When you changed the filter, did you run the line into a catch pan to see if what you had was clean and free of debris or water? There is discussion every now and then of sticky valves on these. A bit of Seafoam in the oil and fuel may help with that. I like to shoot carb spray and or marvel down the carb throat with it running on the ones that I tune in the spring after they sat.

My 425 with the 20hp Kaw needed the time delay module. If yours has this module replace it. It's about $20 from the dealer. It's a common problem with the 425's. Not sure about the 335.

Thanks for the suggestions. I will try the Seafoam and carb spray, Seafoam (not used this before and not sure what it is) and will check bout the time delay module. Also, not sure what marvel is. I see on internet a product called Marvel mystery oil. Is that it?
I didn't let the gas run when I changed filter. My tank is almost empty. Should I just take fuel filter out and let the gas run out of the line and then get new gas? Is the intermittent coughing and sputtering something caused by sticky valves occurr more under load?

Hey Rick. Seafoam is an additive for both the gas and oil. It is good stuff. I've seen old 50+ year old Farmall Cubs run a couple of tanks of gas and Seafoam through them, and run much better. It helps break up varnish, etc formed over the years.

Marvel Mystery oil is good stuff too. It too can be added to the gas or oil.

These are two of the few additives I will use in my engines. I usually shy away from any kind of additive, but like these two products.