Briggs and Stratton pusher runs bad for 20-30 mins then instantaneously runs great.

I have a basic push mower with a Briggs and Stratton motor on it. It's probably 15 years old but I have a normal-sized city lot yard and the mower works okay so I have no reason to buy a new one.

The mower was not used for about 4 years. When I resurrected it last summer I had some issues with some old gas that was left in the tank. I don't know if that could have anything to do with my current problem but I thought I should mention it.

When I start the mower it runs a little rough and sputters occassionally for about the first 20 minutes. It still runs good enough to mow so I just go with it. But then all of a suddent something happens about 20-30 minutes in and bam, it starts to run great. The change is almost instantaneous and it happens every single time I use it.

Can anyone explain this? Thanks in advance!

Maybe the diaphragm is worn that is between the carb and tank causing the choke to not open properly.

spray linkage very good with penetrating oil, something is sticking until it vibrates loose, i was going to say trash in carb but every single time ???

Thanks for the feedback. I will give those things a shot and let you know how it goes.

Also, check the screen on the bottom of the pick up tube in the tank. I have had them rust off and then pieces of crap will block and then shake free.