This has been my first experience with the notorious check valves found on newer carbs. I've been chasing this problem on a FS450 trimmer for a while but luckily it has been between cuts on Water Authority sites. Searching here didn't throw up much but these carbs are used on lots of blowers, trimmers and smaller saws. The carb is a Zama C1Q-S34H which is fixed jet with only what appears to be an air screw (reverse thread) for idle adjustment.Symptoms were hard starting, inconsistant idle which couldn't be adjusted and would run out of fuel with an inch still left in the tank. When operating the primer bulb you could keep pumping without it becoming full and air bubbles were visible in the bulb. At full throttle everything was fine but as soon as you let off it would often stop dead. New lines, primer bulb,fuel filter and a rebuild kit did nothing. The answer was eventually found on Zama's excellent website: http://www.zamacarb.com/tips.html. Before you do anything to a Zama carb I would recommend reading all the Service Tips. Especially DO NOT BLOW COMPRESSED AIR THROUGH A CARB WITH CHECK VALVES. It seems you also have to be careful with aerosol carb cleaner if directed straight into the valve with an extension tube. So here's how it went: The jet at the bottom of the metering chamber is the main jet which Zama call the "Nozzle Assembly Check Valve". Stihl call it the "Valve Jet". The hole in the middle, which is normally hidden by the metering lever leads to the idle circuit. To test both circuits use a piece of hose over the jet and blow with your mouth. If clear then try to suck back. If you can the check valve is defective. This is bad news on the idle circuit as it means a new carb but the main jet can be replaced. Zama do a special tool but I managed to use a cheap drill press and one of my extensive collection of broken drill bits. If you have adjusting screws remove the H before starting. It took a fair bit of fiddling to get everthing lined up and more force than I expected to get the nozzle to move. Here it is out and the new one: The new one went in a lot more easily: A few minutes of reassembly and it fired right up. Ticks over fine now and is responsive to the LA screw.