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Question about fine tuning the carburetor on my saw

Post in 'The Gear' started by tradergordo, Apr 19, 2007.

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  1. tradergordo

    tradergordo Minister of Fire

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    Manual says "When the carburetor is correctly adjusted the machine accelerates without hesitation and the machine 4-cycles a little at max. speed."

    OK - I'm showing my ignornace - but what exactly does "4-cycle" mean in this context? And how exactly do I know its "4-cycling a little at max speed"? Is this an audible thing?


    Is it a bad idea to fine tune the high speed jet without a rev counter?

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  2. DiscoInferno

    DiscoInferno Minister of Fire

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    You can hear it, it sounds sort of rough or ragged. There used to be some instructions and audio files at http://www.madsens1.com/sawtune.htm, but the server seems to be down right now.

    I've adjusted mine by ear using nothing more than those instructions and my manual, but I've been told here than I'm probably losing some power as a result. I keep meaning to pick up a cheap tach to find out.
  3. precaud

    precaud Minister of Fire

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  4. DriftWood

    DriftWood Minister of Fire

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    How a chain saw cuts is the thing when setting hi speed jets. You want to end up with a strong cutting, fast running saw as it is cutting. Play with the hi speed carb jets only at top speed and when cutting.

    First tune with fresh gas and mix.
    Next carb air filter and carb must be clean.
    The saw must be hot, off choke, fully assembled and ready to cut wood.

    Set saw to manufactures spec's, cut some wood to see it it is set right at full speed. If not, stalling or running slow set richer or leaner to get a fast strong cut with a hot saw.

    The idle jet, and low speed are a little tricky as they affect the hi speed setting. I usually set low speed a little rich to avoid a stall on acceleration. The trick is to turn in the jet till it wants to slow down the saw them our till it is rolling rich, then split the difference as a starting point. See how it accelerates and slows down, no stalling.

    The idle speed is set to see the chain stop when idling.
  5. elkimmeg

    elkimmeg Guest

    Great advice unfortunately gas oil mixture is not an exact science this happens more on older saws where I adjust the carb to the mixture as a starting point I turn in the screws hi lo all the
    way in and back them out one turn then adjust them from there. If you do a decent job mixing the fuels tyou should not be making adjustments from factory settings.

    I agree with Keyman I turn the screws to the point it starts loosing power and starts cutting out and back them off one half turn you must have a clean air filter screan to get it correct
  6. Gooserider

    Gooserider Mod Emeritus

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    How hard is it to mix the fuels? I just use a one gallon can and one of those little one-shot bottles of oil w/ gas preservative - I vary the brand, but stick with one of the reputable chainsaw co's packages. I dump the oil into the can and add one gallon of gas from the pump. I always assumed that the gas splashing into the can would mix the oil pretty well, plus I always slosh the can around a little before I fill the saw - is there more I should be doing?

    Gooserider
  7. computeruser

    computeruser Feeling the Heat

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    The audio files at Madsen's are very helpful.

    A good tach costs a few bucks, and most folks aren't willing to shell out $100 to be able to tune their saw; I can't blame them. No problem, though, tuning by ear and feel should get you in the ballpark. Err on the side or funning a bit rich and you'll be fine, especially if running synthetic oil (fewer carbon deposits). After years of tuning by ear I finally bought a tach and found that I was actually running on the conservative side - up to 1500 RPM shy of max-RPM on the smaller saws and about 1000 shy on the bigger ones. Now that I have the tach and have re-trained my ear, I'm able to get within a few hundred RPM of my target just by ear.
  8. tradergordo

    tradergordo Minister of Fire

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    Yea, thanks for that link - very useful. I gave in though and ordered a tach (good excuse for a new toy). But I got a cheap one, so not sure how well its going to work - the manufacturer claims very good accuracy though, and it should easily be able to handle the RPM of my saw... speaking of which, for some reason even though the manual says its best to use a tach for tuning the high jet, they DON'T supply the recommended low load RPM value for some reason (husky 455 rancher) - I checked the owner's manuals of their other saws, and they do supply a number (for example 13,000 for the 359). What's up with that? I have a support question in via their website.
  9. Andre B.

    Andre B. New Member

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  10. tradergordo

    tradergordo Minister of Fire

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    In theory audio based RPM (cylinder firings) should work - (its author says its accurate to 1 RPM but makes no mention of its max detection capability). The PC sound card sampling rate should be able to handle it though. For most saws, you would need to be able to measure up to around 14,000 RPM or slightly lower, that means 233 "bangs" per second, the sound card sampling at 11025Hz with 8bit resolution and mono (1 channel) should sample 11025 bytes per second. I'm not an audio engineer but it seems adaquate to me (?). Since it's totally free, I'll test it out and see how it compares to a measurement from my other tach.
  11. biggins08

    biggins08 New Member

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    Did you ever compare the 2?
  12. tradergordo

    tradergordo Minister of Fire

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    Yes, I tried the audio based RPM detection - I think the software is a valiant effort at a challenging problem (software RPM detection), but not very practical to measure RPM that way. Perhaps it is because my microphone isn't so great? But it was jumping all over the place, I do think its max reading was probably pretty accurate though to what the actual was. It just isn't acurate enough to tune your saw with though, you are probably better off doing it by ear.
  13. Sandor

    Sandor Minister of Fire

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    Trader,

    A two stroke has a power stroke for every crankshaft revolution.

    A four stroke has a power stroke every other crankshaft revolution.

    So, when the saw "4 strokes" at WOT, WITH NO LOAD, your getting a power stroke every other crank revolution, and this usually happens with a slightly rich mixture. Some people like to call it a "burble".

    Two strokes that run lean, will run hot, and will usually result in a melted piston and rings, as well as a scorched cylinder.
  14. Eric Johnson

    Eric Johnson Mod Emeritus

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    Sandor, do they run hot because a leaner mixture means less oil to the vital parts?

    I don't tune my saw by ear anymore, but I've heard that the best approach is to get it to where it sounds right, then back off a little on the air to get a slightly richer mixture.
  15. Gooserider

    Gooserider Mod Emeritus

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    What I know comes from wrenching on bikes rather than saws, but should apply... The theoretically "perfect" fuel air mix is an explosive one, but that detonates which is VERY damaging to the engine, so you need to run richer so that the mix will burn rather than explode. Part of the way this is done is to mix the gas in as an aerosol mist rather than as vapors, so that as the mixture burns it has to vaporize the gasoline droplets. This vaporizing absorbs energy, the same way that vaporizing the water in green wood does, and cools the flames, plus the time it takes to vaporize the drops, slows the combustion process down to the point where it's a burn rather than an explosion.

    When you have a mix that's to lean, it has fewer drops to evaporate, so you don't get as much cooling on the flames, and will run hotter.

    The oil in the mix is still plenty to keep the vital parts lubed, that is pretty much not a factor. However you do get some added cooling effect from the oil, as it also needs to be vaporized in order to burn, and it takes even more energy to vaporize the oil than it does the gas - the oil is such a tiny part of the mix though, that it's hard to say how much difference it makes.

    The mix is very much a compromise situation - theoretically the leaner it is the more efficiency you get, and the more power you get out of the engine, etc. While a rich mix will tend to pollute, foul your plugs, etc. So the general idea is to run the leanest mixture you can while still keeping things from melting.

    Gooserider
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