Compression at 90 psi isn't the greatest BUT all cylinders being even indicates that the low reading may be due to a slow cranking engine OR a defective gauge. The battery and starter MUST be in ideal condition (You state they are newly replaced), and "all" ends of the wiring and cables, including the battery terminals and the black ground cable at the powerhead must be absolutely shining clean... including the components they attach to.... Anything less results in a voltage drop that in turn results in a slow cranking engine.
If that 20amp fuse blows with the key in the OFF position and only when the large RED electrical connector is plugged together....Disconnect one battery lead temporarily, then...................
1 - Remove that 20amp fuse... Now, with a volt meter set to register 12v, insert that meter in place of the fuse (between the two wire ends).
2 - Make a note, a accurate diagram, of the color of the wires that are attached to whatever markings on the ignition switch (B, S, C, A, raised M, reg M).
3 - With the exception of the "B" (battery voltage) wire, remove the rest of the wires from the ignition switch. (Only after making the above diagram!)
4 - Reattach the battery cable to the battery. The voltage meter should NOT move... Does it? If not continue....
5 - If so far, so good... With the key in the "OFF" position... One by one, take whatever wire should be attached to a certain lettered terminal... and touch that terminal with that wire while watching the Voltage Meter...... when the meter registers 12v, you've found the wire that is shorting out and causing the fuse to blow.
Let us know what you find.
