What's the trick to EMP + DVa bomb combo?

Throw the bomb first so the enemies all huddle behind reins shield, emp at the last second so they can't run after they get emp'd

As long as you make sure your sombra is in position for it, you don't need to do anything special for timing the combo (you mentioned the decloak animation). Since you throw the bomb first, and sombra is already right inside the enemy lines invisible, the bomb basically starts your countdown for you. Basically everyone knows how long it takes for a dva bomb to explode. If you throw the bomb first, sombra should be able to time the emp for it on her sleep

I've messed around on Sombra a bit and there is a really weird delay on abilities because of the decloak. If this person is a Sombra main it's kinda weird that they're not used to it but if they're new to the hero I totally get it

Against Goats or Deathball comps Bomb then EMP, against Dive comps EMP then Bomb. Rule of thumb. In truth It's rarely necessary to combo them against Dive comps.

One reason is that in general, lower ranked players don't trust barriers or their teammates as much. In higher ranks, there's not much more trust but it's still expected for the rein/orisa to have a shield up at the right time (be ready to be flamed if you can't stand still and block a bomb). So the emp combo works because it's just unexpectedly instagibbing the barrier. Getting rid of that barrier just doesn't do as much to lower ranked players.

There is a slight delay while decloaking before you can use EMP. Maybe your friend needs to start it a little earlier after you throw your bomb. Here's an example EMP/bomb

Diamond Sombra main here. When I coordinate with DVa for bomb, it's important on my end that either 1) I am already invisible and in position to decloak and EMP or 2) I have my translocator available to throw and there is enough time before the bomb goes off for my translocator to get to where I need to be, to teleport, and to EMP.

Things that a gold Sombra might do wrong would be to be invisible when they throw the translocator, because it's likely to take too long to throw it, teleport, decloak, and EMP. ALWAYS decloak before teleporting for this combo! The 0.7s delay before you can EMP after decloaking is a killer on the timing for this combo.

Like any DVa bomb, the area of the map that you do this in matters too. The best timed EMP + bomb won't get any kills if everybody can hide behind corners.

I prefer for DVa to throw before I EMP to get the timing right.

Here's probably my favorite combo I've been a part of from the DVa's perspective. She waits to throw until I am in position and I don't use my translocator to get there but just run around behind them.

No tricks really, as the D.va make sure your sombra is in a good position before you throw it and make sure that where you are throwing it is likely to get kills, either because people are out in the open or because they are huddled behind a shield. Another note for the sombra is to make sure your emp hits as many people as possible whilst also hitting the shield they are potentially hiding behind. Last but not least emp should almost always go off after you have thrown the bomb.

In lower ranks people dont trust their teammates (rightfully so) so they tend to run away instead of hiding behind barriers

I just hope that people understand there are times when you want to emp BEFORE throwing the bomb. Throwing the bomb will cause the other team to use whatever movement ability they have to escape if they aren't hiding behind a barrier. If the EMP goes off first, the bomb can go in and no one can movement ability out to escape.