BIKE SPUTTERING BAD! WONT GO ABOVE HALF ITS TOP SPEED! HELP!

[webit]

Howdy


I have searched all over the internet for a solution to this problem, been to more than one mechanic and spent quite a bit with no success in solving this issue. As a result my cruiser has been parked for a few months now already. I'm from South Africa though and summer is here - need to get this baby riding! Please assist and advise - any help would be appreciated.

I have a cheapie model cruiser. Its a 2009 MotoMia Cruiser 250cc, 4 stroke, 2 cyllinder. It has less than 5000km on the clock and bought it second-hand. The oke I bought it from has several bikes, so this one remained parked. He only bought it as cheap transport to and from college every morning while finishing exams.

It ran well and with no issues at all for around 3months. I fell in love with it.
Then, one day after taking the longest ride I've ever taken it on - about 40kms - it just started sputtering when I was pulling up to turn into my driveway. After that, it sputters as soon as you reach about quarter throttle.
Opening the throttle fully does sometimes stop the sputtering, but the bike runs terribly and sounds and feels like its only running on one cylinder. I also say this because it never makes it over 60 anymore (and also getting to 60km/h takes a lot of effort) and it used to make 100km/h easily - bearing in mind that top speed is 140km/h. I usually maintained about a cruising speed of about 80km/h, 90km/h maximum.
I've changed plugs, replaced CDI/Blackbox, checked hoses, air filter still looks like new (like most other things on the bike), checked plug wires, checked other wiring, drained fuel over and over and put in new fuel from various fuel stations - no difference.
The spark plugs always seem wet when pulled - I am not completely sure at the moment, but I am almost sure that one plug seems wet and the other normal (not sure as I haven't worked on it for a while and can't fully remember right now).

I should mention that at idle the bike runs fine. Revving the bike, even to full throttle, gives nothing away. It only does this when driven.

I am planning on pulling the carbs tomorrow and checking the main jet and also just do an overall clean, but I don't really want to waste a whole day with no success at the end again. Please, if someone can help me diagnose this problem..

Thanking you in advance for any assistance!
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2009 MotoMia Roma 250cc Cruiser

[Rogerborg]

Just to confirm, you can rag the nuts off of it in neutral and it runs fine? Both exhausts are hot, both plugs are a light tan, not black or Hitler's-ghost-white?

Then you ride it on the road and it won't get up to speed?

Does it run smoothly but simply not rev?

Does it pop, bang, or spit in a regular way?

Or does it missfire irregularly, like it revs then m-m-m-misses, revs, then m-m-m-isses?
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[webit]

[Nexus Icon]

It sounds like one plug isn't sparking under load and, after skimming through the list of things you've changed or inspected, I reckon you need to look again at the HT circuit, especially for the plug that's wet with fuel. Check there's not a hairline crack in the cap or something simple like that.

One of my 125s would start fine, rev beautifully on the stand and would only cough and splutter once over about 3000rpm under load. That was caused by a crack in the plug cap that I could barely see even once I knew it was there.
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Greetings from Shitsville!

[webit]

[ADAMH0]

Try putting 2 of these on your bike. They go inline between the plug cap and the spark plug (there are different ones available depending on what type of connection you have). Ride down the road and if the HT circuit is breaking down under load before the plug, you will be able to see it.

https://s6.postimg.org/xakokyugt/696393.jpg