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Jonsered 525 chainsaw schematic

525 jonsered chainsaw, wont run unless, fuel is dropped down the head, stays going for a few seconds, then shuts off. Just cleaned the carb top to bottom and it is the same as before, blew out all the jets and is spotless inside and out but still the same. Replaced all lines including gas line, carb is sucking gas, and the gas tank is clean and full so I dont know what else that it could be. If you have any help it would be great. Thanks

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Was carburetor cleaner used to blow through the fuel circuits?

Small engine carburetors have tiny polymer or woven Teflon discs that move back and forth within the check valve(s). Carburetor cleaners or dip solutions are not intended for this type of component. The only approved cleaner that I have heard about is non-flammable brake cleaner, like Brakleen. Stihl's tech department recommends this approach to prevent damage to soft components of the fuel system.

It sounds like you are seeing fuel in the metering chamber, so the likely fault is a check valve, especially since this saw may be approaching 30 years of age. It is also important that the impulse passage/line is pulsing properly.

If there is a tiny hole in the intake that matches a hole in the gasket and carburetor, this is an impulse passage. Remove the carburetor, place a drop of oil on the hole in the intake, and turn over the engine. It should draw and expel the oil as this is attempted a few times. I'm not familiar with your saw, but some designs have an impulse hose that attaches to the carburetor's pump cover. I seriously doubt this is what you have, but it can also be checked using a few drops of oil.

The alternation of positive and negative impulses moves the metering diaphragm back and forth, allowing fuel to enter the metering chamber. If the impulse is weak, it may enter too slowly for the engine to fire. If the impulse can be seen repeatedly with the test above, it should be fine to operate the carburetor.

Next, the high speed nozzle needs to be checked with brake cleaner. It should allow a large stream to be blown into the throat of the carburetor from the metering chamber (if the high speed nozzle has a visible hole on top) or by removing the high speed mixture screw and spraying into the seat. It should not allow any cleaner to be blown the opposite direction. This can be tested any most Zama and Walbro carburetors by bending the red nozzle that comes with the cleaner. I usually feed it through the hole in the center of the fuel pump side until it touches the nozzle's outlet.

If the nozzle fails either test, it needs to be punched into the bore to remove it. A new one is simply oiled and punched into place at the same height, but finding this one may be tough. I was unable to find any information about this saw in a quick search.

The carburetor will probably have a model number stamped into it, and that will help to find if parts are still available.

Posted on Apr 23, 2014

  • Lonnie Smith
    Lonnie Smith Apr 23, 2014

    I meant red spray tube that comes with the cleaner.



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SOURCE: fuel line routing help.

You should have one line (with filter) going to the carb inlet & one line going to the bulb then back to the tank.

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SOURCE: McCulloch,Mac 3200 Chainsaw fuel lines...........

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SOURCE: poulan 2000 starts but wont

Two common things are a clogged filter which is usually on the end of the "fuel finder" hose within the fuel tank snd the other common problem is failure of the fuel pump and or check valves. The latter is in the carb and consists of a small diaphram that is driven by vacuum pulses picked off a port in the carb. The check valves of the pump are jsut little flaps. The fuel pump problem is most likely if you have always used clean gas. The fuel pump diaphram seem to be good for about 2 years then it is carb rebuild time. It takes less than an hour to rebuild with new kit. Try not to disturb adjustments and idling screws.

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SOURCE: my husqvarna 350 chainsaw starts but wont stay running for more than a few seconds


Find the gas/air adjustment screws on the carb (next to the the throttle cam where the throttle cable attaches. Turn the screws in all the way... GENTLY... Those are needle valves, and too much torque on them will damage the needle valve seats. turn both of them out one and a quarter to one and a half turns. Start the saw, and adjust one or the other screw in or out until the saw runs properly. If there is a plastic stop installed on the screws, remove it first, and replace it after the saw is running properly. You have to be patient while tuning the carb as it may take a while. If you adjust too much and the saw refuses to start at all, start over with the 1-1/4 to 1-1/2 turns out on the screws.

Posted on Aug 19, 2012

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My jonstred chain saw has no choke lever

Ian Duncan...

I have no idea if you are bragging or complaining...

I WILL TELL YOU ... I have started many many a chain saw... and the chokes are nearly useless...

FIRST RULE: always use only fresh fuel...!!

Proper sequence for an ice cold saw (optimal adjustment)...
- squish the fuel bulb a couple of times
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- completely release the choke and the next yank will get you running

If you have NO CHOKE... JUST leave the THROTTLE ALONE
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525 jonsered chainsaw, wont run unless, fuel is dropped down the head, stays going for a few seconds, then shuts off. Just cleaned the carb top to bottom and it is the same as before, blew out all the jets...

How is the compression on the saw? I see 2 possibilities 1 bad reed valves, or 2 a scored piston and cylinder. To check the reed valves, take the air filter off, and pull the starter rope, listen to see if air is blowing back out of the carb if it is the reed valve is broken or has something lodged in it. As far as the piston and cylinder, you can pull the muffler off and look in thru the exhaust port, what you see should be shiny and slick, if there are any vertical lines in the cylinder, or on the piston it is time for a new top end.

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My 2 year old Jonsered chainsaw starts and runs but has no power. I've cleaned the air filter and replaced the spark plug but still runs the same.

Try playing with the fuel mix screw, one of two that can be accessed on the outside of the saw. one should be marked "T" and the other "S" play with the "T" screw, wind it all the way in and back it off 1 turn as a starting point then half a turn out until you get a result. alternatively... unfortunately, it sounds like the piston rings have gone. go shopping.

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Make sure you are using fresh fuel mix (50:1 ratio). Don't use boat motor oil in a saw--only modern synthetics made for saws. Always premix the oil and fuel thoroughly before adding to the saw. Check the fuel filter for plugging and the condition of all fuel lines, plus the primer bulb condition. Check all carburetor fasteners for tightness. Make sure the air filter and muffler are not plugged. If still no help, you may need to service the carburetor. For an excellent article on Walbro carburetors, go to: Pay particular attention to the placement of the lower diaphragm and the cover gasket--interchanging positions of them cause many headaches. Adjustment of the two jet screws require a special 'D' screwdriver which may necessitate a trip to a servicing dealer for adjustment. Hope this helps!

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PLEASE stop shouting and provide a model number for your saw. I will provide an IPL with some fuel system detail and a schematic to use for hose replacement. Reply below. Lou

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Chainsaw cuts out

  1. Clean the air filter.
  2. Drain the tank and refill with fresh fuel mix.
  3. Replace fuel filter and Inspect fuel lines for cracks.
If 1 – 3 don’t fix the problem, spend $40 for a tune & repair up at your friendly neighbor hood chainsaw shop. HTH

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Been a while since I saw a Jonsered but I think if you bend the choke linkage a little that it will stay in the choke position.

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1985 conrvette

poss. is fuel pressure reg. problem. put a guage on the test port and check press. should be above 35psi. see if it stays running when you spray choke cleaner in the pcv valve steadily while starting. if the engine stays running after a few seconds of running on choke cleaner then probably a bad fuel pump relay cause the oil pressure switch will keep the pump on without the relay when it has good oil pressure.
let me know if ya can do this or if it helps.

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