Help: Jonsered 621 very hard start & stalls

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Someone on here can probably provide you with an IPL (an exploded parts list) for the 621, but changing fuel lines is pretty straight forward and cheap insurance on a 30 year old saw. First drain all the fuel from the tank, and check it for loose debris and/orvarnish. If present, clean it thoroughly using one of the many methods described. Once clean, remove the air filter and top cover (detach your coil wire and kill switch wire) and you will see the carburetor. Remove the choke rod from the carburetor (attached with a cotter pin) as well as the fuel line and you will then be able to remove the carburetor. The carburetor is attached with two bolts at the front of the carb. Remove those and remove the carburetor and air filter mount. Be careful pulling the carb away from the plastic intake manifold. A very important gasket should be there, and you'd do well not to damage it. Now you can see the fuel pump, a white plastic square held on to the body of the saw with four screws. Coming from the pump you'll see one tank vent line that terminates in the area above the clutch, as well as the shorter fuel line that connects to the inlet on the carburetor. Remove the shorter line from the pump body, you'll be replacing it. Now remove the 4 screws from the fuel pump body, and very carefully pry up at the base of the pump. Be careful not to tear the gasket. Once you get the pump up, take a close look at the gasket and make sure its not torn or cracked, if it is, you should probably replace that too. Now you'll be able to see the fuel supply line hanging from the bottom of the pump into the tank, with the fuel filter attached to the end of it. You should be able to pull the whole assembly up and out of the hole in the case. Remove the fuel filter and set it aside so you can reuse it, unless its visibly plugged with any of that old paint/epoxy in the tank, in which case you can probably clean it with some fresh fuel mix and blowing it out with compressed air. Remove the old fuel supply line from the pump and replace it with a length of new fuel line. Check to make sure that you have the right length...if its too short, when you tip the saw over the fuel will slosh away from the filter and the saw will starve for fuel. The filter needs to be able to 'swing' inside the tank so that it is always sitting in fuel no matter which way the saw is tipped/turned. Attach the fuel filter to the end of the new line, and re-seat the fuel pump and gasket on the saw body. You can check to make sure your line is long enough by removing the fuel cap and checking to make sure that at the very least, the bottom of the fuel filter is touching the bottom of the fuel tank. If so, tighten (but don't overtighten...its just plastic!) all 4 fuel pump bolts. Now replace the short fuel line that connects the pump to the carburetor. That should do it for the fuel lines. But while you've got it apart...

Check your intake gasket, this may also be your problem. Almost all intake gaskets that come with the carb kits for these carburetors are incorrect, and I'd be willing to bet that the first shop you brought it to didn't reuse the original gasket and used a new one. This saw relies on an impulse line...the action of the piston moving down in the bore creates a vacuum or impulse, and that impulse needs to be transmitted into the carburetor and fuel system in order for fuel to be sucked into the carburetor. If that system isn't sealed correctly, vacuum will be lost and fuel delivery won't operate correctly. The original carb gasket accomplishes this (it is sort of oval shaped and is a thin paper) however the newer gaskets do not (its a more triangular design and is thicker.) If there is a newer style gasket on there, it won't correctly seal the impulse passage and your saw won't run right, and may not start at all. You'll need to track down an OE style gasket, or just make one. I can scan an old gasket and email it to you, and you can then transfer that to a thick paper, like a grocery bag, and cut it out. However, if the original gasket it there and is not damaged, reuse it. Bolt your carburetor and air filter mount back up, reconnect your fuel line and choke rod. Reconnect your kill switch and coil wires, and bolt your top cover and air filter back on. Fill the tank with fresh fuel mix. Before attempting to start, check your carb adjustments. I used to work in a carburetor shop, and it was almost comical to see how some 'trained mechanics' tuned a carb. On the recoil side of the saw, you'll see a hole in the case with an L and an H cast into it, as well as corresponding mixture screws. Take a small screwdriver and slowly and gently turn both screws in until they are seated lightly. Don't crank them in hard or you will damage the needle or seat, or both, and you'll wreck your new carburetor. Once lightly seated, turn each screw out one full turn. That will be suitable to get your saw to run, and once it starts and warms up, you can further tune the Low speed and High speed settings.

Hope this helps, and if all else fails, send it to CT and I'll get it going for you! :cheers:
 
Flyingtim01: Great explanation and a generous offer. That earns ya about 9 ataboys. JR

Thanks jra! With all the help I've gotten from the AS members, its the least I can do...especially this close to Christmas!
 
Jonsered 621 runs out of fuel on high rpm

Someone on here can probably provide you with an IPL (an exploded parts list) for the 621, but changing fuel lines is pretty straight forward and cheap insurance on a 30 year old saw. First drain all the fuel from the tank, and check it for loose debris and/orvarnish. If present, clean it thoroughly using one of the many methods described. Once clean, remove the air filter and top cover (detach your coil wire and kill switch wire) and you will see the carburetor. Remove the choke rod from the carburetor (attached with a cotter pin) as well as the fuel line and you will then be able to remove the carburetor. The carburetor is attached with two bolts at the front of the carb. Remove those and remove the carburetor and air filter mount. Be careful pulling the carb away from the plastic intake manifold. A very important gasket should be there, and you'd do well not to damage it. Now you can see the fuel pump, a white plastic square held on to the body of the saw with four screws. Coming from the pump you'll see one tank vent line that terminates in the area above the clutch, as well as the shorter fuel line that connects to the inlet on the carburetor. Remove the shorter line from the pump body, you'll be replacing it. Now remove the 4 screws from the fuel pump body, and very carefully pry up at the base of the pump. Be careful not to tear the gasket. Once you get the pump up, take a close look at the gasket and make sure its not torn or cracked, if it is, you should probably replace that too. Now you'll be able to see the fuel supply line hanging from the bottom of the pump into the tank, with the fuel filter attached to the end of it. You should be able to pull the whole assembly up and out of the hole in the case. Remove the fuel filter and set it aside so you can reuse it, unless its visibly plugged with any of that old paint/epoxy in the tank, in which case you can probably clean it with some fresh fuel mix and blowing it out with compressed air. Remove the old fuel supply line from the pump and replace it with a length of new fuel line. Check to make sure that you have the right length...if its too short, when you tip the saw over the fuel will slosh away from the filter and the saw will starve for fuel. The filter needs to be able to 'swing' inside the tank so that it is always sitting in fuel no matter which way the saw is tipped/turned. Attach the fuel filter to the end of the new line, and re-seat the fuel pump and gasket on the saw body. You can check to make sure your line is long enough by removing the fuel cap and checking to make sure that at the very least, the bottom of the fuel filter is touching the bottom of the fuel tank. If so, tighten (but don't overtighten...its just plastic!) all 4 fuel pump bolts. Now replace the short fuel line that connects the pump to the carburetor. That should do it for the fuel lines. But while you've got it apart...

Check your intake gasket, this may also be your problem. Almost all intake gaskets that come with the carb kits for these carburetors are incorrect, and I'd be willing to bet that the first shop you brought it to didn't reuse the original gasket and used a new one. This saw relies on an impulse line...the action of the piston moving down in the bore creates a vacuum or impulse, and that impulse needs to be transmitted into the carburetor and fuel system in order for fuel to be sucked into the carburetor. If that system isn't sealed correctly, vacuum will be lost and fuel delivery won't operate correctly. The original carb gasket accomplishes this (it is sort of oval shaped and is a thin paper) however the newer gaskets do not (its a more triangular design and is thicker.) If there is a newer style gasket on there, it won't correctly seal the impulse passage and your saw won't run right, and may not start at all. You'll need to track down an OE style gasket, or just make one. I can scan an old gasket and email it to you, and you can then transfer that to a thick paper, like a grocery bag, and cut it out. However, if the original gasket it there and is not damaged, reuse it. Bolt your carburetor and air filter mount back up, reconnect your fuel line and choke rod. Reconnect your kill switch and coil wires, and bolt your top cover and air filter back on. Fill the tank with fresh fuel mix. Before attempting to start, check your carb adjustments. I used to work in a carburetor shop, and it was almost comical to see how some 'trained mechanics' tuned a carb. On the recoil side of the saw, you'll see a hole in the case with an L and an H cast into it, as well as corresponding mixture screws. Take a small screwdriver and slowly and gently turn both screws in until they are seated lightly. Don't crank them in hard or you will damage the needle or seat, or both, and you'll wreck your new carburetor. Once lightly seated, turn each screw out one full turn. That will be suitable to get your saw to run, and once it starts and warms up, you can further tune the Low speed and High speed settings.

Hope this helps, and if all else fails, send it to CT and I'll get it going for you! :cheers:

flyingtime01,

My Jonsered 621 stalls out at high rpm. I have replaced both fuel lines, cleaned fuel tank filter (what is the ball inside the filter) replaced gasket under white fuel distribution block (thought it was air leak but nothing has fixed it yet) I also took the fitting out that has the check ball in it. I noticed a plastic seat area that it looks like this ball sits in. If the ball does not seat properly what would the result be? I checked the carb gasket also, it is the correct one. Replacement carb worked fine originally but now I can't keep the thing running at high rpm.

Any help would be appreciated. I am leaning toward a new check ball fitting.

Thanks,

Joel:chainsaw:
 
Great ideas and suggestions, I wonder why the shops never thought of the fuel line, air leak or vent prior to all the costly useless repairs, I even wondered if it was getting starved of fuel.

.....

Have you got the problem solved yet? :confused::popcorn::givebeer:

Was the tank vent ever checked?
 
Hey jstamb,

I'm a little unsure of what you mean by stalling out at high rpm. Is it bogging in the cut, or are you revving it outside of the cut and the engine stumbles? At first read I'd say that it sounds like your high speed adjustment is lean. If the culprit was your tank vent, then saw would stop running altogether and be extremely hard to re-start without cracking open the fuel cap. What sort of symptoms does the saw exhibit before it dies? How does it start and idle? How is the acceleration? When did it start having problems? Let me know. Happy to help.
 
Saw a no go

Have you got the problem solved yet? :confused::popcorn::givebeer:

Was the tank vent ever checked?

Vent was checked to be okay, the fuel line was checked by first shop and they reported they thought it was okay but they replaced fuel filter also and also used faulty gaskets on a non tillotson carb.

Took saw to another 3rd repair place since 2nd one hung up on my dad when we had questions for them. They found that the elastomer impregnated cloth was again distorted even though we use fresh gas mix. Compression was tested and found okay. Jonsereds is not answering his email regarding new modern fuel compatibility with old design parts. There is also only 3 repair dealers within 250 miles of us and we have used up 2 of them without satisfaction.

NO more money is being put into the saw and he plans to sell it. Too bad it was our old reliable.
 
i would replace all ur fuel lines, very very easy on the 621's inspect your fuel pump very well, take the top cover off and its all exposed there will be an little plastic square box going into ur tank four little screws, pull it off install new pickup tube new line from the pump to the carb and a new impulse line and check the intake block going form the carb to the cylinder those come loose alot and will cause it to not idle at all or rev really high if so retighten down using some locktite check gasket from cylinder to intake block and from block to carb make sure the little wire coming from the points up to your coil does not have a bare spot on it where it can be grounding out
 
Hey jstamb,

I'm a little unsure of what you mean by stalling out at high rpm. Is it bogging in the cut, or are you revving it outside of the cut and the engine stumbles? At first read I'd say that it sounds like your high speed adjustment is lean. If the culprit was your tank vent, then saw would stop running altogether and be extremely hard to re-start without cracking open the fuel cap. What sort of symptoms does the saw exhibit before it dies? How does it start and idle? How is the acceleration? When did it start having problems? Let me know. Happy to help.

If I run it at full rev outside of the cut it will just slowly fade & quit. Same situation if you are at full rev & cutting, it just fades.
I just checked spark, that is pretty healthy. I also noticed a small divot in the top of the piston but must be nothing, compression check shows 135 psi. Any idea what the spec is?
Both low speed & high speed I have tried at 1 & 1 1/2 turns, same result. It will start & idle & acceleration is good, just seems to run out of gas.
I took the top half of the carb off & it was full of gas. I have not messed with the bottom half. I am starting to suspect dirt somewhere. Do the seat valves stick in these like a bowl float valve would?
Also using 50:1 fuel mixture, that OK?

Thanks for your help! This is frustrating!

Joel
 
Hey joel,

Firstly, yes, 50:1 with good gas and modern 2 stroke oil is fine for your saw.

Secondly, if its gradually stalling at high speeds in and out of the cut like that, it sounds like its starving for fuel. Try to richen up the high side by another half a turn or so and see if that helps. If not, you may want to put in a new fuel filter. I know you said that you cleaned the old one, but they're super cheap and I'd feel better knowing that in addition to all my new fuel lines (you did use the green Tygon ones, right?) I have a brand new and modern fuel filter. How long ago was the new carb put on? Also, a quick way to rule out the tank vent is to loosen the fuel cap on the saw while you're running it. If it acts the same, its not the vent. If it straightens out and runs right, its the vent.

Once you've ruled out the fuel issues, you may need to look at the spark. Sometimes when coils get hot, they fail intermittently, and you'll be chasing what you think is a fuel issue endlessly and without success. To be honest, I never did figure out how to conclusively test the coil on my 621. And if its failing intermittently at high rpm, it could test out fine on the bench and then fail on the saw once it gets hot. The success stories I've heard always end with "I bought a known good coil, and now the saw runs great."

Let me know how you make out with the fuel filter and richening up the high speed setting a half turn or so, and then we'll look at the coil.
 
Vent was checked to be okay, the fuel line was checked by first shop and they reported they thought it was okay but they replaced fuel filter also and also used faulty gaskets on a non tillotson carb.

Took saw to another 3rd repair place since 2nd one hung up on my dad when we had questions for them. They found that the elastomer impregnated cloth was again distorted even though we use fresh gas mix. Compression was tested and found okay. Jonsereds is not answering his email regarding new modern fuel compatibility with old design parts. There is also only 3 repair dealers within 250 miles of us and we have used up 2 of them without satisfaction.

NO more money is being put into the saw and he plans to sell it. Too bad it was our old reliable.

That saw must have some nostalgic value (at least mine does) - don't let him sell it!
 
Hey joel,

Firstly, yes, 50:1 with good gas and modern 2 stroke oil is fine for your saw.

Secondly, if its gradually stalling at high speeds in and out of the cut like that, it sounds like its starving for fuel. Try to richen up the high side by another half a turn or so and see if that helps. If not, you may want to put in a new fuel filter. I know you said that you cleaned the old one, but they're super cheap and I'd feel better knowing that in addition to all my new fuel lines (you did use the green Tygon ones, right?) I have a brand new and modern fuel filter. How long ago was the new carb put on? Also, a quick way to rule out the tank vent is to loosen the fuel cap on the saw while you're running it. If it acts the same, its not the vent. If it straightens out and runs right, its the vent.

Once you've ruled out the fuel issues, you may need to look at the spark. Sometimes when coils get hot, they fail intermittently, and you'll be chasing what you think is a fuel issue endlessly and without success. To be honest, I never did figure out how to conclusively test the coil on my 621. And if its failing intermittently at high rpm, it could test out fine on the bench and then fail on the saw once it gets hot. The success stories I've heard always end with "I bought a known good coil, and now the saw runs great."

Let me know how you make out with the fuel filter and richening up the high speed setting a half turn or so, and then we'll look at the coil.

Well, I decided to dig a little deeper, removed the carb, completely disassembled & blew every passage way out & put it back together. Then I thought about blowing out the pulse hole that goes to the crankcase when I noticed a little bit of gasket sticking up into the fuel inlet area so I removed the black spacer block & found a broken gasket. I am guessing at High RPM I was sucking air. I hope this is it! Know anybody that I can get gasket number 4131401.
Does this sound like the culprit to you?

I know one thing for sure, it's time for a brew!

Joel:cheers:
 
That certainly could have been the culprit, as well as the piece of gasket blocking the fuel flow. I wouldn't search too long and hard for an OE gasket, I'm sure they've been discontinued for years. It would be time better spent to trace out and cut a new gasket. Let me know how it goes back together.
 
That certainly could have been the culprit, as well as the piece of gasket blocking the fuel flow. I wouldn't search too long and hard for an OE gasket, I'm sure they've been discontinued for years. It would be time better spent to trace out and cut a new gasket. Let me know how it goes back together.

Made a gasket, reassembled & it fired right up & doesn't die out at high rpm. That was the culprit!
One job finished this spring, 300 left to go!:chainsawguy:
 
Is the 621 really all that slow? Yeah, it weighs a ton, but its faster than some of the old Homelites and Macs of that generation.
 
That saw must have some nostalgic value (at least mine does) - don't let him sell it!

Yes it is nostalgic, it is the first saw family owned after buying the property but I don't see him storing a saw that is dead slow meaning dead in not running at all. It will get sold to help pay for the new saw.
 
Is anyone still responding to this thread? My Jonsered 621 seems not to be pumping fuel, although I can manually pump fuel by applying vacuum pulses with my mouth. If I blow into the hole in the engine casting that is supposed to communicate with the crankcase, it seems that my breath just escapes to atmosphere easily. Is there a gasket inside the engine blown? BTW, I may have started a new thread on this same subject a few minutes ago. I am not familiar with websites like this, so I may have made some kind of mistake in posting a new subject.
 
I'm working on the same saw. I've completely re-conditioned it, for a customer. It will fire and run with a squeeze bottle prime. I see no fuel coming up the fuel line to the carb. I am going in to see what the fuel pump really does.
 
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