Briggs And Stratton Genuine Diaphragm And Gasket Sets
The most common part I replace on all the lawnmowers I repair for profit is a diaphragm and gasket on a Briggs And Stratton Lawnmower.
This little part causes so many problems as when it becomes old and wrinkled you will experience running problems such as hunting and surging and white smoke problems. Take a look at my article for more information 
https://repairlawnmowersforprofit.com/petrol-lawnmower-hunting-and-surging/
Replacing this diaphragm and gasket set fixes 90% of all Briggs And Stratton running and not starting problems.
The correct genuine part for this is Briggs & Stratton Carburettor Diaphragm/Gasket Kit. 795083
Never attempt to fit aftermarket diaphragm and gasket sets. They rarely if ever work correctly and I’ve even seen them last less than a minute. If you have a Briggs and Stratton plastic style carburettor that sits directly on the gas tank with a primer bulb on the side this is the correct part.
They are all the same size.
You can see my article that links to many more gaskets here https://repairlawnmowersforprofit.com/where-can-i-buy-genuine-briggs-and-stratton-lawnmower-diaphragm-and-gaskets/
Here is a video showing how to replace this gasket set
Briggs And Stratton Head Gaskets
From time to time, you will need to replace the head gasket. If you are having running problems and have replaced or repaired the carburettor I suggest changing the head gasket.
The most popular head gasket I replace is on the Briggs And Stratton 35 classic engine. This is a common engine fitted to lots of lawnmowers over the last 20 years.
The correct genuine part is Briggs and Stratton Genuine 698717 Cylinder Head Gasket
I would definitely suggest replacing or servicing the carburettor before the head gasket.
Honda Gasket Sets
The most common gasket set I replace on Honda Lawnmower is Honda GCV135/160 Lawnmower Eengine Gasket Set
This is a common part on Honda lawnmowers fitted with the popular Honda GCV 135 and GCV 160 series Engines.
Most Honda IZY lawnmowers use this gasket set.
Often when attempting to service the carburettor the gaskets appear worn and many times have started to perish.
The Honda carburettors have various linkages and springs attached to the carburettor and I usually replace the gasket set when servicing or replacing the carburettor.