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Boring out chainsaw carbs

Discussion in 'Saw Building 101' started by trappermike, Apr 21, 2018.

  1. trappermike

    trappermike ArboristSite Guru

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    Since the beginning of the 60's the Mikuni carb is still the best carb to run on most 2-stroke engines. However they are too large and bulky to fit on most chainsaws and would also require a fuel pump. But you can get a good idea of how big a carb you need on a particular size chainsaw motor by looking at what size of carb manufacturers use on their high performance engines such as go-karts and MX models and similar size motors.
    Of course Mikunis are float type carbs and cannot be run in any position.
    If you have a large Hot saw and it doesn't have to look stock and you can physically fit one on I would definitely use one.
     
  2. Huskybill

    Huskybill ArboristSite Guru

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    There’s a lot of adjustments on the mikuni carbs. I put a 40mm mikuni carb on a 80 husky 390 cr. I had to lean the needle jet besides the low and high speed jets. I installed the thunder products “UFO” on the bottom of the slide. It wasn’t a bike for beginners after that.
     
  3. Ketchup

    Ketchup Smells like 2-stroke.

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    Can you bore a strato carb effectively?

    How about an Autotune or M-tronic carb?

    Just curious. I've never bored out a Venturi before.
     
  4. trappermike

    trappermike ArboristSite Guru

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    I will only refer to normal carbs. But as I've seen the strato carbs that just add air,for performance I would like to replace it with a normal fuel/air carb,somewhat like a 4 barrel carb with secondary barrels on a V-8,and try to get more fuel/air mixture into the motor for more power,however I have not experimented with these systems,but in theory it should work.,but won't help emissions of course.
     
  5. Leafy

    Leafy ArboristSite Operative

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    So zama 3c carbs can only really only take a 11/16 drill bit through it and you really cant bore either the choke or throttle side any larger without breaking out into ****.
     
  6. cus_deluxe

    cus_deluxe Thats what she said.

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    The rwj-4 (372 x-torq carb) is considered an upgrade for a lot of saws. When the filter holder is swapped out or gutted, it no longer feeds fresh air to half the carb. Bigger bore than the hs-228 that came on 288s. No need to bore those. Other “true” strato arrangements (5xx huskys) have the clean air come in through a totally seperare butterfly and so boring the carb really wont gain much i wouldnt think, as they already have a ton of intake area simultaneously available.
     
    deaves61 likes this.
  7. Huskybill

    Huskybill ArboristSite Guru

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    Do you change the jetting in the carb after boring the cfm larger? If your 25% larger on the inside diameter than we need to increase the jet size to take the full advantage of the increase or were running on the lean side a tad.

    I often thought of putting a mini muffin fan to pressurize the air box. I think the heated handle generator could supply the mili amps. I'm weird ok.lol
     
  8. motor head

    motor head Addicted to ArboristSite

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    Chainsaw carburetors play a large part in keeping the engine from over heating,Be careful adding more air to the fuel,more air is more heat.:)
     
    Chainmale likes this.
  9. Huskybill

    Huskybill ArboristSite Guru

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    Gas/oil is part of the cooling process. The amount of air flow matters too we could lean it more. Lots of things to factor in when playing with the intake setup.
     
  10. Chainsaw Jim

    Chainsaw Jim Addicted to ArboristSite

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    WJ-69..
    20180818_175213.jpg

    HR-1 on 85cc's..
    20180818_174836.jpg
     
    cus_deluxe, psuiewalsh and Ketchup like this.

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