Hello group. I have a Chinese GY6 49cc scooter that I am very determined to fix.
The scooter broke recently. It stopped starting because of the stator.
I changed the stator after some time and it ran well but something did not feel right. The moped was very slow that day. It used to go up to 50 km/h and today it was 30 km/h. It had poor acceleration. After a few days, it got worse. I was starting but when I tried to pull the throttle even to its max it did not even move. I pushed it a bit and it started to move slightly. Then I expected the moped and it was because of the fueling system. The vacuum fuel pump was not working quite right. I could see it after I changed all the fuel and vacuum houses. The fuel was hardly being pulled from the fuel tank which is located on the bottom of the feet on the moped. I tested for leaks but I could not find any. I managed to fix the vacuum fuel pump enough so it runs enough (20-30 km/h but with slow acceleration). I decided to make the moped with performance parts since I right now need to fix it. I ordered a new carburetor 24mm - a bigger one for better performance (PWK Mikuni carb that was not that expensive), I got a NIBBI air filter 48mm (the carb air side is 50mm but it fits good), NIBBI intake manifold (33mm rubber fit for PWK24mm), new electric fuel pump (CarBole Micro Electric Gasoline Fuel Pump Universal 5/16 inch Inlet and Outlet 12V 1-2A 35GPH 4-7 P.S.I.) and a TEMSONE Adjustable Carb Fuel Pressure Regulator Kit Universal Manual 1-5 Psi Fuel Pump Pressure Regulator FPR for Carburetor Engine with 8mm 10mm adapters.
I did not know and did not find any information what is the optimal PSI for the carburetor I am going to install so I got the fuel pump with more PSI just in case so I do not damage the carburetor I got the fuel pressure regulator in order to find out what pressure is good for the moped.

With all that said I should also mention that I have made some replacements:

• New variator rollers 6 grams (mine were not round and very worn out)
• New variator cooling fan because I broke mine when taking off the variator front pulley
• New kick starter
• The new stator I talked about and the new houses both for the fuel and the vacuum
• I changed the oil
• New belt
• I bought an iridium spark plug

Even with these changes, the moped was pretty much the same, just a bit better. I tested the spark and played with the air/fuel ratio. The last thing I was afraid that it could be is the compression of the engine. I removed all the plastics and opened the valve cover to adjust the valve's clearance. The intake valve - 0.004 and the exhaust valve - 0.005. Before I regulated them they were very tight. Everything went nice except I noticed a crack on the cylinder head. I bought parts to replace the heads of the engine and the piston which was very worn out. Now when everything is new and set up the timing is checked according to me right (I watched numerous videos about it, the T mark and the cam shaft position etc......) I assembled everything from the engine but left the exhaust so it is easy to remove the cylinder head if I need to afterward. I installed the old carb without the air filter and tried to run it but the engine did not start. I can hear it turning and the sound that it makes is quite weird. It sounds like bubbles popping. I saw that in the valve cover where the kicker and the valves are in the bottom, there is a bit of engine oil and I do not know if that is normal and if that is not because of a faulty gasket or something (all the gaskets are new) I again checked valves and timing but the result was the same. I changed the carb with the new one with the air filter on this time and used the choke (I do not have the throttle cable for this carb yet so I cannot rev it up I did not think about that when I ordered the carb) and put on the electric fuel pump by connecting it to the battery terminals. I could see the fuel from the transparent houses going into the carb but the moped did not start and sounded again the same. The carb was full of fuel and I removed the fuel house from the carb inlet I could see the fuel in the inlet to the top of it. I played with the psi but nothing significant happened. Also, the vacuum houses are installed correctly I think. The intake manifold is connected to the fuel petcock and to the EGR system and the house from the valve cover is not connected to anything since I removed the stock air filter (the box one) but I do not think this is a problem because from this house only gases from the compression of the cylinder are going out.

I am out of ideas at this point. If someone can give me a tip or tell me what I have done stupidly wrong please help me I am open to judgment. I am by no means a professional or a mechanic. I am just keen on this project and I just started to understand things so I am still a noob in this field. I would highly appreciate any criticism and feedback on how to fix the moped.

Thanks a lot !!!
I am attaching some pictures and videos as well.

Changed the cylinder

The new carb

The old cylinder head

top view of the old assembly of the moped before the engine rebuild

The old cylinder head

 

Ok so I bought a scooter brand new not even six months ago. Added a big bore kit. Everything was great. Added variator, clutch, CDI box, ignition coils, rectifier, new rollers, new stator, new plugs, new starter solenoid, new fuel filter, new gas cap and vent, new petcock, new enricher, I mean everything. It was awesome for awhile.. Then.. Back to the drawing board.. So as I was riding along one day to go buy a trunk for my buddy's scooter..I was in the parking lot at idle, and it just died and wouldn't start, after awhile if holding the starter button it did fire up but as soon as I have it any gas at all, it would die.. It was getting fuel and spark, spark looked weak but it would still idle and nothing more. My problems have now begun and have been fighting with this ever since. I bought a new stator and carb the next day and it seemed to work, but after about a mile it just bogged out and died again and I would have to wait till it would start.. Like it would warm up and die. So I changed the enricher.. Now it's erratic and problem persists..I took gas cap off in case it was vapor lock and that didn't help.. I've gone through six carbs now and can't seem to get over a mile away without breaking down. So that's my predicament.. As far as your problem goes..I went through that as well, and fixed it, see.. I'm willing to bet that when you changed your stator, everything was fine for a little while right? And then it started misfiring and losing power right? I'm willing to bet, that you didn't tighten the flywheel enough with an impact wrench. Then as you ride, the fly wheel would slide in and out a little bit on the woodruff key, slowing chewing it up and screwed up your crankshaft keyway so that now your timing is constantly skipping around making your motor run like crap. Probably starts right up fine and sounds fine until you give it wide open throttle, then after a few min.. pop bang poop.. Take your flywheel back off.. Use the right puller tool so you don't mess up your stator. If the crankshaft keyway is worn out or shredded, then week's some metal into it and use a Dremel to restore it to it's former glory, or split the case get more puller tools and tools to do your bearings, whatever it takes to change your crankshaft, I personally don't have the right tools myself so I did the welding trick and it works. In fact you really only need to line the keyway up and then tighten the flywheel with impact wrench, it will work without a woodruff key as long as it is tight.. But yep..I been in your shoes..I bet you didn't tighten the flywheel with impact and now it's chewed up and timing is floating around.. Good luck