Hello!
A month ago I had a sour crop problem with a hen. I took her to the vet and they emptied and washed her crop with activated carbon. I cannot say it completely worked because at her first little and soft meal her crop became bubbly again. Good thing my hen is fat because it took me 5 days to fix the problem.
First day: I only gave her water with apple cider vinegar with (Heinz, I didn't have any other) 15ml per 4L. No food.
Second day: I gave her water with pre and probiotic (Probiolyte), no vinegar. In the morning a teaspoon of egg yolk but her crop wasn't happy about it so no more food.
Third day: Vinegar again and one teaspoon of egg yolk. This time, the crop stayed cool (good size, no bubbles or sounds).
Fourth day: Water with ProBiolyte and more egg (the whole egg). Everything went super smooth. Feces started to look normal (before it was very watery, greenish and super extra smelly) so I gave her her chicken feed but with no corn at all and with a lot of water, it was like a creamy soup.
Fifth day: Same formula as Day 4. Crop was happy, feces were beautiful, hen was happy.
I kept the wet food for two more days and not corn at all for at least a week. And that was it.
I hope it helps.