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.

 

[Old2stroke]

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.

 

[cbfarmall]

I don't think that carb has a check valve or even nozzle. You'd see it in the picture down the bore.

 

[cbfarmall]

#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.

 

[cbfarmall]

From Tillotson, they scanned it upside down.

 

[Old2stroke]

#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.

View attachment 1245412

#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.

View attachment 1245412

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.

 

[cbfarmall]

I don't see an accelerator pump in his pictures. Didn't a Homelite 1020 or 1050 have that?

 

[cdahl383]

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 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]

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.

 

[cdahl383]

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.

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.

 

[cdahl383]

Well, I tried messing with the saw again today on lunch. Still no luck. I just don't get what's going on with this thing.

Put carb back on, fired it up. It literally fired up on the first pull with choke. It died, I turned the choke off and it started. I let it warm up for just a little bit and then started to rev it up. It seemed to idle and rev up fine. Dialed in the low speed screw. Got it set.

Then I revved up the saw and remarkably it actually revved up and held at full throttle! It's not a very high revving saw, but it did rev up quite well. Played with the high speed screw a little bit and tried again. It revved up good again. Then I let it idle for a little bit.

Then I noticed the saw started to run different, then it started to stumble, then it died. I pulled it over several times and it would run for a bit then crap out. Got it running again, it idled and revved a little bit, but then it started doing the same thing as before. As soon as I would crack open the throttle, it would just bog way out and die. No amount of adjusting the high speed screw did anything. I know that high speed channel is clear. I had it all cleaned up this morning and the saw did actually rev up and hold at full throttle.

Any ideas?

1.) Carb or fuel delivery
2.) Ignition

The carb was cleaned and it's virtually spotless. Needle/metering arm set properly. I tested the pop off pressure it was about 15psi for the needle. Seems about right.

Tested for spark with the inline tester after it crapped out, it was still lighting up. The plug didn't look fuel soaked or anything like before when it was flooding out.

I just don't get it. It seems like that carb should be good at this point.

Could it possibly be ignition related? Faulty condenser or coil? Warms up and then doesn't work as well? I know on my car when the coil or ignition module was going out it would quit working when it got hot. Once cooled down it would be fine again.

I know the saw has the potential to run right as it did for a brief period when I first started it up. It actually revved up and ran good. But then shortly after that all of the sudden you could hear it started to run different and then it started to stumble and crapped out. It never ran right after that. Maybe some ignition component got warmed up or hot and started to fail?

This thing is really testing my patience.

 

[cbfarmall]

There is a joke I heard about this stuff. 90% of fuel problems are ignition. I think I change that condenser just because it's old. Sounds like you ruled out the carb other than trying a known good one. Carbs are weird like that.

Did you try playing with the choke while it was stumbling?

 

[cdahl383]

There is a joke I heard about this stuff. 90% of fuel problems are ignition. I think I change that condenser just because it's old. Sounds like you ruled out the carb other than trying a known good one. Carbs are weird like that.

Did you try playing with the choke while it was stumbling?

Yes, I've heard that before in the car world. It's true too. People will spend countless hours changing things on their carburetor when the real problem is the ignition advance is wrong (too little initial timing, timing curve too slow or fast, too much total timing, etc).

That's kind of how I feel. I pulled the carb off again to take it apart but then I thought, what am I going to do this time? At some point you're just doing the same thing expecting different results (i.e. insanity). I tested the pop off pressure and noted that. The needle did leakdown slightly but it wasn't horrible, not enough to make the saw run like that.

When the saw is running too rich, it generally smokes like a freight train. That's what it was doing a while back when it was flooding out. It would run but smoke like crazy. Now it has a little smoke coming out like normal but nothing crazy. When it was bogging out and running goofy, it wasn't smoking like crazy. I don't believe it's running too rich.

The carb and all passages in it are clear. The needle is set properly, the metering arm is set properly, the low speed screw does work and affect how the saw runs, so I believe that rules out an air leak.

I wish I had another carb to try on it that I knew worked. I'm supposed to buy a couple saws from a guy on here in a few weeks (I think he commented on this thread too a few times). One of the saws he has is a Pioneer 600, which is basically the predecessor to the 620 I have. I believe it has the same carb on it, so maybe when I pick that saw up I could swap it on there and see if it runs any different.

Barring that, I think the carb is good like you said.

Maybe I'll try the condenser first. If that doesn't help, then maybe it's time for a brand new coil.

No I didn't get a chance to mess with the choke. Once it started running goofy it was hard to get it to run long enough to mess with it.

 

[cbfarmall]

Condensers are easy enough to find. Stick with NOS not aftermarket.

 

[cdahl383]

Condensers are easy enough to find. Stick with NOS not aftermarket.

I'll see if I can find a NOS one.

What about the coil? Maybe try condenser first, then if no luck with that, try a new coil?

 

[cbfarmall]

That's what I would do. Coils fail less frequently than condensers in my experience.

The Wico # for the condenser is right there in your picture. X11181