i had the same problem with kanger cartos. the ones that are $5 a box. fill them until liquid runs out the bottom, take a few puffs then burnt taste,
Exactly NOT how you fill a stuffing carto. You should be be filling a carto TO CAPACITY. You need to know the how many ml the carto is.
Then you fill to capacity, cleanup, settle. My method is - block the threaded end hole (with finger and tissue or a carto spike) and make it take at least 90% of it's stated capacity, wait 20 seconds or so then drip in however many more drops it will take. I make a 1ml carto take 18 drops, then I wait, then I drip in 2-6 more - whatever it will soak in. Then I clean it up. Then I lay it on all sides and ends for 15-20 minutes (enough time for thin juice) to make sure every bit of stuffing is damp. THEN I puff it.
And while vaping I am on guard for reduction of flavor or vapor, any change or lack in the flavor, any unusual warmth of the carto - if any of those happen I check if it could use some juice. I use a plastic or delrin drip tip as a mouthpiece instead of the endcap and I use thin clear usually light-color juices with no flavoring sediment. So my carto's stuffing goes white on top when it could use more juice. I also check/top off a carto before leaving the house, before bed, and before putting it on a fresh hot battery.
Re 5v box mod - on your Ego 3.4v, a 3 ohm carto is barely warm. On a 5v box mod is near the low end of low resistance for 5v. So of course it is MUCH hotter on a 5v box mod and it will be probably use juice faster and it will be more critical to keep the carto damp.