I have several 5hp gas engines. I kind of collect them. The problem is that they all have a problem with surging. They all start up ok. One of the lawn mower engines runs fine for about a minute or so and then begins surging. The surging becomes worse until the engine stops. Another lawn mower engine runs fine until I try to put it in the idle position. It then starts to surge. My power washer runs fine while under a load, but surgers when at idle. The carbs have been cleaned. I suspect that the there is a problem with the needle valve and seats which have not been replaced, or there is a problem with the float settings. This is driving me crazy. Do you have any suggestions.

adjust the air/gas screw... and do yuor suggestion and check the needle valve and seats...

What is the air/gas screw? There are no adjustments on the carb except for the idle which is on the outside body of the carb. I plan to order a carb rebuilding kit which has a new needle valve and seat. I will post the results when I get kit. Thanks.

It sounds like your engines aren't getting the required amount of fuel for one or another. Dirty carbs , plugged fuel filter or vac. leak at one of the gaskets.
You can test for leaking gaskets by squirting carb clean in the gasket areas while engine is running and if the engine smooths out you have found a source of the leak.
I assume you are using fresh fuel.
Be careful with carb cleaner because is burns your eyes and skin.

Hope this helps.