Pioneer 620 - Won't rev up clean to full throttle

[Old2stroke]

He's probably referring to what would be the main nozzle. It would be 31A in the picture. Not sure if those have check valves, Walbros definitely do. Did you blow cleaner through the needles and see it spraying in the bore? The main nozzle will have a spray pattern, while the separate idle ports are just tiny streams. If you choose to remove it, they just press out into the bore. Make sure to remove the needles first.

Carb is too old to have that type of nozzle check valve. Probably a tiny screen over a disc. Welch plug probably covers idle ports

 

[hotshot]

Now, I've had a saw that would idle but would miss at speed and run terrible. Turned out to be a faulty condenser. It was pretty easy to hear that miss, though.

+1, needs to buy or borrow a cap tester or a DVOM with capacitance function. Should measure about 0.2 microF.

 

[WoodmanSr]

Carb is too old to have that type of nozzle check valve. Probably a tiny screen over a disc. Welch plug probably covers idle ports

What is the shape & condition of your High Needle Screw.
My Pioneer 600 is set 1/2 turn open on H screw. Easily revs to full spec WOT of 5500 rpm.

No H screw response seems more wide open, broke off tip, or wrong H screw.

Pictured proper Tillotson HL-22A high screw.