triggered by a switch located around the edge of the bucket.

Could those wires be from the saddle switch? The switch that checks if you're properly seated and which prevents from unlocking the rol-lock and revving.

Welp, cleaned the air bypass port best I could then adjusted the valve clearances (what a PITA). They were already pretty much spot on. No change in the starting issue. Re-installed the air cleaner box and it's even harder to start, but when it does, it seems to idle reliably. Throttle response still seems a little sluggish, though I don't really know how it should be since the scoot was dead when I bought it. I'm just happy I've been able to get this far with it.

Now I'm wondering about the TPS (Throttle Position Sensor) or maybe the O2 sensor. The spark plugs are pretty black after a bit of idling. I haven't actually rode the scoot yet because I have the front end tupperware all removed.

One thing I haven't tried is to spray brake cleaner all around the intake/throttle body area while it's running. If there is an air leak somewhere, that should make itself known.

Also, there doesn't seem to be any kind of switch in the pet carrier to turn on the tub light. Oh well, not really that important anyway.

Make sure the seal on the airbox cover is present snd correctly installed. If not, it will be very hard to start.

I concur on checking on vacuum leaks, but I use propane. Easier and cleaner to use. Any change in idle confirms you have a leak

Brake cleaner is very hard on plastics and tends to discolor them and/or make them brittle

It was hard to start even without the airbox in place, but easier than when it is in place. Strange.

The brake cleaner we have here in California, USA is mutated to meet the states environmental laws, which means it's pretty weak and won't harm the air or plastics. It also doesn't clean as well as the good stuff I used in years past.

Maybe I'll pull the O2 sensor to see what it looks like. If it's heavily coated, I'll try burning it off with a torch then cleaning it with solvent to see if that helps. The scoot has fairly low mileage (kilometers) considering it's age though. About 14K miles, or 22,500 km.

With that mileage the sensor should still be fine, but make sure that seal between the airbox halves is there and correctly installed

Let's hope you're not dealing with the dreaded mp3 cracked piston ring problem

Cracked piston ring problem!??! More info please.
Will a leakdown or compression test find this?

Search the forum or use Google.

It's one of the big weak points on the mp3

I'd start with a compression test to check if it holds. When a engine compression is low, it inherently becomes hard to start

Spraying brake cleaner around the intake and throttle body made no difference to the idle. Guessing everything is well sealed. I'll have to buy a new compression tester since the one I have will not accommodate the small sparkplug hole size. I bought it back in the early 70's when I was working on and hot rodding American muscle cars with their big V8 engines.

OK, did some research regarding the cracked piston ring issue. Seems like it's actually a cracked piston ring land, in other words, it's the piston that's cracking and not the rings. Either way, it's bad news. And since replacing just the piston is generally not a good idea since one would have to know the exact bore size of the cylinder and then a hone job, a complete matching piston/cylinder set is preferred.

Something like this:

https://www.ebay.com/itm/236333629742

Also, since the MP3 500 engine has a compression release mechanism to make starting easier, a normal compression tester will not work. Correct me if I'm wrong. What's needed is a leak-down tester.

I read that one of the indications of a cracked piston is the air filter box filling with oil, along with the hard starting. While there is a little bit of oil in mine, I wouldn't say it was filling up. Maybe it's because the engine hasn't been run long enough for it to collect. I noticed when I had the air filter box removed that there is a LOT of oil vapor/smoke coming out of the crankcase vent that normally gets fed into the airbox. If the piston ring land was cracked, the rings won't seal correctly and combustion blow-by would cause that kind of smoke. It would also explain why the brand new sparkplugs are turning black in such a short time. Sigh.

This is turning out to be a very expensive scooter purchase.

I've dropped GY6 engines from smaller scooters and rebuilt them, but while the Piaggio engine is similar, it's a whole new kettle of fish. I wonder if it's possible to partially drop the engine, just enough to remove the head and cylinder. Maybe remove the gas tank to give better access. Need to do more research.

Sorry for the late response. I think if the seat pressure sensor were to be disconnected, there would be an error light on the dash or some kind of reaction from the ECU. At least that's what I've heard.
I'm thinking those wires were connected to the seat switch that turns on the tub light because the switch itself seems to be missing. There's a hole there, but no switch. Perhaps a previous owner (I'm the third caretaker) removed the switch for some reason.

Have you noticed more oil in the airbox? My 2008 was one of the ones that suffered the cylinder eating itself. The early signs of it was an increased about of oil in the airbox that gets worse with time until failure.

Oh I will take a look for the blue wires on mine when I get home as I currently have two 500s apart in my bike barn lol

There is some oil in the airbox, but not a lot. Though there is a lot of oil vapor coming out of the crankcase vent when the engine is running. This is my first MP3 and I haven't really been able ride it yet since its been dead from the day I bought it so I don't know what is normal or not.

Basically, I either got a great deal on an MP3 500, or the biggest paper weight I've ever seen.

I didn't get chance to check the blue wires yet.

If you are concerned about the top end health maybe get a compression test done.

The 2008 is the one with this problem for the most part but according to an informal poll I did with other owners at the time the suspect build months were between March and July. Here is a thread where I accumulated the data. 500 owner's input wanted

As far as I now Piaggio has never acknowledged any build defects.

Did a leak down test. Failed miserably. I could even hear and feel air coming out of the dip stick hole.

Ah well. A new piston/cylinder/gasket set isn't too expensive at around US$500. Now to see what a shop will charge for labor. If it's too expensive, then I'll try tackling it myself. I've dropped and rebuilt GY6 engines before and the Piaggio engine doesn't look too much different. A lot heavier and water cooled.

Life is an adventure. It's a good thing I didn't pay very much for this thing. Besides, I like this scoot, even though I haven't been able to ride it much.

I'd think more than twice before spending the money on repairing the engine.

Considering the age of the thing, they did make improvements along the way, you'll probably be better served with a newer mp3.

As of this writing, the best bang for the buck seem to be the early 3rd gen mp3 circa 2015-2017. The 3rd gen is a more reliable and dependable bike without major engine problems.

The upside? You can still find early 3rd gen mp3s for not too much money due to their age that are still very low mileage.

And if you do decide to replace the jug and piston, it's best to use a proven part like those from Malossi

I checked Malossi. Seems they no longer offer piston/cylinder sets for the early MP3. Other than the engine and non functioning roll lock, my MP3 is in excellent condition with no rust, corrosion, or worn out seat. Even the shocks are clean with no indications of leaks. The paint is perfect and there are no cracks in the tupperware. I'd like to save it and give it a second chance at life. Most importantly, I like the styling of this generation MP3. I like the Mad Max look. Besides that, I've already ordered a new piston/cylinder. I also have a aftermarket muffler coming that's lighter, louder, and de-catted. I like old things. They remind me of me. And besides all that, MP3 500's are rare as hen's teeth here in the States. They usually have fairly high asking prices because of their rarity. In the many years of perusing local ads for used Piaggio MP3 500's, I've come across no more than six, and most were older ones like mine. As a matter of fact, there's a second one of the same age at the dealer where I found mine. Like mine, it's there for repair but is in worst shape plus the owner is not interested in selling anyway.

Scooterwest has the OEM kit. https://www.scooterwest.com/cylinder-piston-pin-ring-group-873812-b019019-mp3-500-2007-2019.html

It is not a bad job. Hardest part is breaking it in half after getting the plastic off lol. Here is a thread where I documented my nooby attempt and ultimate success doing it. Got the engine dropped today. Head off. NOW GOOD NEWS!

Thanks sticky. Scooterwest is where I ordered the set from, along with various gaskets needed for the swap.

And thanks for the link to your 2015 post. Still reading through it but there's a lot of good info there.

I looked at the build plate for my scoot and it turns out it's a 2008 manufactured in May. The scoots paperwork says it's a 2009 sold in 2008. Confusing.

Your MP3 looks exactly like mine. I hope the head on mine isn't damaged too badly. I plan on doing some porting work on it since it'll be out anyway. And Maksor mentioned that the engine doesn't have to be entirely removed to change the piston/cylinder, just tilted. I hope that's true. Sure would make life easier. Though I'm wondering if the fuel tank needs to be removed.

Yeah he told me that, after I had pulled the scooter apart lol. Should have asked him first about it since he is the MP3 wizard.

Yep that puts it in the "failure zone" based on my informal polling data.

What's the best way to drain the coolant so that it's easy to capture rather than just dumping it on the ground?

Remove the cover of the water pump impeller (lowest point on the bike) after placing a tray on the ground

Have a new gasket on hand before starting

Make sure you loosen the bleeder needle valve during refill (engine running) to avoid warping the head

My new piston/cylinder just arrived today. I have my new Dominator muffler too. Now to find time to get the swap done. My tiny garage is full so I'll have to do all the work outside on the city sidewalk. I don't have a driveway. Hope I don't run into any major problems so I can get it done ASAP. I'll take some pictures along the way and post them if anyone is interested.

So just pulling off the hoses from the pump cover isn't good enough? I mainly want to drain enough coolant to allow removing the head and cylinder without making a mess. And thanks for the tip about the bleeder valve on the head. I was wondering what that was for.

You don't need to remove the hoses