I’ve been harvesting Carambola for awhile. And I’ve discovered a fool proof method to determine when the fruit is at its prime.
So when the fruit starts to change color, smell it. If you can detect a very faint, complex floral fragrance, the fruit is at its prime. At this stage the fruit is crisp, slightly sweet.
The fragrance enhances the flavor. And it should be consumed as soon as possible, certainly within 24 hours.
There’s some latitude in how Carambola looks when ripe. Color change often peaks a half day after the fruit is past its prime. Most often a prime fruit will be more toward the golden-yellow stage than the orange-yellow. And it will have a just slight hint of green on the ridges.
Before its prime, the fruit is firm, bright yellow, odorless with green ridges, is bitter, not very sweet, and acidic. An odorless fruit should be checked again in 12 hours.
When past its prime, the fruit will pick up a slight off odor along with a waning floral scent. And it begins to darken and soften near the stem.
At this point, some fruit, if not damaged when harvested, can be salvaged by cutting of the softer, darker parts near the stem and immediately consuming the firmer, less ripe parts. But I’ve never had to resort to this since incorporating the smell test.