Little quiz to make you think about the magic of how engine things work:
What powers a carburetor?
1) - electricity
2) - gravity
3) - fuel
4) - air movement created by engine vacuum pressure
... correct answer is #4) air movement created by engine vacuum pressure. Carburetors operate off engine vacuum pressure and without that controlled movement of air they just sit there and do nothing.
If your engines intake vacuum pressure is low, your carburetor will?
1) - do nothing
2) - perform poorly or not at all
3) - not operate
... pick any one, they are all the same difference. Compression in the gasoline powered internal combustion engine is every bit as important as air, fuel and spark. Because your engines compression and vacuum pressures are directly related, a problem contributing to reduced cylinder compression will similarly contribute to reduced intake vacuum pressure.
Why do valves need a set clearance?
1) - to accomodate dimensional changes in the valve train components when they are operating at normal operating temperature.
2) - to frustrate the operator and make less mechanically inclined owners take their motorcycle in for shop servicing.
3) - because metal expands with heat.
... best answer is #1) & #3) with #2) being an unfortunate side effect.
What is the valve clearance at normal engine operating temperature?
1) - same as when it's cold
2) - as close to zero as possible while still allowing the valve to completely close and seal.
3) - large enough for burning gasses to leak past the valve seat and burn metal parts.
... hopefull #2) because #1) is physically impossible and #3) is what will happen if you don't set your valves correctly.
Can you crack an exhaust valve seat by running the engine with no headers or exhaust system in place?
1) - well Ya! sure you can, the various metal parts shrink at different rates so when you shut the hot engine down, cold air contacting a hot valve seat can destroy it.
... if you don't believe that to be true, you're welcome to test the theory.