I have a gravely ZT34 with 17 hp engine 31E777 that seized and broke the block, connecting rod, counter weight, etc. I have since replaced the engine with a larger 21 HP Briggs. The problem I am having is that the newer engine ran great on the old mower it was salvaged from. On my Gravely frame, it runs for about 15 seconds then dies. That lead me to conclude that the only difference is the location of the fuel tank. I placed the fuel tank from the older mower back onto the engine on the gravely frame above the engine and it runs.
I know I do not have a clogged filter or anything. I think it's because of orientation. The older mower had the fuel tank above the carb and then gravity would do the work of delivery. On my ZT it sits level. There was a positive pressure line (is my guess) from the air intake side of the carb to the gas tank. Maybe this isnt pressure and is just a way of venting. I'm not sure. I switched carbs from the broken engine onto the 21 HP older engine to leverage that extra line (fitting was not on the 21 HP engine) but it makes no difference in the way it runs.
I don't understand why a very similar engine wouldn't work in the same configuration. Neither appear to have a fuel pump anywhere. What's the best way to solve this other than mount some sort of crazy gas tank above the engine? Perhaps add a fuel pump? Any type of pump better than another? Maybe I need larger lines? I'm really at a loss as to why this engine is not working with this config.
Anything else I might need to consider in this swap? Drive shaft was same length and size as old mower, drive pulley and electric clutch fit perfectly.
In digging a bit deeper, I see the 3 line goes to a small box that says rollover vent (kelch.com) from there it goes to the tank. The tank has a ventless cap (click three times - like a car). I'm still not sure if that's affecting my fuel delivery.
If your tank is lower than the carb then it needs a pump
The line into the air filter is the tank vent ( EPA Regulations ) so it lets air into the tank so the fuel can get out.
Put a pump on it
