Modern Vespa

Ok, so my GTS 300 was acting weird last night so I figured I would run it past the Modern Vespa crew for some ideas.

I'll do it tech support style first,
2012 Vespa GTS 300 SS
19,500 miles
Dr Pully Sliders at 12k miles
Belt change at 12k miles
up to date oil changes
Akropovic Exhaust at 14k miles "does not backfire under normal riding"
Mike's Airbox Mod at 17k miles
I am a bigger guy, 350lbs.

Bike has almost 20k and has always performed flawlessly. Went on a 120 mile ride yesterday, cruising 55 mph through the country, stopping every 15-25 miles to check out little towns, take pictures and what not, not a demanding ride by any stretch of the imagination.

Twice during the ride the check engine light popped on for 5-7 seconds and then went off, never ran funny or anything. This happened a bit around 16k miles, turned out to be one of my battery posts was loose.

The last 20 miles though things went weird. Bike started shuddering on take off, smooth, fast, didn't matter. Around 7 mph the bike would shudder hard then smooth out.

Also, during that 20 miles, the bike started backfiring violently anytime I was decelerating. Now with the pipe, I've always been able to leave a tiny bit of gas during deceleration and get a little pop or two if i didn't let go of gas all the way. But this was anytime i tried to coast it started popping VERY loud, enough that I was worried it was hurting something. So I kept some gas on to do a bit of a powered stop and keep the thing from barking so loud.

Right before the 20 mile problem there was a 5-10 min section stuck on 2 blocks of stop and go, got the bike hot enough to turn the fans on a few times. But it always cooled back down and stopped, then later when i was having the problems, I was moving fast enough to keep the temp down.

I've seen some mention of brake clean for the shudder, and possible needing the clutch roughed up. Could that clutch issue somehow be causing the back firing though?

Curious as to what people think?
Thanks for the listen.

UM... Road trip picture from the ride so its not all doom and gloom.

The shuddering sounds like the clutch.

The backfiring is maybe valves?

I change my belt around 6,000 miles and yours is at almost 8,000. Like LEMUR said the shuddering sounds like the belt. I'd change it out and see if that helps. I know some people go way more than 8,000 miles but i'd rather change it a tad early than have it shred and leave me stranded in the middle of nowhere. Haven't a clue about your check engine light. I just have a little old carbed bike. Good luck

okcgravity, sorry to hear you have an issue/s.

Moving off with a shudder, as everyone is saying could be the clutch needing a clean and deglaze. BUT...from your description I think this particular shudder is caused by a misfire when moving off. Popping and backfiring in the exhaust is due to weak mixture (probably due to lack of backpressure in the after market exhaust). This recently may have become exacerbated by perhaps a leaking exhaust manifold gasket. They do burn away over time on these bikes and this can cause the symptoms you describe. It's an easy fix though. It could also be the valve adjustment needs doing because the exhaust valves are going tight when the motor is hot causing misfiring. Other things to check are: the oil in the engine is not overfilled as this can be sucked into the air filter causing the wrong mixture. Loose spark plug. Try to check these things and report back to us. Good luck!

Well, updates to all of this.

Bike has 19,800 miles.
At 13k miles it got a new belt, Dr Pully sliders, and an Akropovic exhaust.
At 17k miles I did Mikes air mod

Original problem.

After bike warms up, 10-15 miles. Starts the shudder when starting slowly between 0 and 7 or so mph. If I grab the throttle its not as bad, but normal parking lot stuff was very... clunky feeling.

Then when decelerating the bike would sometime backfire uncontrollably until stopped, or given throttle.

Check engine light would come on occasionally.

Took it in to the shop, they put it on computer, error code is O2 sensor, the replaced it and told me to come get my bike.

Nothing was fixed by this repair, all of same problems exist.

They tell me it may be because of my aftermarket exhaust and open airbox causing problem but not sure. Also, shudder probably needs a new belt.

Visit to shop #2, they check computer, get same error code, change the belt.
I asked if the belt was bad, no, it was kinda worn, but not bad at all. Said the clutch sliders "not Dr. Pully, but actual slider" were a little worn, but they didn't have any in stock, and they should be fine.

The shudder was maybe a tiny bit better, but back firing and engine light coming on sometime persist. Weird clatter type noise at idle, very soft, probably just me being a super picky owner.

Visit to shop #3. They open it up, somehow while checking things timing get thrown off. They fix that, adjust the valves slighty, check exhaust, change air filter out, oil change, replace clutch sliders, de glaze clutch, clean engine up while in there. 16 mile test drive, "It's good now" I asked what was causing the problem since, "not sure, maybe the valves"

Mehanic said it kept hitting rev limiter and wouldn't go over 55 mph "something that has never happened" and its probably the Dr. Pully sliders. So they replaced the Dr. Pully sliders with stock rollers and it fixes this rev limiter issue that I never experienced.

So tonight I take the bike out on a 20 mile stop and go mostly under 40 ride to really warm up the tranny. And I get this awesome scrape noise coming from transmission sometimes. Happened about 20-30 times while doing a slow pull away from stop. After that the "clunkyness" was back at slow pull away. Then 2-3 times I had the super loud back firing while decelerating....

I don't know what to do anymore here. I've now replaced an O2 sensor, a Belt, my Dr. Pully sliders with stock, and clutch sliders, de glazed the clutch, adjusted timing, adjusted valves and NONE of these things seem to have fixed the problem. I'm out money everytime it goes in and nothing is getting fixed. Heck, with this weird noise I feel like we've gone backwards. And I have no clue what was going on with the "hitting rev limiter at 55" issue since it never happened to me and I wasn't talked to before replacing the rollers.

Anyone have Any ideas???? I'm pretty open at this point.

Also, live in an apartment where we are not allowed to work on cars in lot, so I don't have an option to do anything really myself.

And to clarify, this is a good shop, they've never done me wrong in past, mechanic working on bike knows his stuff but I don't know whats going on here.

Mine does the same noise.
2014 GTS 300SS.
Stock engine, stock exhaust.
Zelioni variator, zelioni flywheel, zelioni clutch, zelioni clutch bell, belt from a GT200.
I'm at 18,400 miles, but they're all city miles.
The flywheel,clutch, clutch bell, belt, and rollers have all been changed within the last 4000 miles except for my variator. It's been like 12,000 miles since I put a new one in and my mechanic says it's time for a new one again. I'm hoping it solves the problem.

Im no expert on engines nor Vespa but I've worked a lot on mc and car engines.
I learned that a petrol engine needs 3 things, air, fuel and ignition. If any of these 3 is out of balance you will have problems.

Originally you had backfire problems which is a combustion problem. Then you have shuddering which is also a combustion problem unless it's in the transmission, also the O2 sensor error can often be due to gases passing is so out of range you get failure codes, I always start with the first 3 air/fuel/ignition

I had the similar problem in a few engines and it was a bad spark plug most of the time there was shuddering and backfire. Even on brand new plugs I had problems. There has been fails inside the isolation. It sparks ok but intermittent it doesn't and that's when the engine shudder usually on acceleration. Anyways it's a good place to start the fixing of the problem.

So if I were you I'd start by just replacing the spark plug (very easy to do anywhere by anyone)

If that doesn't help at all let me know and I'll read through the repair journals I kept for more answers. Good luck:)

Acceleration chatter could be from clutch pulley assembly not cleaned and greased properly, between the to half of the clutch pulley are roller bearing ping which run in ramps between the tow sections, these are rarely cleaned, unfortunately, see service manual. They need to be cleaned, inspected, and greased along with the needle and ball bearings of the clutch hub. at 19,000 I would replace the mainspring which tends to pull the two pulleys together as you decelerate and attempt to resist opening the pulley, keeping the scooter in low gear longer, to accelerate faster from a stop. I usually get a new clutch hub, shoes and springs, these springs help resist the clutch making contact until you reach higher speeds, more torque/power from the stop. If any one of these springs weakens, the clutch will chatter, and if the big mainspring becomes weak, it will shift into high gear prematurely and lug the engine. Poor engagement of the clutch will cause slippage and hitting the rev limiter before you get up to speed. The aftermarket exhaust is sealed properly should not pop, but with all exhaust, the exhaust gaskets can burn away or become loose letting air into the exhaust causing a lean air-fuel mixture and popping sound. You popping sound sounds like a recent problem, not experienced back when install, suspect it may be a gasket. If not gasket, the ECU may have to be remapped to make the fuel mixture rich to compensate for the aftermarket exhaust.

You do realize there is about 4.5 years between your reply and the last post on this problem?

It's a pity sheepdog has chosen to post his reply here. His diagnosis to one of the reasons for the cause of clutch chatter and its remedy is insightful and it would be useful information to others to help address their own clutch rattle issues. Perhaps it could be edited and put into the wiki section.