This summer we picked 14 lbs of crabapples (the nice, large variety) from a single tree. They all got turned into various jellies (some with watermelon added, some with grape added, some just plain crabapple). Because crabapples (and apples) are naturally high in pectin, we didn't have to add store-bought pectin. But it made adding the right amount of sugar more of a guessing game. Especially because the water-pectin-sugar ratio is constantly changing as water boils away. By the end I had it down, but several of the jars came out runny--more like syrup than jelly (and really sour too). One jar came out like a solid piece of candy!
Fortunately, you can revive those runny jellies and jams by adding sugar. Heat up the syrup right in the jar in the microwave or on the stove and add sugar a half cup at a time, until you think it will be the right thickness after cooling down. Watch out for bubbling over. You can always add more sugar later. Instead of re-canning these, I reinvigorate one jar at a time, keeping it in the fridge after doing so.
Viva la jam!