How does dilution affect result of titration of jam?
You can do this at any dilution of both titrate and titrant, as long as you "keep the books" up to date when calculating the results you should be fine. I would probably only dilute the titrate though, unless you have good reasons. (If the dilution of jam is to make it less viscous then that is a good reason for doing it)

Whenever I needed to perform titration something of unknown pH and contents (and I had a large supply of the solutions) I would do a low resolution "sweep" with high concentrates and low tech (old school burettes are usually the fastest, valve fully open and stirring at... vigorous). I'd do two or three of those, just to pinpoint the area of interest. (Sometimes there are several, but then you just do the below for all of them). Assuming one specific point of interest was at 100% addition of titrate, I would add 80% of the titrant to a sample of titrate, then do another titration at 10x dilution of titrant. Three fast samples. Repeat the above, and now at 100x dilution i'd put it in the autotitrator and get some graphs.