Quoted from TimMe:

Here are some things that can cause differences in flipper strength when all parts are new, besides what you already mentioned.
If you haven't done this yet, you can check your coils (out of circuit) with a multi-meter to see how many ohms you have on the power windings. If the ohm reading between the two coils is much different, the one with the lower ohm reading will generally have the stronger kick.
If the return spring tension is different on the two flipper assemblies, the flipper with the stronger return spring will have a weaker power stroke.
If the plunger pivot on the flipper crank is binding on one crank but not the other, the crank with the binding pivot will seem weaker (yes you can have a binding pivot on a brand new crank).
If the flipper crank is installed on the shaft of the flipper such that the bushing of the crank is too tight against the bottom end of the nylon sleeve that runs through the PF, that binding point will make the flipper weaker. If one flipper has good end-play between the crank and the sleeve bottom, and the other flipper is too tight, then the tight flipper will seem weaker.
If the coil mounting bracket for the flipper coil is positioned such that the plunger path into the coil is not lined up quite right, the plunger can bind against the inside wall of the coil sleeve. This mis-alignment can sometimes be caused by attaching the flipper crank onto the flipper shaft too far down (ie, towards the end of the shaft). If one flipper plunger is binding in this way and the other isn't, then the binding flipper will seem weaker. You may not notice this type of binding problem unless you check the action of the flipper assembly while the PF is in the normal playing position.
- TimMe

Quoted from HowardR:

If you'd like to test relative coil strength, swap them left for right.

Quoted from TimMe:

Here are some things that can cause differences in flipper strength when all parts are new, besides what you already mentioned.
If you haven't done this yet, you can check your coils (out of circuit) with a multi-meter to see how many ohms you have on the power windings. If the ohm reading between the two coils is much different, the one with the lower ohm reading will generally have the stronger kick.
If the return spring tension is different on the two flipper assemblies, the flipper with the stronger return spring will have a weaker power stroke.
If the plunger pivot on the flipper crank is binding on one crank but not the other, the crank with the binding pivot will seem weaker (yes you can have a binding pivot on a brand new crank).
If the flipper crank is installed on the shaft of the flipper such that the bushing of the crank is too tight against the bottom end of the nylon sleeve that runs through the PF, that binding point will make the flipper weaker. If one flipper has good end-play between the crank and the sleeve bottom, and the other flipper is too tight, then the tight flipper will seem weaker.
If the coil mounting bracket for the flipper coil is positioned such that the plunger path into the coil is not lined up quite right, the plunger can bind against the inside wall of the coil sleeve. This mis-alignment can sometimes be caused by attaching the flipper crank onto the flipper shaft too far down (ie, towards the end of the shaft). If one flipper plunger is binding in this way and the other isn't, then the binding flipper will seem weaker. You may not notice this type of binding problem unless you check the action of the flipper assembly while the PF is in the normal playing position.
- TimMe

Quoted from pins4u:

Early pinball machines (of every brand) that have directly controlled flippers using the EOS to break the high current circuit, rely very heavily on accurate and careful cleaning and adjustment of the EOS switches. This is the most critical part to having flippers working well in these machines.
You need to adjust the contacts close enough to shunt power to the pull side of the coil as long as possible BUT make sure the EOS is able to open reliably at the end of stroke to prevent burning out of the coil.
Naturally there are many other considerations such as flipper bush wear and plunger/sleeve wear but, as you have replaced all of these, it probably comes down to SUPER critical EOS adjustment or replacement of you haven't already?

Quoted from pins4u:

Early pinball machines (of every brand) that have directly controlled flippers using the EOS to break the high current circuit, rely very heavily on accurate and careful cleaning and adjustment of the EOS switches. This is the most critical part to having flippers working well in these machines.
You need to adjust the contacts close enough to shunt power to the pull side of the coil as long as possible BUT make sure the EOS is able to open reliably at the end of stroke to prevent burning out of the coil.
Naturally there are many other considerations such as flipper bush wear and plunger/sleeve wear but, as you have replaced all of these, it probably comes down to SUPER critical EOS adjustment or replacement of you haven't already?

Quoted from TimMe:

It's best to detach one solder lug of a coil so that you are testing it totally out of the circuit. Since you only need to check the high-current winding of your flipper coils, you won't need to unsolder all the wires. If you are not sure which two lugs represent the high-current winding, look at the magnet wires coming up from the coil winding, where they attach to the solder lugs. The two lugs with the thicker magnet wires on them are the two lugs for the high-current winding. Detach the game wiring from one of those lugs to get the most accurate ohm reading from the coil.
If the two readings you posted are really correct, that is a pretty big difference in power. It surprises me that those two coils are the same part number. The part number literally represents the wire gauge and the number of windings, and so I would not expect two coils that are wound the same to be so different.
I would not consider those two coils to be a matched pair, and I would not put them in the same game. If you don't want to replace one of the coils with something that creates a better matched pair, you can compensate for the power difference by adjusting the EOS switch of the stronger coil to open sooner than the EOS switch of the weaker coil. That will make the two flippers have the same power level, although it accomplishes this by making the stronger flipper become less strong.
In your situation (again, assuming your ohm readings are correct), if you want to make the weaker flipper stronger, you'll probably need to put in a different coil that matches the ohm reading of your strong coil more closely.
- TimMe

Quoted from Rikoshay:

if you want to keep the stronger/weaker flipper coil setup but have the stronger coil a little weaker you can add in a ceramic resisitor to the EOS circuit, maybe a 5W 1.5ohm to start?

Quoted from TimMe:

Some additional info on this - I got curious so I just checked the high-current windings of the spare Bally flipper coils I keep on hand. All my 25-500s landed at 3.1 ohms. All my 25-600s landed at 3.8 ohms. So your flipper coil with a high-current winding that is 1.7 ohms seems to be the odd one out.
- TimMe