I have a Keihin KTB on a 2007 Honda Rancher 400. The engine will run (not perfect) and I can ride it around the yard when I remove the spring from the piston/plunger assembly. When I reinstall the spring, the engine will not run unless I remove the air filter and manually lift the plunger. I have cleaned the carburetor a couple times and have reassembled it with the service manual as my reference. In case it helps, the engine is mildly difficult to start with the spring out. With the spring installed, it will not start unless I manipulate the plunger. The engine has a new piston, cylinder, rings, head, valves and timing chain. The timing appears correct and the valve clearances are good.