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It seems like you are right about it not being rich. I tried smaller jet. It run terrible on WOT. Tried bigger than original jet, it runs probably better. But still nowhere near good as before. Tried different sizes but still don't know. It seems to me that it runs better without air filter on, so will buy a cheap open one.

But I still don't understand why it's not working as before. So confusing.

Yeah, I tried different diaphragms and noticed that one of them is not rising. Unfortunately I now don't remember which one was originally there few weeks ago when the problem happened, but definitely could be the cause!

The scooter now drives much better.. can even get to 70 kph (43mph) But still lacks power - I'm pretty sure it's running rich. I read somewhere than now more free-flowing exhaust could cause rich condition. So I ordered smaller jets and will test it. Will report here, but will take a few weeks till they come from china.

Ok, I checked the cam timing. Not just against the mark on the flywheel but also checked the real position of the piston. It's not a timing issue. I'm really really running out of options!

Can a CDI be faulty in such way that it would cause these symptoms?
I'm really lost.

Ok guys, bad news. I sawed the exhaust into two halves, got rid of the catalyst inside. I tested the scooter today with which was basically just a manifold with one half of the muffler - without any air restrictions, basically just a pipe. It didn't work well. It was pretty much the same. Now I'm really confused.

When I disassembled the engine, I noticed a tiny tiny wobble in one of the bearings on the camshaft, so the bearings are definitely not in the best condition, but I don't see that causing that big loss of power.

I'm running out of options. Honestly, I can't imagine what can be the cause. Can it be something electrical like CDI?
Or is it running bad just because it had no backpressure?

So to sum it up:

The engine runs smoothly and nicely but has no power.
It's bad with and without the muffler.
Valves are good.
Valve gaps are good.
The timing chain is correctly set.
The engine has new oil.
Runs bad with and without the air filter.
Runs bad whether the needle in the carb is in the lowest or highest position. 
I checked valve springs and they are not broken. But I can't say for sure for the inner one, I couldn't visually check that one.
I can imagine how this could become a sudden problem with results similar to mine. But pushing them with my fingers is very difficult so I don't know....

One of the main important things to note is the way the problem developed. 

I was running WOT on the city highway when it SUDDENLY started losing power and lost most of the power in a few seconds.  I was happy that there was an offramp in 2 minutes.Since then it basically has no power. It's sluggish and slow. But engine runs nicely, no bogging or anything. But during my first ride after the problem developed, it was actually firing into the exhaust sometimes and losing power completely. Now it works fine, but has no power.

New test: Today I unscrewed the exhaust from the engine so it wasn't tight and so it doesn't seal properly. It definitely run better! Therefore I assume the problem really lays in the exhaust. I will try to get a new one, but finding one for gy6 153qmi is a pain in the arse. Atleast here in Slovakia.

Thank you guys, your help is tremendous , will report as soon as I buy new exhaust.

EDIT: Seems like getting a new one which fits will be a real trouble. Is there something I can do right now? Like drilling holes in the muffler...  or something ? Thanks

Hi guys, so I put it all back together. And even did a test ride. Nothing changed. (Well I did nothing so I'm glad it's still the same )

So, to sum up my recent test:

• Don't forget that this problem  happened all of a sudden during a 10 minute highway ride at 90 kph (55mph). Suddenly it lost power and I was happy I was able to get off the highway doing about 40-50 kph (25-30mph). 
  
• Air filter is not clogged or anything
  
• Timing chain is correctly set
• Valve clearances are correct
• Valves and valve seats are sealed properly
• Compression seems good (But I don't have a tester, but valves don't leak, piston rings seem OK, and rotating the flywheel creates adequate resistance in compression stroke)
• Transmission has no problems (vario OK, belt OK, clutch OK). Wheel is easy to rotate by rotating the clutch.
• Sound when running is normal, nothing unusual
• I wouldn't have noticed that it is "weak" or has not enough power when on the main stand freewheeling.
• It has no power when driving. It drives like a lazy 50 cc. Lazy acceleration and max speed of 50kph (30mph) but otherwise runs nicely without any hiccups bogging or anything.
• Spark plug after the ride was black and sooty indicating rich condition (but maybe the soot got there when idling for a few seconds after the ride)
• I have problem with idle, I can't get a nice stable idle to work. It seems to work the best when the idle screw is all the way in - which is strange (In the past I used to have idle screw about 2 revolutions out for nice idle). And auto choke works OK.

And about the exhaust issue: Yep, new exhaust would be great. But I see prices of around 200 euros for this exhaust. And that's way too much. That's the value of the whole scooter right now. It has an 153qmi engine, does anyone know if mounting exhaust for 152qmi or other type will be a problem? 


Here's another picture I took. It looks just like when I bought it new from china.