Big boy scooter - at wits' end

[Walterp79]

Please help I am desperate. I blew my gy6 engine, I think the timing was out. The piston burnt and the chain adjuster broke. So I replaced the piston, barrel, head, chain adjuster, chain guides,oil pump. It cranked without starting. Then I cleaned the carburettor. It cranked without starting. I checked the fuel flow. It spurts when I turn the petrol. The carb bowl has fuel. I checked the spark. I also did the valve clearance properly. But it has a weird plunging sound when it cranks. Any ideas?
Oh, I also opened the starter motor and cleaned it. I thought that the plunging sound could have been oil inside the starter. There was some black stuff at the bottom. Not sure if it was oil.
I have 5 bar of compression. The guys here says that's the best I can hope for it should start.

[Jack 807]

Is there fuel on the spark plug after you try to start it? Have you tried spraying a little starter fluid in the air cleaner to start? How old is the fuel?

[Walterp79]

Thanks for the response 😁 I did try other fuel but this came from a canister so now I went and got fresh fuel again. That didn't work. I tried spraying carb cleaner into the air cleaner because I did not have starter fluid. I saw this in a video. I don't know if I should get starter fluid specially? The spark plug is also dry. The spark looks tiny but regular. Is that something that can happen? That the spark is too small? (scraping the bottom of the barrel I guess) I understand why you suggested the spark plug though. That plunging sound does sound wet, but maybe that's the new piston that has to work in🤷 also, I've been doing these things with the air cleaner completely off till now. I thought the engine should start without the air cleaner attached. I did put it back in now before I did these things. And the exhaust is off but I can't see how that could be a problem?

[Jack 807]

The dry spark plug suggests there is no fuel getting to it. You can pull the plug and drop a little fuel into the head replace the plug and see if it pops. What kind of carburetor is on it?
What are your valves set for? Mine are .004 for intake and .005 for the exhaust. Were the timing marks in the correct position to set the valves? What type of scooter is it? 5 bar equals about 72 PSI My scooter has about 145 PSI which is about 9 Bar. You replaced the piston did you put the rings on it, so the ring gaps are staggered?