Popping out Passenger side Exhaust

[Phil Gillespie]

Not my car but trying to help out.
12v negative earth H & H flathead fitted with electronic ignition. Fitted with new 4 barrel 390 carb. Distributor is mechanical advance only. Checked advance and only shows 3 to 4 at idle and only around 12 at 3000 rpm. Know it should be around 24 degrees.
Pops out passenger side only mainly at low speed.
To check firing by removing a lead to see where the popping occurs I understand you need to ground the removed carbon lead t avoid damage to ignition unit? Hope to locate cylinder and go from there.
Plan is a compression test, lead down check to locate the cylinder at fault.
Possible this may be valve train related, sticky valve, a vaccum gauge at idle should show this up?
Appreciate all comments.
Phil NZ