Is smell from a siphon always caused by the siphon itself?

No. Sometimes the issue is sewer venting, a dried-out water seal, bad slope or a blockage further down the line.

If a siphon drips, will tightening the nuts harder fix it?

Not always. Overtightening easily damages plastic threads and distorts seals. The real leak point should be identified first.

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Siphon repair in Israel — leaks, odours and bad drainage | KABLAY