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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.
     
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  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
     
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  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.:)
     
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  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 CJ Saws

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    WJ-69..
    20180818_175213.jpg

    HR-1 on 85cc's..
    20180818_174836.jpg
     
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  11. trappermike

    trappermike ArboristSite Guru

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    Mikunis are great carbs,but just too large to fit on anything other than a 090.
    With the carb bored you will find you will have the HS screw further open,sometimes 1.25 turns or more with a good work porting.
     
  12. Justin Taylor

    Justin Taylor ArboristSite Member

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    I have a poulan wild thing its a 42cc saw thats ported and everything and its at the point were i have the h screw maxed out and it needs more fuel and i just dont know what to do i have a part saw ms 311 that i will have to cut a lot and make custom brackets to mount the carb and air filter housing in the wild thing or i could try to bore out the stock carb what do u think would be beter?
     
  13. Leafy

    Leafy ArboristSite Operative

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    What carb is stock on it? Walbro Wt? You can go up to an HD sized carb pretty easy since the bolt spacing is the same it's just like 2-4mm longer in the flow direction, you'll just need to pick the right one or be willing to make your own linkage.
     
  14. Justin Taylor

    Justin Taylor ArboristSite Member

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    I think its just a Walbro WT 89 891
     
  15. Leafy

    Leafy ArboristSite Operative

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    Or if you just need more fuel on the H you can just get a pin vise and micro drill bit set and got a drill Size or two larger on the feed to the H needle. To figure out what hole to drill take the H screw out and take the meter side cover off and spray wd or brake clean into the hole and see which hole in the metering side it comes out.
     
  16. Justin Taylor

    Justin Taylor ArboristSite Member

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    Id might give that a try but also i may need more air flow
     
  17. Ketchup

    Ketchup Smells like 2-stroke.

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    I have a rule that I never modify anything unless I'm prepared to replace it. Surf some pawn shops and flea markets until you find a donor saw. Then hack up the air box or bore the carb knowing you can go back to the earlier configuration.

    In your case you might be maxing out more than the carb. The transfer capacity is probably at its limit. It can't hurt to put a bigger carb on and find out. Maybe even just a bigger jet in the current carb. Not a lot of people spend time on Wild Things, so you have to do a lot of the experiments yourself. If it's turning 20,000 others would probably appreciate a full thread about your saw.
     
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  18. Leafy

    Leafy ArboristSite Operative

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    With how carbs work the more air you're trying to draw through the carb the more vacuum you make in the venturi and the more fuel comes out. So if you have the H screw all the way out then you're probably drawing less airflow than normal. Guessing you really widened the intake and extended the port duration, so it's getting more air but over a longer period of time.
     
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  19. Justin Taylor

    Justin Taylor ArboristSite Member

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    yes that is probly correct
     
  20. Justin Taylor

    Justin Taylor ArboristSite Member

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    Ya i like to do that too having replance ment parts on hand or a donor saw but i got the thing for free and i have 2 more motors on hand and i already cut a big hole in the top cover next to the air filter and go some good results with doing that
     

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