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
Pioneer 620 - Won't rev up clean to full throttle
- Thread starter cdahl383
- Start date
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+1, needs to buy or borrow a cap tester or a DVOM with capacitance function. Should measure about 0.2 microF.
WoodmanSr
ArboristSite Member
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.
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cdahl383
ArboristSite Operative
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.
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cdahl383
ArboristSite Operative
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.
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.
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You have probably solved the high speed running problem. Now the issue will be about the nozzle check valve, if it had one it is now destroyed and will have a serious affect on the idle. If the idle is still good and not too lean to run, you're in luck and the main nozzle doesn't have a check valve. Most carbs (all newer ones) do have a nozzle check valve that prevents air from bleeding back into the fuel chamber at idle but if there is enough isolation between the feed sources of the two circuits they can work without them.
I don't think that carb has a check valve or even nozzle. You'd see it in the picture down the bore.
#2 in the diagram I think plugs the main port into port and it's just an expansion plug. Later carbs, as you pointed out, replaced that with a nozzle assembly you could press out.
From Tillotson, they scanned it upside down.
Yeah, that #2 is just a cap to plug what is probably just a drilled passage through a raised bump in the venturi, no special orifice. Interesting that the diagram shows one version has an accelerator pump, I wonder if this one has it, could be another problem.
I don't see an accelerator pump in his pictures. Didn't a Homelite 1020 or 1050 have that?
cdahl383
ArboristSite Operative
I looked at the schematic that came with the carb kit and I did not see any check valve or ball in there. It just shows the two needles, the passages, and the ports going up into the carb. I've heard of carbs that have those check valves like you mentioned, but it didn't seem like this old carb had that. Hopefully I'm right.
cdahl383
ArboristSite Operative
I brought the carb in and put it in our little ultrasonic cleaner. It's really only for jewelry and glasses, etc but I was able to fit the carb in there most of the way haha! I filled it up with hot water and some dawn dish soap. Set it for 10 minutes and the water was filthy afterwards even though the carb looked pretty clean beforehand. Dumped that water out and put it in some fresh hot water/dish soap for another 10 minutes. A little more dirt came off but not as much. Then I just let it sit in some hot water with no soap for a bit.
Brought it back outside, blew carb cleaner through everything again, compressed air, and verified the high speed channel was unblocked. I put the rubber gasket piece over the port in the carb throat and when spraying compressed air it shot way up. Put some carb cleaner in there and shot compressed air in there again. Saw a nice spray of carb cleaner come shooting out of that port.
Put the carb back together, dumped some fuel inside the carb so there was some stuff in there to start with. Got it back on the saw and all hooked up again. I pressure tested the carb needle before putting it back on the saw and it held, very slight leakdown. I've found that sometimes you get a false leakdown when the carb isn't completely full of fuel, so I wasn't too concerned with that.
Hoping that did the trick. If not, I'll have to do some more thinking on what to look at next.
Brought it back outside, blew carb cleaner through everything again, compressed air, and verified the high speed channel was unblocked. I put the rubber gasket piece over the port in the carb throat and when spraying compressed air it shot way up. Put some carb cleaner in there and shot compressed air in there again. Saw a nice spray of carb cleaner come shooting out of that port.
Put the carb back together, dumped some fuel inside the carb so there was some stuff in there to start with. Got it back on the saw and all hooked up again. I pressure tested the carb needle before putting it back on the saw and it held, very slight leakdown. I've found that sometimes you get a false leakdown when the carb isn't completely full of fuel, so I wasn't too concerned with that.
Hoping that did the trick. If not, I'll have to do some more thinking on what to look at next.
cdahl383
ArboristSite Operative
Correct, there are basically two plugs. The larger welch plug covers the idle ports for the low speed channel. I didn't touch that as it seems to be functioning fine. The smaller gold colored one covers the port for the high speed channel. It's directly below the port inside the carb throat. The needle goes into the bore and closes off the fuel inlet hole and meters the fuel that can be pulled up through that channel via vacuum in the venturi.
I did not see anything like an accelerator pump on that carb when I had it apart several times. The carb on my old car has an accelerator pump, but I don't believe this carb has one.
