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Thanks for advice. Why is it so? I changed my piston and piston rings last year, I mostly wanted to gain some experience and it was fun to do.

After the job I got an advice about not using G clip - (They said the extra part on G-clip in comparison to C-clip can break off and damage my piston/cylinder). (The original piston came with a C clip but the new one had a G-clip in the package)

I definitely don't think it's necessary to open the head and do this stuff, but I must admit I kind of enjoy it

Will post pictures

Thanks

Hi czmdiver!

It has more than 40 000 km (25,000 miles) on it. 

If by current top end you mean speed (sorry I'm not a native speaker), it is about 50 kph (30mph).

I'm gonna dive into it right away and will post some pictures.

Yep, the day after that thing with losing power happened, I checked the valves and intake valve had zero lash. I tried to check if valve spring is not broken but I haven't found a problem. I probably made one pretty bad mistake when adjusting the valve when I think about it, I rotated it while I was pushing the spring with my thumbs. So when I adjusted the lash, the valve might not have been seated properly.   --- DEFINITELY have to recheck that ASAP.

My exhaust is like 4 times welded. But never caused any visible problem before. Will check that too. 



Sorry I don't understand what that means. Can you explain it in some other words please? 


About it running different on center stand and without stand at all (in riding position):

It has higher rpm when on idle and on center stand - The wheel doesn't rotate and clutch works well. When I put it on the ground, the rpm drops. This is probably due to the fuel level changing a bit in carburetor.

I serviced the vario last year. And yes clutch is releasing. 

Thank you, Will post pictures and updates. 

Hi guys, thank you in advance for your help!

My scoot:
Keeway ARN 125
GY6 153qmi - 4 stroke

The problem:

I was driving WOT at about 90kph (55mph) for about 10 minutes and suddenly the scooter lost power and started decelerating, the engine was still working but I was only able to manage about 40-50kph (25-30mph) after that.

Since then, it can not exceed 50 kph (30mph), and having half throttle and WOT doesn't make much (any) difference.
The exhaust gasses smell horribly - I suspect lean condition. I don't have a thermometer but engine seems to be much hotter after a short ride than ever. The spark plug shows signs of lean conditions.
Engine was also firing into the exhaust on deceleration. 

It seems to rev just fine on stand. But doesn't have enough power on the street.


At first, I adjusted the valves. The intake valve had no valve lash at all.
Only effect I noticed was it is not popping or firing into the exhaust anymore, but this might just be a coincidence.

The engine idles, but it's pretty rough, but I have long term problems with setting this correctly.

The first 0-20 kph seems ok, but then the power diminishes and runs terribly. It does not boggle or anything, the sound is nice, but it just doesn't have the power.


Then I checked the carb.
Carburetor OK - all jets clean, I changed the diaphragm just to be sure. It definitely seems to be working OK. And anyway, it would be pretty strange if it suddenly "broke down". I can only imagine three ways carburetor can make a problem like mine: 
1. Damaged diaphragm (It's not)
2. Clogged main jet (It's not)
3. Problems with fuel level (This might be it, but I don't see it happening "suddenly". The float level is not adjustable on my carb, and it definitely seems to be OK to me. I would say this is NOT the problem)

Air leaks: I checked for air leaks, and found none. I've had problems with air leaks before, but they were mostly present when idling. When the butterfly in the carb opened under WOT, under these low vacuum conditions the vacuum leaks never were a problem. So I'm pretty sure air leaks are not the cause.

I recently changed bad CDI for a new one, which worked flawlessly, so I don't really see how this could cause a lean condition or loss of power all of a sudden.

My exhaust is in a pretty bad shape and I wouldn't be surprised if there are problems there, but how could it make engine run lean? I don't know.

Can engine running lean be caused by something wrong with the engine itself? Like piston rings or something? 

I'm pretty lost. I'm thinking about opening the engine and checking the piston etc. (I've changed my piston and piston rings before) but it's quite a lot of work and if it's useless, why do it. 

Feel free to challenge my conclusions. I'm definitely not a pro and might be interpreting the symptoms all bad - maybe it runs rich in the end..

Thank you for your help! 

So I exchanged the CDI for a new one today, and it works. I have some carburetor problems and difficulty starting, but the scooter works without dying. And I can honk without engine dying. Will see after a week or two if it is resolved by this.
Thanks for all your help!

It doesn't shut off when the horn is unplugged and I press the horn button. But I will test it all again when new parts arrive and will keep you updated.

tried a new ignition coil today. No change at all.. might try taking a more thorough look at the pick up coil.

EDIT:

Ok, I plan to do this now.

Wire CDI directly from battery +12V and ground.
Ground engine directly to battery. 

I've already done this but I will try once more. If it still doesn't work that leaves several options:

Bad CDI - how could I diagnose it?
Bad pickup coil or ground or signal from pickup coil.

Connection to the Ignition coil is already made new and should definitely be fine.

I'm pretty lost, but will try to play with it some more and come up with solution.

Hi guys, thank you a ton for all your answers. It's pretty difficult to work on a scooter right now as I don't have a garage and it's cold and snowing outside . I thought several times that I found the cause, but it never is the correct one. I tried checking grounds, connectors - but I'm pretty sure I missed a lot of stuff. So definitely have to recheck that. 

It's hard for me to diagnose because it's sometimes working perfectly and sometimes not. For example the horn trick does not work now - engine won't die when honking. 

But now it has come to the point when I have this one persistent problem - that might help me find the real cause. 
It's the starting problem. Engine runs only as long as I hold the starter button (Seems like a ground problem to me). (After trying to start for like 5 minutes or maybe 10, it will suddenly start working without the starter button pressed and works ok, till the next stop, just a minute is enough and starting is problem again. Sometimes it just randomly dies too. )

Another thing is that it starts fine on first try, but I let go the starter button, it dies, and then it doesn't start. I have to wait for about 30 seconds to crank it because it doesn't fire at all. - This might possibly be caused by the spark plug getting soaked in the unburned fuel after the engine died? - If not, then I have no idea what might cause this unwillingness to start without a waiting period of 30 seconds or one minute.

I don't have a CDI photo right now. But I am pretty sure it is a DC cdi. I only have one 12v power cable connected to the CDI which gets juice as soon as I turn the ignition key. I even tried wiring it directly to + of the battery to see if there is any power delivery problem. But I only tried it on the stand and it worked most of the time with or withous direct connection so I don't really know. 

I think you already advised me plenty of useful ideas, so I should definitely recheck all connectors and grounds and I will let you know then. I'm not sure when the weather will allow me to do that.

Thank you guys for all your help! :-)

Here is a photo of the front where I suspect the problem lies.