Only a little over a day has passed since I switched from cigarettes to my new Joye 510 L and I have already made my first modification. It seems that by thinning out the filler in most cartridges and inserting a small straw to allow trapped air at the back to escape you can greatly increase their capacity (rather, how much juice they are able to take without being removed) and the wicking mechanism which results in much creamier puffs.
Credit is due to opusMorian who started this thread: Cart Mod for better refilling and created this diagram:
The weapon of choice seems to be the brown double-barrel (sliced in half) plastic stir sticks of the sort you might expect to find at Tim Horton's. Unfortunately, it's too cold out for a Timmies run this late so I cut up some thin drinking straws, sliced them down the middle and rolled them as tightly as I could.
Gripping the roll in a hemostat I applied heat by way of a regular butane lighter waved back and forward about an inch from the tube.
A little gentle massaging and you should end up with a relatively even tube; whether the side seals or not is unimportant as long as the tube is thin and retains its shape. Drop the straw into your empty cartridge reservoir.
Cut right across the top to bring it down to about a millimetre above the cartridge lip, remove the straw then cut it down a further 3 or 4mm.
Put a diagonal cut in each end of the tube going opposite directions on the same side, such that the tube resembles a rhombus when looked at from the side.
The bottom cut allows air to flow through the tube from the back of the reservoir and the top cut keeps as much plastic away from the tip of the atomizer as possible. The cuts must face inward when the straw is situated against a wall of the reservoir. Cut one of the long edges off of the removed filler (or roll your own out of new material with consideration for the reduced volume) and use a pin or similar tool to stuff it back into the reservoir, leaving 1-2mm of fill protruding past the end of the cartridge. Cut off this extra fill then use your finger to push any frayed ends in and evenly spread the remaining fill over the straw.
You should now find the apparent capacity of your cartridge much higher and the hits much more consistent.