[Maddevill]

Not sure what the problem could be but you can get a LIFAN engine in various sizes for around $400.

Mad

 

[Maddevill]

Does that have an automatic centrifugal clutch ? Maybe the clutch is seized. Sometimes a clutch can stick if it's been sitting. What does it smell like ? Oil smell or electrical ? If it's electrical look for wires sitting on the exhaust or melting insulation from a short. If it's oil, some smell is normal after an overhaul. Look for leaks.

Mad

 

[Maddevill]

If you can get it to idle slow maybe it will work there. In the video it seems like it just revved right up. This would engage the clutch. Can you rotate the rear wheels when it's not running ? If you can't then the clutch is for sure stuck. Sometimes you can just push the quad back and forth jarring the clutch plates until they break free. I've had to do that on several old dirt bikes.

Mad

 

[Maddevill]

Put the chain on and see if it rolls. It should although some drag is normal.

Mad

 

[Maddevill]

Hmmm ok see if it starts with the idle set much lower. Maybe run it with the chain off until you get the carb tuned then try it with the chain on.

Mad

 

[Maddevill]

You didn't mention if you cleaned the carb. Only running on choke usually indicates your pilot jet and/or passage is plugged. That could also explain the dark plug. As for smoke out the exhaust, since its air cooled it's probably oil smoke. If you can do a leak down test on it, that can tell you if your rings or valves are leaking. I think you put rings in this, yes ? Did you measure the cylinder to see if its worn or even round ? Ring end gaps ? It may take a little time for the rings to seat but idling for a long time doesn't help them do that.

Mad

 

[Maddevill]

During a top end rebuild, you should always lap the valves into the valve seats at a minimum and then check for sealing. With new rings, you use the old piston to push them down in the bore a bit and measure the gap in the end of the rings. Too small a gap and the ring can break. Too much and you lose compression and burn oil.
A leak down test goes briefly like this. You will need a way to hold the crankshaft from turning and a way to introduce air into the cylinder. I use a hose from my compression tester.
Set the engine up at TDC on the COMPRESSION stroke. Have someone hold the crankshaft there. Put the air hose into the spark plug hole and snug it down. Hook up an air source. Don't need much. 40-60 psi is fine. Watch the crankshaft because it will want to turn. Now, open the throttle fully. Listen at the carb mouth or air box. If you hear or feel air, your intake valve is not sealing. Now, go listen at the muffler. Air here indicates the exhaust valve is leaking. Find the crankcase breather. It's usually a hose coming off the crankcase to either the air box or overboard. If you hear or feel air there then your rings are not sealing.

Mad

 

[Maddevill]

As long as the crank moves smoothly and there's no play in the rod bearing, putting a new top end on might be
the way to go. As long as it includes
New cylinder
New piston and rings
I'm going to assume new valves and seals ? You'd still want to check ring end gap and lap and check the valves for
sealing.
Mad

 

[Maddevill]

I think you can get an entire Lifan engine for that thing for around $4-500

Mad

 

[Maddevill]

I'm sorry but Seafoam will not restore the rings or the bottom end bearings. The only thing it does is remove fuel
and some carbon deposits.