Picked up a 72CC Buyang BMX Mini Quad for my son pretty cheap.

The previous owner said it only needed the valves adjusted.

So I ended up tearing it apart, adjusted the valves to .004”, replaced the fuel filter and line, spark plug, cleaned the carb and put some fresh gas in it.
The quad would only stay running if I had the idle turned all the way up. By doing so it would creep forward without my hand on the brake. It also had a horrible bog without feathering the throttle.
After many carb adjustments trying all the all 5 clip positions, turning the air screw anywhere from ½ turn out to 2 ½ turns out.
I ended up buying a new carb online.
Now after putting the new carb it idles great but still has a bad bog when hitting the throttle or putting a load on the motor. It won’t go up any sort of incline without stalling out.
I noticed the new carb air screw setting is ½ turn out. If I turn it out any farther it will stall on the stand. Seems the bog gets worse as the motor gets warmed.
I’m starting to pull my hair out with this little thing.
Any advice would be much appreciated.

 

Well finally got around and got 10mm fitting for my tester and compression check on the 70cc.

The best I could get out of it is 62 PSI.

Would this make sense why I can seem to keep this this running?

 

yes. needs a minimum of 95 psig

 

Ouch.

So would I need a whole upper half? Piston, rings, cylinder, and valve assembly? Where would you recommend getting the parts from?

Would this fix the bog and lack of power issue that I can't seem to fix

 

I ended up taking the motor apart and found out that I had a piston with worn down rings.
I bought a new cylinder and piston and rings.
I checked the valves and they are tight with alot of carbon build up on them. I poured some alcohol in the combustion chamber to see if they leaked.
After getting it all back together I still have a bog.
With the float set at 10 degrees above with the carb upside down.
The needle clip in the middle and the air screw turned out 3/4 turn.
It won't even start with the air screw turned out 1 1/2 times.
The plug is BLACK. It is a stock carb (PZ19)
My head is spinning.
Thanks
Jay