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

[cdahl383]

Start with just cleaning and setting the points. If it still acts up, I'd try a new condenser as the next likely problem. This is all assuming there isn't some more obvious problem like an unintentional ground or shorted wire.

I started working on it last night but ran out of time to finish. Will work on it more this weekend.

On first glance, it appeared that the one points arm (not sure what it’s really called), was up a little higher on its pivot point. This was causing the two contact points to not meet up exactly even. I pushed it back down and got them lined up properly.

Then I took some 600 grit sandpaper and cleaned the points up. Finished with a shop towel soaked in brake clean.

Next I rotated the piston to tdc by looking through the exhaust port. I found that when I did that, the lobe on the cam was not at the center high point like I thought it would be. I guess this makes sense as the timing is set for 30 degrees before tdc. The cam is keyed so I don’t see how you could be off with this anyway.

I checked the gap and it was much too large. It was closer to .028” or so instead of .020”. I closed the gap up to .020” where the feeler gauge was just dragging a bit.

Cleaned up a little more inside there, put a few drops of oil on the felt pad and closed it up.

Will put it back together today and see what happens.

 

[cbfarmall]

I haven't really messed with these 600s much--mostly the RA models and 700s. Anyways, I pulled out 2 manuals and both point out the timing is set to 30 BTDC but neither say explicitly where you set the points gap. I ran into this once on my Solo Rex and set the gap at TDC. I didn't know the points were already closing by the time so the gap was much wider and the top of lobe. This made the timing too advanced and the saw kicked like a mule when cranking. Once I set the points correctly, the saw fired immediately and smoothly. So, set the points to .020 at their widest, at the top of the cam lobe. When you check for spark, just set the flywheel on the crank snugly, by hand and give it a quick spin, by hand, spark plug out. If everything is right, you will get spark. Unlike modern electronic ignitions which need a minimum cranking speed to produce a spark, these old mag ignitions are capable of hot spark at low RPM. I also like to use a drill with a socket adapter to check for spark at speed, with the plug out. A drill won't undo the nut like an impact will.

 

[cbfarmall]

From a wico manual.

 

[cdahl383]

I got it running. Unfortunately it’s doing the same thing as it did before. Starts on the first or second pull. Idles nicely. Revs up a little but then bogs down. It won’t die, it just bogs down. Then it recovers and idles again.

Are you saying to set the points gap at the highest point of the cam lobe? Previously you said to set them at TDC.

Any ideas? Ignition? Timing? Carb? Maybe still has an air leak?

Here’s a video of what it’s doing…

 

[cbfarmall]

I thought about it, rechecked and realized the points should be set at their widest not necessarily TDC.

Sounds like the saw spins up when you get on the trigger but then bogs. I'd check fuel. Did you go thru the fuel line. The bowl gasket can allowing air in or either of the fuel lines. Did you go thru that stuff?

 

[cdahl383]

I thought about it, rechecked and realized the points should be set at their widest not necessarily TDC.

Sounds like the saw spins up when you get on the trigger but then bogs. I'd check fuel. Did you go thru the fuel line. The bowl gasket can allowing air in or either of the fuel lines. Did you go thru that stuff?

Ok, so maybe my points are set wrong then? So you just eyeball the highest part of the cam lobe and set them there?

It has brand new fuel line, new fuel filter, etc.

You may be on to something with that bowl gasket. Still using the old one, but I noticed it’s leaking a little fuel. Might also be pulling in air there? I’ll make a new gasket for that and replace that. Easy enough to do.

Do you think the points gap being off would cause it to run like that?

 

[cbfarmall]

If it's leaking fuel at the bowl then it's also sucking air in.

 

[cbfarmall]

Regarding the points gap, if it started good for you and it sounded like it wants to rev up. Seems OK. For myself, it would bug me and I'd go back in. I wonder what the actual difference is?

 

[cdahl383]

Regarding the points gap, if it started good for you and it sounded like it wants to rev up. Seems OK. For myself, it would bug me and I'd go back in. I wonder what the actual difference is?

I’ll reset them. Not that hard to do now that I know what im doing.

Another guy on FB said to raise the metering arm to be flush with the gasket on the carb, not even with the chamber floor. Have you heard of that before?

 

[cbfarmall]

I’ll reset them. Not that hard to do now that I know what im doing.

Another guy on FB said to raise the metering arm to be flush with the gasket on the carb, not even with the chamber floor. Have you heard of that before?

Not for an HL carb. Flush with the floor. Raise it as high as that guy suggested, your saw will never run and whizz gas out till the tank is dry.

Set the points the correct way, replace that bowl gasket and try again.

 

[Old2stroke]

I got it running. Unfortunately it’s doing the same thing as it did before. Starts on the first or second pull. Idles nicely. Revs up a little but then bogs down. It won’t die, it just bogs down. Then it recovers and idles again.

Are you saying to set the points gap at the highest point of the cam lobe? Previously you said to set them at TDC.

Any ideas? Ignition? Timing? Carb? Maybe still has an air leak?

Here’s a video of what it’s doing…

Bogging on acceleration is usually caused by the low speed set too lean or plugged transfer ports. There are 2 transfer ports behind the throttle plate that feed extra fuel as the throttle is opened, allowing the engine to transfer to the high speed circuit. If they are plugged, you often have to remove the welch plug covering them and clean them with a tiny wire. Low speed "L" has to be set richer than lean-best-idle.

 

[cdahl383]

Not for an HL carb. Flush with the floor. Raise it as high as that guy suggested, your saw will never run and whizz gas out till the tank is dry.

Set the points the correct way, replace that bowl gasket and try again.

Ok that’s kind of what I thought too.

 

[cdahl383]

Bogging on acceleration is usually caused by the low speed set too lean or plugged transfer ports. There are 2 transfer ports behind the throttle plate that feed extra fuel as the throttle is opened, allowing the engine to transfer to the high speed circuit. If they are plugged, you often have to remove the welch plug covering them and clean them with a tiny wire. Low speed "L" has to be set richer than lean-best-idle.

Ok, I’ll try richening the low speed.

I haven’t removed the Welch plugs. If this other stuff I try doesn’t work then I’ll try popping those off to inspect and clean.

 

[cdahl383]

Managed to reset the points today. Set the points to .020” at the highest point on the cam lobe. Prior to resetting I noted the points were about .023” gap at the widest point. Not a huge change. But now it’s set properly.

Removed the old sediment bowl gasket which was in terrible shape and made a new gasket using the old one as a template. I’m not sure if the gasket material I have will hold up well to the fuel. Guess I’ll find out.

Didn’t get a chance to fire it or test anything unfortunately. Spent most of the weekend doing family stuff and fixing our dishwasher. I was able to fix the dishwasher myself so that was a plus.

Will try firing it again tomorrow and see if carb tuning will get it to rev up properly.

I’ve read where this issue can be ignition related as well. Bad coil, bad plug wire shorting out, bad condenser, etc.

I’m also going to try running it with no air filter and gas cap loosened just to rule those things out too.

 

[cdahl383]

Tinkered with the saw a little bit today on my lunch break. It's running, but still not right. Here's what I did:

1.) Cleaned/gapped points the other day to .020" at their widest point. Nice big fat spark now.

2.) Cut out a new gasket for the sediment bowl. Got my rubber gasket sheet out and cut out a new gasket using the old one as a template. Installed that, seems to be holding up, no leaks as of yet.

3.) Got the saw running and removed air filter assembly. Seemed like it changed how it ran slightly, but did not make any big changes.

4.) Loosened gas cap while running, no change to how it ran.

Got the saw fired up and let it warm up a bit at idle. It was doing the same thing. It would idle, give it gas and it would bog down badly. I tuned the L speed screw to get a nice idle and good throttle response. The L speed screw is responsive. It seems like the engine will idle nicely now and it will rev up a little with quick throttle response. However, beyond that when you open the throttle more it just bogs down and if you hold it long enough it will die. You can then start it back up and it'll idle all day.

I started adjusting the H speed screw and noticed that no matter what I did, it had zero effect. I turned it all the way in, 1 turn out, 1.5 turn out, 2 turns out, didn't matter, did the same exact thing no matter what. That didn't seem right to me. Even if something is off, it should change how it runs a little bit right?

My new thought is that maybe the H speed port is clogged up with something. If something is obstructing the passage, it doesn't matter how much you turn the screw, no fuel is going to get by. That would explain why when you open the throttle beyond what the L speed screw and transfer ports can handle, there's no more additional fuel to support the increased airflow, so it bogs down and eventually dies out.

Am I on the right track, or way off? Could it still be an ignition issue causing it not to rev up?

The lack of adjustment with the H speed screw leads me to believe that is the issue now. Thinking maybe the next step is to remove the Welch plug and really clean out the H speed passages in the carb.

I've never removed and installed a Welch plug before. Any trick to that? Use a little screw or punch to poke a hole in it and lift it out? Install the new one with no sealer or with some sort of sealer? I've read where nail polish can be used to seal it?

Here is a long video of the saw running earlier today and me fiddling with the carburetor screws. You can hear how it runs differently when I adjust the L speed screw, but basically runs the same no matter what I do to the H speed screw. Pardon my frustration, I was getting a little fed up with it earlier today haha!

I had the idle speed set a little higher than normal just so I could fiddle with it and not have it die out constantly.

 

[cbfarmall]

As a further experiment, if you play with choke and give it throttle, see if that helps it wind up. If yes, you definitely have a fuel delivery problem. A good idle, responsive to adjustment pretty much rules out air leaks.

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.

 

[Old2stroke]

If that saw has a rubber disc in the nozzle check valve, it could have turned to goo and is plugging the valve. Not sure what form the valve is in that carb, it could be a screen in the fuel well held in with a ring clip and the disc would be under the screen. I've had to repair a number of these by making a new disc.

 

[cdahl383]

As a further experiment, if you play with choke and give it throttle, see if that helps it wind up. If yes, you definitely have a fuel delivery problem. A good idle, responsive to adjustment pretty much rules out air leaks.

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.

That's a good idea. That would restrict the airflow and potentially help it rev up a little more. I'd have to put the choke on as I give it gas though as it will die if I give it choke at idle once warmed up. I'll give that a try.

I agree though, I don't think it's an air leak issue any longer. If it were, then the L speed screw wouldn't do much. It's actually set pretty close to the factory setting too (3/4 turn out).

Yes, I could see that, but you can pretty much hear in the video that the engine is bogging down from too much air and not enough fuel. I don't think it's running rich, otherwise it would load up, burble, and sort of four stroke a bit. That sounds like a blockage in the carb not allowing fuel through somewhere.

 

[cdahl383]

If that saw has a rubber disc in the nozzle check valve, it could have turned to goo and is plugging the valve. Not sure what form the valve is in that carb, it could be a screen in the fuel well held in with a ring clip and the disc would be under the screen. I've had to repair a number of these by making a new disc.

This saw has a Tillotson HL22A carb on it. Pretty sure it's the original stock carb. I cleaned it and rebuilt it with a new kit, but I did not remove the welch plugs as I've typically not had to do that on other saws and they always ran fine. I guess this one is going to be difficult for me haha!

I have a parts diagram that came with the kit for this carb. I'll see if I can find any sort of check valve or disc. I know the kit came with some extra parts that I didn't use, like the welch plug, and some little clip looking thing.

Maybe I should just try soaking the carb in some Berryman Chem Dip overnight, let it really soak good, and then blow everything. If that doesn't work, then remove Welch plugs and other stuff and clean it that way.

 

[cbfarmall]

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.