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.

 

[cdahl383]

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.

I think mine is okay. I don’t really have a point of reference. This is my first really old pioneer saw I’ve messed with. It looks similar to yours though.

Here’s a couple photos of the needle when I first pulled the carb off to rebuild it.

 

[cdahl383]

Took the carb off tonight to clean it a bit more. Found that there was no real good way to shoot carb cleaner or compressed air through that passage. It either went back out where the needle screws in or back into the mixing chamber.

I cut a small piece of fine wire and poked around in the port in the Venturi and a little bit in the fuel inlet in the mixing chamber.

I found a grommet that fit perfectly in the chamber slot to block off that hole. Then I took my compressed air nozzle and shot air through the high speed needle bore. Did that several times along with spraying more carb cleaner in there.

Then to verify air and fluid could pass through there, I set a small piece of rubber gasket over the main discharge port inside the carb. I sprayed compressed air and the rubber piece shot up. I put some carb cleaner in there and sprayed again. I could see the carb cleaner spray up into the carb. Now I know that passage is not obstructed.

The only thing I’m thinking now is perhaps there is a fuel flow issue at higher rpm. Maybe a problem with the fuel filter or loose fit fuel line inside the tank. Maybe that’s why no fuel was coming through the carb. I’m hoping that’s not the case and that it was simply obstructed and now it’s clear.

Will have to do another test run tomorrow.