My FH500V is on a DR field and brush mower. It runs fine once warm but is very hard to get started, particularly when its very cold out. I have to crank it a long time and then it only starts to fire when I pull the throttle all the way to idle which also turns off the choke. It seems like the choke is not working. I took off the air filter and noticed the flapper in the carborator was mostly closed with the choke handle all way on, but not completely closed. Could that be the problem? Or could it be in the fuel pump?