GP100 Spring upgrade issue

I recently acquired a used GP-160 (Blu, 6-inch half lug barrel). The hammer pull was pretty stiff but seemed to work well.
I replaced the hammer and trigger springs with lighter ones from a Wolff spring kit.
The pull is in fact lighter but now there seems to be a quirk when the hammer gets to the cocked position. It takes an unusual strong force to finally cock the hammer and a lot of trigger force (single-action) to release the hammer. Basically the same with double-action pull also.

One thing I noticed which I hope is a clue is that the original trigger spring is 18 mm long. The Wolff springs are about 22 mm.

This particular GP-160 was made in 1996.

I also have a 2006 KGP-141 and the spring upgrade went very smooth. Its original spring (stock) was also 22 mm.

Is it possible that older models required a shorter trigger spring and that the longer spring is too tightly compressed when reaching full-cock?

Or does anyone have any other suggestions?

I think you are on the right track. It sounds like the trigger spring is almost fully compressed when the hammer approaches the fully cocked position. This would explain the extra heavy trigger pulls.
I would first first try replacing the trigger spring with the original and see what happens.

I trimmed a couple of coils from the hammer return spring so the length matched the stock spring and it works perfectly.

Excellent!

Get mcarbo spring kit sp101 and gp100. No fuss no muss they have figures out weights ro get you nice trigger pull plus Vet owne and 100 percent life time warranty.

Next you may have switched the paw spring and detent with the cylinder lock detent and spring. Switch them and see If that doesn't fix the issue.
The cylinder lock detent and spring are the longer of the two. If I am not mistaken .

Also check out the video on the mcarbo page as it does into 1 hour long in depth description of parts disassembly and reassembly.

https://youtu.be/txbCmzM4v6A