You're shooting in the dark 'cause you didn't figure out what was going on inside.
The only thing you changed by changing springs was the shape of the curve.
My suggestion is either
a) take the dizzy to the distributor machine and pay the man to work on it.
b) Call FBO and have him send you a distributor.
c) get some paper and pencil, measure, and figure out what is going on so you can make the changes needed.
- since its on the car, start by measuring the curve with the timing light. With the car running, set the timing whereever it runs OK. Measure the timing at as low of an rpm as possible, then measure every 100 or 200 or even 250 rpm until it stops advancing. Use the throttle screw if you dont ahve a helper. Reset your idle speed and PLOT the curve on graph paper or a spreadsheet program while the engine is cooling down.
- Take the distributor apart and move the weights to see when the springs engage. Check the pins in the slots for side play. Measure the starting points of the slots - compare with the other plate you have. Examine the springs to see if the lengths are different from the other distributor. Measure and write it down.
- With the above, you can then figure out what is going on and what needs to be done.
- You may still want to take it to the distributor shop because using two short loop springs in that slot take some time and experimentation to get the 18 at idle. It will never be a stable timing at idle for reasons I explained earlier. Also idle timing will be different than your initial as explained before. This will make more sense after you plot out the curve.
- If you happen to have a super heavy long loop spring of just the right loop length, you may be able to use the 30 degree slots as is. This is still a lot of testing easier done on the machine.
- the simple way to set up the distributor is to shorten or buy the slots needed for this motor. Knowing which side of the slots to shorten depends on the relation of the springs, weights and slots.