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

[cdahl383]

I have been following along with this thread and have a suspicion about the troubles that now exist, a compression test is needed when the cylinder is warmed up. I have not so far seen a pic of the cylinder/piston on the exhaust side. Why I ask is that my family and myself ran these Pioneer saws from 1957 on up through 69-70. I myself have a 600,610,620,620Super and several 650`s. During the years running these saws I have seen many different problems pop up and repaired all of them here at home, when the cylinder warms up the engine can loose compression if the piston is worn undersized or the rings are about worn out. We had one RA the predecessor of the 600, have .022 ring end gap it would run ok at cold idle but as soon as it warmed up it became impossible to tune it and keep it running.

I’ll try to get some better pics of the piston and cylinder later. It looked good when I checked it a while ago.

I’ll try to get it fired up and warm and then run a compression test just to compare cold vs hot.

Here’s a brief video I took a while ago checking the piston and cylinder…

 

[cdahl383]

Ok I ran out the garage real quick and checked the compression.

Cold was 145psi (first photo)

Hot was 145psi (second photo)

I took a couple videos as well. Will post those up later tonight.

Took one of it running. It runs goofy once warmed up. Kind of wondering if it gets an air leak once warmed up or something.

 

[buzz sawyer]

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

Yes, my 1050 has the pump.

 

[cdahl383]

Here’s a video of it running from earlier today…



Started right up. Ran good at first but after it warms up it starts running inconsistent. It revved up good prior to when I started the video. You can see how it sort of fluctuates while running. Almost seems like an air leak or fuel starvation. Then at the end it just craps out.

 

[cdahl383]

Here’s a video I did checking the compression once warmed up. It was about 145psi cold and 145psi warm. Didn’t seem to drop at all really.

 

[buzz sawyer]

I just saw this thread and followed it all the way through. First, I admire and applaud both of you for your patience and perseverance. I restored an RA a few years ago so I am familiar with most of the issues. Most questions or comments I thought of have been addressed. All I can add is - I had a similar high speed bogging problem with a small Husqvarna saw. It idles fine and revved up but bogged soon after. Would up being crap under one of the welch plugs. If I let it rest, the foreign matter would move away but when I revved it, the increased fuel flow would move the FM back over the passage. Found it when I pulled the welch plug.
FWIW, here is the Homelite 1050 with the accel pump - circled in blue.

 

[buzz sawyer]

Actually, I just thought of something - make sure the carb mounting flange surface is flat - where it mounts to the engine. This area is often warped a little and I believe the crankcase pulse passage goes through the flange. If leaking, it may not get a strong enough pulse to work the fuel pump efficiently. I flatten the carb mounting flange with a large flat file or with progressively finer grades of sandpaper on a thick piece of plate glass. I also flatten the carb mating surfaces, except the fuel pump side with the sealing ridges.

 

[cdahl383]

I just saw this thread and followed it all the way through. First, I admire and applaud both of you for your patience and perseverance. I restored an RA a few years ago so I am familiar with most of the issues. Most questions or comments I thought of have been addressed. All I can add is - I had a similar high speed bogging problem with a small Husqvarna saw. It idles fine and revved up but bogged soon after. Would up being crap under one of the welch plugs. If I let it rest, the foreign matter would move away but when I revved it, the increased fuel flow would move the FM back over the passage. Found it when I pulled the welch plug.
FWIW, here is the Homelite 1050 with the accel pump - circled in blue.

That’s a good point. Could be some crap under the Welch plug. Never thought of something moving back and forth like that inside that area.

What’s the best way to remove those plug? The low speed looks like it has the bigger plug. The high speed has the little gold one.

 

[cdahl383]

Actually, I just thought of something - make sure the carb mounting flange surface is flat - where it mounts to the engine. This area is often warped a little and I believe the crankcase pulse passage goes through the flange. If leaking, it may not get a strong enough pulse to work the fuel pump efficiently. I flatten the carb mounting flange with a large flat file or with progressively finer grades of sandpaper on a thick piece of plate glass. I also flatten the carb mating surfaces, except the fuel pump side with the sealing ridges.

Ok, I could pop that off and check it with a straight edge.

Do you think it could be ignition related though? That’s the route I was sort of heading towards before after messing with the carb.

 

[cdahl383]

Check this video out, near the end. It sure sounds like it has an air leak or starving for fuel. Second guessing my thoughts about it being ignition related now.

When I pressure tested the saw I had blocked off the intake right up against the engine. Maybe it’s pulling in air around the intake?