John Deere L120 with ~ 150 hours on it. Briggs 20 HP V Twin engine. Regular service at the proper intervals - oil, oil filter, fuel filter, plugs, air filter / pre-cleaner all about 10 hours ago. This just started happening - runs great then after cutting for less than 30 minutes the motor begins to surge as if the choke is engaged. I can get it to run smoothly for less than a minute by giving it full choke, then it dies. If I let it cool it will run OK for less than 15 minutes, then begins to surge again. So far I have tried gumout carb cleaner in the tank and have run about 1/8 of that tank through. I've also tried spraying carb cleaner in the air intake. Any thoughts?

Clean the carb and complete fuel system. From the fuel tank cap all the way to the carb.

clean the carb and complete fuel system. from the fuel tank cap all the way to the carb.

I'll give it a shot.

I've also noticed pressure build up in the gas tank after running the engine, which is new. Maybe there is some kind of vapor lock going on or the gas cap simply needs to be replaced?

I'll give it a shot.

I've also noticed pressure build up in the gas tank after running the engine, which is new. Maybe there is some kind of vapor lock going on or the gas cap simply needs to be replaced?


The surging started to happen again the other day, but it only lasted a little while, then a puff of black smoke, then it ran fine. My guess is whatever was in the carb must have dislodged. Previous to that cutting I cleaned the gas cap because of the pressure build up by using a tooth pick on the vent holes, but I bet it was all the carb cleaner I dumped into the tank and sprayed into the carb and onto the linkage.