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This seems like it should be a simple problem but...

I have a Briggs ES45, which has had very little use (it's only for the lawn edges alongside a riding mower). It was fine last year but this season is very unhappy.

First start, it was surging and tricky to start. I removed the carburettor, replaced the gasket and made it worse. It would then only start with fuel directly in the carb - and still surging.

I've since then replaced the whole carb but it's no different. I ran through a whole tank of fuel - surging all the way - and changed the filer, oil and plug. How much deeper into the engine do I need to delve?

Thank you .

Surging is caused by a engine that is running lean, meaning the air fuel mixture has more air than fuel.
I'm going out an a limb and thinking it has a plastic intake pipe with a split in it, allowing more air in PAST the carb.
What's the engines Model number? Should be stamped above the muffler or spark plug

That sounds very likely - I'll look out for some plastic after the carb.

The model number is 127707-1723-B1.

Thanks.

I've downloaded the parts manual, and it appears there's a plastic intake manifold...

Thanks for the pointer.

also, there is a O-RING that seals the carburetor to the intake pipe, Number 617 in the list. check to be sure it is in place and not cracked and dry rotted. I've repaired a few that were running wacky due to a small split in the plastic, finally had one one time it spit enough and wouldnt start so the fuel leaked out of it onto the block.

Thanks, I did change that O-ring when I swapped the carb.

Time to investigate the manifold...

Well, the manifold was fine. But it will now run only with fuel squirted directly into the carb.

The primer bulb feels weak. Could it possibly be causing the problem?