new guy needs some advice

[Weldangrind]

Baron,

Thanks for asking clear questions. We can be as detailed as you wish in the answers. To begin:

The combustion chamber is the area above the piston where combustion takes place. This occurs when the piston is at the top of its stroke, referred to as Top Dead Center (TDC). The spark plug screws into the cylinder head to ignite the fuel / air mixture once it has been compressed by the piston. When I say "above the piston", that is a reference to a cylinder that is arranged vertically; as a point of trivia, your 90 has a horizontal arrangement.

The combustion chamber needs a rather specific ratio of air to fuel to run properly and safely. Too much fuel results in a poor running engine, generally identified by a bog condition and a black spark plug (referred to as rich). Too much air can burn up a piston, and it results in a white spark plug (referred to as lean). The ideal air / fuel ratio will produce a tan / grey plug colour. Incidentally, the theoretically ideal air / fuel ratio is 14.7:1, although it's generally richer in reality, like 12:1. We don't have an efficient way to measure the ratio, so we go by colour and feel.

The valves (in your machines) are in the cylinder head, adjacent to the spark plug. As the name implies, they open or close as necessary to allow fresh air and fuel in or allow exhaust out. I recommend that you don't go there yet.

Yes, the switch at the back of the quad is a tether for young riders. It has been problematic for some, resulting in a no-spark condition.

Please tell us how you know that the 90 has spark.

When you cleaned the carb, did you adjust the air / fuel screw and the idle screw? If so, we can guide you towards a good starting point.

Welcome to the world of Chinese bikes and quads. Buying new stuff won't necessarily make your problems go away; even a new quad could have these concerns, and you have to be comfortable enough to tackle the issues (you sure seem to be). A good sense of humour helps a lot.