The best thing for cleaning them is caustic soda (sodium hydroxide) - but not the small in-line flame traps that LR use - they have aluminium mesh.
You can buy it a the supermarket - I get mine from Woollies. (If your local meths drug manufacturer hasn't bought all the stock.

)
Here's some pics I took of the flame trap from one of my cars - before and after:

Guess which is which!
After an overnight soak, I washed it out with a hose. It might need two soaks.
Caveats
Caustic soda (commonly used as a drain cleaner) is dangerous. Absolutely follow the instructions on the can.
Do not add water to the caustic soda, add the chemical to water. To do otherwise can lead to the water boiling and spitting up into your eyes. Making up the solution is highly exothermic - it creates heat - it can even ignite flammables.
Do not get it on your clothes - it will eat through them (ask me how I know - I learnt this as a 13-yo)
Wear gloves and eyes protection - it is highly alkaline and as its name says, caustic. It will burn.
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