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

[cdahl383]

Recently picked up a non-running Pioneer 620 chainsaw. Had spark and 150psi compression and piston/cylinder looked good. Fuel system was wiped out.

Cleaned fuel tank out, replaced fuel lines, fuel filter, cleaned/rebuilt carb, cleaned air filter, and got a new spark plug. Got the saw to fire up, but it ran very poorly, smoking a lot, running rich, belching oil out the exhaust. Plug was wet and black. Pulled carb off and replaced the needle/metering lever with the originals (the ones from the kit were obviously flooding the saw out for whatever reason). Made sure the lever was flush with the chamber floor of the carb.

Fired it up today and after running rough for a bit at first, it cleared itself out. The plug was no longer fuel soaked and it wasn't smoking like a freight train anymore. After running for a while and revving it up, it idled on its own. Can shut it off and restart it easily. Idles nicely. Revs up off idle nicely with good throttle response. However, when I give it full throttle, it just doesn't want to rev up good. It sort of blubbers, misses, and starts to bog down. I let off the throttle and it cleans up again and idles nicely.

I adjusted the high speed screw out another 1/4 turn, then 1/2 turn, then 1 full turn out from its initial setting. It basically had no effect which was odd.

I'm thinking it either has an air leak somewhere (i.e. crank seals, intake by carb, base gasket, etc) or it has a fuel starvation issue (maybe the filter is obstructing fuel flow, felt filter in sediment bowl is still clogged after cleaning, or maybe still an issue with the carb somewhere).

I'm new to working on chainsaws, so I'm not 100% sure where to look next. I do have a Mityvac air pump that I recently bought, so maybe I could run a pressure/vacuum test on it? I've never done that before, but there are plenty of Youtube videos out there. But maybe it's just something simple I'm overlooking, or maybe the carb needs to be cleaned better still?

Any suggestions are welcome. Happy I got it running now, but would like to get it running good.

Below is a video of it running earlier today...

 

[cbfarmall]

That is probably ignition. I'd be cleaning the points and possibly changing the condenser. A good idle and smooth return to idle usually rules out air leaks.

Ignition misses at high speeds are usually the condenser in my experience.

 

[farmguywithasaw]

That is probably ignition. I'd be cleaning the points and possibly changing the condenser. A good idle and smooth return to idle usually rules out air leaks.

Ignition misses at high speeds are usually the condenser in my experience.

Recently picked up a non-running Pioneer 620 chainsaw. Had spark and 150psi compression and piston/cylinder looked good. Fuel system was wiped out.

Cleaned fuel tank out, replaced fuel lines, fuel filter, cleaned/rebuilt carb, cleaned air filter, and got a new spark plug. Got the saw to fire up, but it ran very poorly, smoking a lot, running rich, belching oil out the exhaust. Plug was wet and black. Pulled carb off and replaced the needle/metering lever with the originals (the ones from the kit were obviously flooding the saw out for whatever reason). Made sure the lever was flush with the chamber floor of the carb.

Fired it up today and after running rough for a bit at first, it cleared itself out. The plug was no longer fuel soaked and it wasn't smoking like a freight train anymore. After running for a while and revving it up, it idled on its own. Can shut it off and restart it easily. Idles nicely. Revs up off idle nicely with good throttle response. However, when I give it full throttle, it just doesn't want to rev up good. It sort of blubbers, misses, and starts to bog down. I let off the throttle and it cleans up again and idles nicely.

I adjusted the high speed screw out another 1/4 turn, then 1/2 turn, then 1 full turn out from its initial setting. It basically had no effect which was odd.

I'm thinking it either has an air leak somewhere (i.e. crank seals, intake by carb, base gasket, etc) or it has a fuel starvation issue (maybe the filter is obstructing fuel flow, felt filter in sediment bowl is still clogged after cleaning, or maybe still an issue with the carb somewhere).

I'm new to working on chainsaws, so I'm not 100% sure where to look next. I do have a Mityvac air pump that I recently bought, so maybe I could run a pressure/vacuum test on it? I've never done that before, but there are plenty of Youtube videos out there. But maybe it's just something simple I'm overlooking, or maybe the carb needs to be cleaned better still?

Any suggestions are welcome. Happy I got it running now, but would like to get it running good.

Below is a video of it running earlier today...

Hard to tell if you don’t hold it wide open ie quit fingering the throttle. I personally couldn’t hear a miss it just sounds really fat (rich) in the high jet. That said cleaning the points and replacing the condenser is not bad idea anyway

 

[cdahl383]

That is probably ignition. I'd be cleaning the points and possibly changing the condenser. A good idle and smooth return to idle usually rules out air leaks.

Ignition misses at high speeds are usually the condenser in my experience.

Ok, that's a good point. I've been focusing solely on the fuel system so far.

It seems to run kind of erratic and inconsistent. Yesterday I was able to get it to idle and rev up off idle great. Today I messed with it some more and it ran okay for a bit but now it doesn't really want to idle that well either.

I pulled the two needle screws out to replace the o-rings on them as the originals were pretty rough. I put the screws back to their original settings that I had the other day but now it seems to run goofy. When it starts to run rough, it starts puffing smoke and blubbers and loads up and eventually just dies. As though it's flooding out or something.

I'm going to try pressure testing the saw and the carb as well to see if there are any air leaks and if the needle is seating properly in the carb. Maybe that's part of my problem.

I'll have to get a flywheel puller and pull the flywheel to get to the points. I haven't checked those yet, I just saw it had spark and figured I could go from there. But if the points are dirty or the condenser is going bad or is bad, then maybe that's the root of the problem.

 

[cdahl383]

Hard to tell if you don’t hold it wide open ie quit fingering the throttle. I personally couldn’t hear a miss it just sounds really fat (rich) in the high jet. That said cleaning the points and replacing the condenser is not bad idea anyway

It will die if I hold it wide open for too long. It starts to blubber and then starts to die. If I let off, it comes back to idle again and smooths out. At least it was doing that yesterday. Today it was running a little worse it seemed.

I'll check on the points and condenser next. Will have to get a flywheel puller first though.

 

[farmguywithasaw]

It will die if I hold it wide open for too long. It starts to blubber and then starts to die. If I let off, it comes back to idle again and smooths out. At least it was doing that yesterday. Today it was running a little worse it seemed.

I'll check on the points and condenser next. Will have to get a flywheel puller first though.

That plastic disc on the bottom of the carb that the fuel line goes on can cause issues if it’s not perfectly sealed as well. It won’t pump fuel well and suck air

 

[cdahl383]

That plastic disc on the bottom of the carb that the fuel line goes on can cause issues if it’s not perfectly sealed as well. It won’t pump fuel well and suck air

I'll check that. I snugged it down pretty good but didn't go ape on the those little screws, didn't want them to break.

I'm going to pop the carb off and pressure test the needle and see if it's good or not.

I'll get the saw ready for a pressure test too. Need to block off the intake and exhaust ports.

 

[cdahl383]

Took the carb off again last night. Cleaned it thoroughly with carb cleaner and compressed air. The carb is spotless. Adjusted the metering lever to be perfectly flush with the chamber floor of the carb. Reassembled everything. Pressure and vacuum tested the carb through the fuel inlet on the carb. It held 7psi and 14hg vacuum. The needle is good.

Also double checked the fuel line. I found I had originally cut it too long and the fuel filter was curled up at the far end of the tank. Cut about 1.5" off the fuel line and put it back in the tank and now the filter is resting flat on the bottom of the tank. I don't think that was causing an issue but it wasn't right so I fixed it.

Put it all back together. It fired up on the second pull with choke on. Turned the choke off and pulled it over and a couple more pulls and it was running. It will idle and rev up a little bit, but if you hold the throttle open for long it will start to bog down. It will recover if you let it idle. I also noticed the idle would vary. It would rev up a little higher, then go lower, then higher, all on its own. Seems like unmetered air is entering the engine somewhere. I don't think it's a fueling issue.

I'm going to do a pressure/vacuum test on the saw later this week. Just waiting on a spark plug adapter so I can screw that in to pump air into the engine. I'm pretty sure that will tell me what's going on.

At least I can rule out the carb and fuel system now. Must be an air leak somewhere or perhaps something funky going on with the ignition (points gap, condenser, etc).