Have you verified that the choke is completely closing?

you know what tomplum....I haven't!
Thanks...I'll check.
I never suspected something simple since it has happened on two brand new units. I will check in the am.

The choke adjustment should be checked.
here are some more ideas.
If the fuel tank is in the rear, the engine may need to crank for those 20 seconds to pump fuel to the carburetor. This occurs if the mower is unused for a while and fuel in the carburetor evaporates.

You did not mention how old the fuel is - or if it is summer fuel or winter fuel. The Briggs power portal has a training video that describes volatility of fuels. If the fuel in the tank was purchased last summer, it may have been blended to vaporize in warmer temperatures. If this is the case, the engines cranks for 20 secondes before it will start. The repeated comprssion strokes are needed to generate enought heat to vaporize the fuel.

Another possibility is old fuel deteriorates and becomes less volatile. My opinion is to start the engine and run the tractor empty. If there is old fuel in your spare gas can, pour it in your car to get rid of it. Add about 1 oz of fuel stabilizer to your spare gas can and purchase some fresh fuel.
See what happens.

Another thought is to check the condition of the fuel line. Look for cracking rubber and loose clamps. If there are any defects, the fuel pump may be drawing some air. Since the tractor is almost new, this condition is less likely.

Bluemower
Thanks for all the info. The fuel is new. I always run it empty and winterize it. The choke seems like the most likely candidate to me, since there is plenty of fuel in the filter.
I followed the accelerator cable and it seems that in order to access the carburetor I need to remove a good portion of the upper covers. The carburetor is not visible from the air cleaner port as I am used to seeing.
Not sure if I want to dissassemble that much with it still being under warranty, but Id love to get this thing starting right.
The wierdest part for me to understand is why after changing the fuel filter and plugs, it stated immediately for 3 days of testing. Then it went back to usuall.

capriow - are you keeping the engine's RPM up at full throttle while it's under load? Operating under load at mid to low throttle could be causing engine to run very hot...

spark plug changes are old sckool todays design rarely require new spark plug IMO. I have 21 year old vanguard that's had the plugs changed three times and twice it didn't need it. But, got caught up in the tune up brainwashing! I clean them, check for cracks, and install them, if I question where they are firing? I remove them connect them to the spark plug wire and crank the engine so I can see if the are sparking and the color of the spark. If its nice blue spark I know the ingition is good and the plug are good also. I spend my money else where like on oil ans filters. that's what's going to keep and engine running along time.

As far as starting even with the choke working properly it still make take awhile to fire, I have 17 hp els that's been that way since new and I usually have to prime it after it sets couple of days and dam sure have to do it after winter storage. Why is this? IMO piss pour design weak vacuum fuel pump system. After few years it becomes weaker due to sucking air somewhere and pumping little fuel up to the carb. until the engine fires and has stronger vacuum, but that's my opinion. I just live with it and prime it before I start it with carb. cleaner. I get bonus using the cleaner by getting chemcial cleaner everytime I start it from storage weather it be couple days or months.