Like the above person said, only be totally inflexible on the times you’re willing to fly out and land. Specifying the airport (not choosing “all airports”) and specifying the airline couldn’t hurt, either.
You can use google flight for this
https://www.google.de/flights/#search;f=SHA,PVG;t=TIJ;d=2016-08-01;r=2016-09-25
Click on the Start Date and then click on the Price Graph.
You could give a start date (1st Aug) and a duration (55 days). Then the price graph would give all possible sliding windows of length 55 days starting from 1st Aug.
Try with Kiwi. I have written about Kiwi in my answers here and here.
You can not specify the flight numbers but you can specify the origin and destination as well as direct flights only which for your case of AM99 and AM98 boils down to the flight numbers you were looking for.
Datewise, you can specify a date range of several month for both departure and return or a duration of stay of 2-31 days. The cheapest I get for departing August – September 2016 and staying 2- 31 nights is EUR 1079.
Funnily the graphics of Kiwi make it look like you were not flying over the Pacific ocean but the other way round 🙂
Credit:stackoverflow.com‘