Can a 20 pin power supply operate a 24 pin motherboard ? or do I need an adapter,I've read conflicting messages but no real detailed yes or no

cap

You'd be better off replacing the PSU with a more modern one that conforms to the latest ATX12V standard & has a 24-pin or 20+4 pin main ATX plug.

You may not have the other necessary connector on your old PSU. Most new MBoards need the square 4 pin connector that plugs into the MBOard in addition to the 20 + 4 Power connectors.

Some mboards can use either a 20 or 24 "pin" (position) main connector from the PS - see the manual.

If the PS with the 20 pin main connector has enough capacity, and if the mboard requires a 24 "pin" connector, you can get 20 to 24 "pin" wiring adapters.

However, newer mboards often also require an additional 4 position power connector from the PS to the mboard elsewhere. A PS with a 20 position connector may not have that, and you would need a wiring adapter for that too if that's your case. If you had to get both wiring adapters, it probably wouldn't cost much more to get a new PS with a 24 position connector that also has other required connectors.