Engine missfire

[AlvaSpeed]

My engine developed a missfire from mid to high rpms.
Checked the coil gap..O.K.
Changed the sparkplug.
The car starts fine.

Can the carb be cloged? haven't installed a fuel filter yet.
Can the lightweight flywheel be a problem?
What is the matter about the fuel tank not pressurizing or something?
Anyn ideas before I take everything apart?

 

[Bigger the Better]

I would think that you are missing spark somewhere? Even if the tank pressurizes you should not miss spark.
You said you changed to a lightened flywheel? Are you sure you reinstalled the flywheel pin and the flywheel has not moved?