If I start to get a burning taste what do I have to replace?
It depends on what you are using but assuming that your kit has been working OK and if the coils are replaceable then you probably need a new one.
If your atomiser is not rebuildable then you may need to replace it.
If you are using a variable voltage battery you should ensure that it not set too high for your coil resistance.
If you are using a new coil/atomiser you should ensure that the resistance is compatible with your battery.
I'm not getting as much vape as I used to.
Usually this would be down to a duff or dirty/worn-out coil especially if varying the voltage has no effect. Are you able to check the resistance is correct?
It could also be down to a missing or poor seal, make sure everything is still in place as they can easily get lost or moved during cleaning and reassembly. You should also make sure that any breathe holes in the base are clear.
My coil resistance measure high.
If it's vaping OK then there's not much wrong and more likely you have a duff reading which is usually caused by dirty contacts. Resistance measurements are taken without power and so are vulnerable to contact issues. When you vape power is applied to the coil and so it can work fine even if the reading appears wrong, if it really was 8 ohms it would not work well at all. Tried not to be too technical so I hope this makes sense.
Gurgling and Leaking
If this only happens once left over night it seems the coils have flooded, i.e. over wicked. This can happen with thinner liquids or poor quality/faulty coils. Otherwise there may be a seal problem or the air holes in the base might be blocked.
My e-juice is burning
Sounds like you are using too high a voltage for the coil resistance you have. We recommend no more than 3.8 to 4.0 volts for resistances below 2.0 ohms, for voltages above 4.2 use coils of around 3.0 ohms. You will need to experiment depending on the e-liquid you are vaping.
e-liquid is toxic, is it safe to use?
Pure nicotine is toxic but e-liquids contain less than 2.5% in and so it should not even need to be classified.
Variable Volts or Watts, what's best?
If you want a consistent vape when you swap atomisers over (which will have different resistance values) then you should use variable wattage. But first experiment with your favourite atomiser and e-juice to find your preferred setting making small changes at a time. Then the theory is that the wattage will stay constant but the voltage will be automatically adjusted for different atomisers. Or if like me you prefer to adjust the setting each time you change atomisers until you find your sweet spot then use variable voltage, it's simpler. Unfortunately it's a bit difficult to understand unless you are an electronics engineer but I hope this helps.