G'day

Like Alan, I have not used filters on my engines (and I have had lots of them) for more than 30 years. I just make sure that the fuel that goes into the tank is clean and does not contain moisture. A filter either inside my fuel bottle or after the fuel pump does the filtering for me. To keep the moisture out, keep your fuel in sealed containers and if you can, avoid opening them when the weather is humid or rainy. If you find you can't get your engine to idle reliably, always try some fresh fuel before changing settings or reaching for a new plug.

As for fuel coming back out of the carby air inlet, the only engines that I have seen that did this were Super Tigre G15 high performance glow engines which had a huge air intake and they would spit a bit back if they were tuned rich. (It is usually best to tune your engine a little rich (about 1 quarter to 1 half turn back from max revs as that way it will last longer and you are less likely to have "dead stick" landings. Remember, as your tank empties, there is a slight drop in fuel pressure and if you have tuned too lean, the engine may stop or worse, overheat. Are you sure the fuel is coming out the carby air intake? Some engines throw a little out the front bearing (not really normal to do so) and you will always get some mist of unburnt fuel/oil mixture coming out the exhaust which will splatter all over your plane. Some mufflers leak so that some fuel comes out the joins of the muffler as well but usually not very much.

But ... and this is really the important bit ... if it is running well and performing well, not stopping in flight and has plenty of power, then I would not be too worried. My Saito 72 four stroke in my big lazy Kadet Senior leaves a nice white vapor trail as it snails through the sky. (Well it is not very fast and it leaves a trail like a snail.) So does my Saito 100 in my Four Star 60. I get worried if I can't see at least a bit of a trail from my engines when they are at or near to full power.

All the best. Have fun and keep flying.