Chain saw won't stay running under load

Discussion in 'General Off Topic Forums' started by Andyman, Jun 2, 2013.

  1. Andyman

    Andyman Scroungus Stereophilus Subscriber

    My $25 14" Might Mite yard sale chain saw won't stay running once it hits the wood. I've had it a few years and have used it occasionally for some pruning and yard work, but not much. We did take out the neighbor's 20' birch a few years back, so it could do the job. the project at hand is a 15' Mountain Ash with a 12"-14" trunk and 4-5 6"-8" limbs. Probably 10 minutes to trim and fall and then half an hour or less to munch down into baggable curbside waste.

    What happens is that it starts with some coaxing and I can get it screaming a bit with some careful throttle work, but once I think it's good to go and touch a limb, it just bogs down and dies. I'm thinking it could be a fuel supply/filter issue, as my two two stroke trimmers both suffered from rotted lines, and one still won't stay running. Some online sources suggest a carb kit as there may be diaphragm issues too

    Any ideas? It's a cheapie and doesn't get used much, so I'm not looking to spend a bunch of cash on it as I'm sure I can find another. I figured I'd ask here before I move on.
  2. Oerets

    Oerets AK Subscriber Subscriber

    Derby City U.S.A.
    Guess you have cleaned the air filter? New plug, good gas/oil?

    Then there should be two adjustments on the carb one for the fuel air mix an other for throttle speed. The internet might have a diagram for your model. They are very touchy so make note of position. Only move them one at a time no more then a 1/8 turn and back at first. Might just be dirt in a air passage and with it running place your hand over carbs throat and quickly burp it a couple of times.

  3. grillebilly

    grillebilly Empty Head Subscriber

    Fairfax VA
    Little engine problems are almost always fuel related. Previous suggestion would be the first step.
  4. Andyman

    Andyman Scroungus Stereophilus Subscriber

    Pulled the air filter. No difference.
    Cleaned the plug. No difference.
    New gas. No difference.

    It probably is something buggered up in the carb/fuel line, and I'm thinking the fuel line as it does bog sometimes when I raise it up to cut the chest high limbs.

    Maybe I'll pull the filter as perhaps it's too clogged to flow enough gas at full throttle under load???
    Last edited: Jun 2, 2013
  5. skippy_ps

    skippy_ps AK Subscriber Subscriber

    Palm Springs, CA
    Yep, sounds like a fuel delivery problem.

  6. rswojo

    rswojo AK Subscriber

    Crack in fuel line?
  7. Sam Cogley

    Sam Cogley Last of the Time Lords Subscriber

    SW Missouri
    If it has a fuel filter, replace that. Otherwise, you're probably looking at a carb rebuild.
  8. jimbofish

    jimbofish for TRULY GREAT MUSIC

    NW lower penninsula Michigan
    :yes: Most likely. Could just need adjustment of the idle and high speed screws.

    I'd also look at the exhaust ports and muffler to see that there's not a build-up of carbon. That causes a die-off of rpm's at high speed and load.
  9. Chip Chester

    Chip Chester Super Member

    Central Ohio
    Double check to make sure the auto blade brake is not engaged -- it's triggered by the bar in front of your top hand. It can easily get pushed forward, engaging the brake. Saw will run when slow (below centrifugal clutch speed) but takes all kinds of gas to run above it. Apply to wood, and it all stops.

    Worth a look before you tear it all apart.

  10. gadget73

    gadget73 junk junkie Subscriber

    Southern NJ
    Pull the muffler off and make sure it isn't clogged up too. They often have a baffle plate in there, and if the wrong oil ratio is used, the plate gets plugged up and chokes the motor down pretty badly. Usually just washing it down with carb cleaner will un-clog it.
  11. Andyman

    Andyman Scroungus Stereophilus Subscriber

    Yep, did that too. Actually, it was pretty clean.

    This I haven't done and it sounds like a worthy look
  12. bktheking

    bktheking Gitter Done!

    Ottawa Ontario
    Put a carb kit in it, should have a number stamped on it. The fuel pump is in the carb, it may have a perforation.
  13. SAE2922

    SAE2922 got sae?

    North Texas - DFW area.
    About the worst possible scenario that could cause your bogging down problem would be a crankcase vacuum leak. Either the down-the-middle crankcase gasket, or a crankshaft bearing seal behind the clutch or behind the recoil starter could have an air leak.

    I once had an old, hand-me-down, Homelite XL chainsaw from my dad that exhibited the same symptoms as your saw, Andyman. I rebuilt the Walbro carb using a new carb kit and the saw still had the same problem. So, I slathered some gas-oil pre-mix over the top of the spit-case gasket and then closely examined the case gasket all around the saw as I was finessing the throttle to make the engine run, just as you did with your saw. Didn't take me but just a minute to see the oily film ever-so-slightly bubbling on the bottom of the engine case directly over the gasketed case seam. A vacuumed leak was confirmed.

    I tightened the case screws hoping for an easy fix, but it only helped slightly. I still had a vacuum leak, although smaller in size.

    Your machine may NOT have a vacuum leak, but if it does, a teeny tiny air leak will throw a big monkey wrench into the fuel-air mixture ratio.
  14. Oerets

    Oerets AK Subscriber Subscriber

    Derby City U.S.A.
    Move the choke in a bit to see if it helps. This would indicate air fuel mix.

  15. Andyman

    Andyman Scroungus Stereophilus Subscriber

    I've been messing with the choke a ton. Actually, it runs best on half choke and won't run much on no choke.

    Tomorrow I'll dig into it more as the rest of today is too full already cleaning up some loose ends. This can wait as trash day isn't until Friday, so a Thursday would be the cutting day.
  16. Andyman

    Andyman Scroungus Stereophilus Subscriber

    Well, I found a long wood blade for the SawZall, so the limbs are down and a tall stump is left.

    I'll take care of most of the mess and do the chain saw later.
  17. petehall347

    petehall347 the brandy coffee man

    uk.. the middle bit
    when a 2 stroke is left with fuel in the oil drops to the bottom and fills the carb with oil .
    this might be the problem .
  18. mr greg

    mr greg Active Member

    Near Cohoctah, Michigan
    One thing that does happen with older saws is they can loose compression due to sized-up piston rings. Usually a good clue is if you have trouble starting it. Remove the spark plug and put your thumb over the plug hole and pull the starter. You should feel quite a bit of compression. It should be kind of hard to hold your thumb there. The ring / rings, often will get built up with carbon and freeze in the ring grove.
  19. toxcrusadr

    toxcrusadr AK Subscriber Subscriber

    Central Missouri
    I had a similar problem with both of my good Stihl saws, one after the other. They would start and idle, but not run at high speed or under load. They have a spark arrester screen inside the muffler that gets gunked up with carbon (as mentioned above). The saw will breathe OK at low speed but there's not enough throughput at high speed.
  20. iloveamps

    iloveamps "Back in my day..."

    Boulder, Colorado/Bozeman, MT
    I'm 90% sure you've got a dirty carb. Like the other suggested, find a set of any new gaskets/diaphragms it might have and have at it. It's really easy.

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