You seem to be working on the right system here anyway. An easy and inexpensive thing to do would be to replace the cap, line and clean the tank. It is best if you pull the tank, rinse a few times with fresh fuel and catch what came out to confirm that there isn't debris from assembly or earlier funky fuel. Another commonly overlooked place for debris is the fuel line itself. Water/ debris can accumulate in the dip in the line or enough cleaner was ran through the line to "break it down". This is what you see when you see that black fine debris in the carb bowl many times. Another possibility is the fuel cap itself which isn't always venting.

This is a common problem on lawnmowers produced during the past ten years. Emission engines are calibrated right on the edge and cannot tolerate any dirt in the fuel system. Additionally, fuel deteriorates in as little as 30 days. Fuel varnish forms on internal passages in the carburetor and reduces the amount of fuel delivered to the cylinder. The main fuel jet located on the bottom of the float bowl must be clean for the engine to run properly. A lean fuel mixture causes the surging and stalling you are describing. There are some good comments in the previous post. I assume this is a typical Quantum engine with the primer system. When servicing a mower, I clean the entire fuel system by removing the fuel tank and draining. Spray some carburetor cleaner in the tank to better clean the tank interior. Let the carburetor cleaner drain out and use air pressure to dry the tank interior. I recommend a new fuel line since your original line is probably deteriorated. When fuel lines are installed or pulled off, tiny particles of fuel line materials may be left in the fuel line so it is very important to flush the line. Cleaning the carburetor with a spray carburetor cleaner dissolves the fuel varnish. Attempting to clean with gasoline or kerosene will not be effective. When cleaning a carburetor with spray cleaner, the viton seat for the needle valve may swell up from the carburetor cleaner. This results in lower fuel level in the float bowl and the primer may not work as it should. Additionally the fuel may have a slightly greater distance to be drawn into the carburetor venturi area. Replacing the needle and seat is a good idea. The bolt that holds the float bowl in place also contains the main fuel jet. The jet must be cleaned with the spray carburetor cleaner to remove all varnish. Replace the o ring that seals the carburetor to the mounting plate. Finally, install a new gasket between the carburetor and the air filter housing. Sometimes the air filter mounting area warps and the gasket does not seal effectively - causing a weak prime.Get rid of the fuel in your spare fuel can. If there is evidence of rust, replace the can. Make sure there is no dirt or grass residue in the spare fuel can. Add about 1/2 ounce of fuel preserver to the can - then add 1 gallon of fuel. If this fuel is not consumed within 30 days, get rid of it by pouring in your car. While this step may seem extreme, I am sure lawnmower techs will verify the importance of using only fresh fuel that is treated with fuel preserver. The old B&S Quantum engine was a simple flat head engine that generally gave good service. Good luck.

If the mower was stored with gas in the tank over winter that gas may have lost a lot of it's volatility. Simply topping off the tank with new fuel won't take care of the fuel in the bowl. You or your mechanic should drain all the fuel and start over with fresh.

For sure you need to use a fuel stabilizer - your issues sound like that type of problem. Even fresh fuel 30 days old can be a problem.

Also ensure you are using only 87 octane fuel with no E10. I would suggest trying a different fuel station - could be the gas quality is suspect and your mower is showing signs of this.

I use Sta-bil, but there are other good choices like Seafoam.

Jim

Jim-

I have had better better luck with running 92 octane. I think all fuel here in TX has some level of ethanol content. I do know in IA 87 is available as non-oxygenated or no ethanol

Can anyone else comment on their fuel choices 87 vs 92?

KYLE

All I can buy here is E10 and I have had no problem with it in any gasoline equipment including 2-cycle and 4-cycle, plus cars and trucks. I use a 1:1 mixture of STA-BIL/Marvel Mystery Oil in the 4-cycle gas cans at 1 ounce/gallon. The 2-cycles get 1 oz/gal of MMO in addition to the regular amount of oil (most 2-cycle oil contains fuel stabilizer). You must at least use a fuel stabilizer/cleaner like STA-BIL in today's gas. I don't say this here as much as I used to because of all the derisive comments about how unnecessary it is, but my stuff runs right all the time, including the cars/trucks.
The use of old gas is deadly to equipment. My late bro-in-law bought several big cans of gas in late 1999 for 'Y2K'. After nothing happened, he should have just used it in his cars, but he saved it. Then years later, he decided to start using it in the nice old mower I had given him. This nearly ruined it but I was able to bring it back to life with no mechanical work other than a spark plug and oil change. The only reason was it has a Pulsa-Jet carb which holds no gas after shutdown, so there was no varnish in the carb. Just using my fresh 4-cycle mix got it running fine. For modern 4-cycle mowers with fuel bowl carburetors, the metering orifices get clogged with gum and varnish. If you don't use a fuel additive, you are bound to keep having trouble.