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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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    The fact is that chainsaw carbs are pathetically small compared to any normal high performance 2-stroke engine of the same cc's.
    A 100 cc motorX engine may have a carb of 30mm,but a chainsaw motor may only have a carb of 15 or so mm. Pretty darn small considering the saw revs even higher. So getting a bigger carb on is a must for more power. The problem is we cant just go buy and stick on a bigger carb 'cause they don't exist. So since the mid 80's I just "bored out" the stock carb larger. This instantly gives a great boost in power to any saw,I've done hundreds. You do have to strip the carb down to the basic block,remove everything,throttle shaft etc.,but doesn't cost you anything.
    Tillotson HS carb- bore out to 3/4"
    Walbro WJ carb- bore out to 49/64"
    You can have a machinist do it but I did it in my shop with a drill press and drill bit.It's a bit touchy,you have to do it carefully and slowly and spray oil on it,no sweat.
    A bigger carb and more power for not a penny spent!
    I don't know why some people are afraid of carb boring,I've done it since 1986 with no failures.
    Also used to bore out the big tillotson HT carb too.
    You can port the motor and do everything,but if the carb isn't bigger you severely restrict the motor...
     
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  2. Bobby Kirbos

    Bobby Kirbos ArboristSite Guru

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    Do you reuse the factory butterfly or do you need to make a larger one to match the new I.D.? (Or is the opened up part farther in the body and still smaller than the inlet?)
     
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  3. trappermike

    trappermike ArboristSite Guru

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    No,you are only boring the venturi larger (the smallest part of the carb bore)r,nothing else is affected and the carb runs as good as stock.The butterfly is still much larger than the smallest part of the venturi you bore. It works perfectly without any other mods or problems. I've been amazed all these years that no one else was doing it. Of course for racing you can "thin" the throttle shaft a bit etc.,but not necessary for normal use. Just strip the carb completely and re-assemble it after. Much more airflow like a bigger carb,as good as it gets.I used to do it in the shop in about a half hour...
     
  4. trappermike

    trappermike ArboristSite Guru

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    If anyone in northern B.C. ever finds a Mag2,064,066 or 084 with an "X" stamped on the nameplate,it's one I modified and bored the carb...
     
  5. TheTone

    TheTone ArboristSite Operative

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    Quite a novel idea. Do you smooth the bore where the drilled area meets the stock bore?
     
  6. trappermike

    trappermike ArboristSite Guru

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    It's not really needed for normal use,there is no real sharp corner left,but you can smooth or polish the bore and venturi more of you wish.
    The WJ on the 066 cannot be bored because a lump exists in the venturi housing a governor circuit and you cannot cut that open,use a 064 carb.
    When boring on a drill press a drill press vise is mandatory,center the bit in the carb bore by turning it backwards by hand,then clamp the vise down. Use about 1/3 drill press speed and spray lots of oil.
     
  7. trappermike

    trappermike ArboristSite Guru

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    You can practice on an old carb first to learn how to do it...
     
  8. BlackCoffin

    BlackCoffin ArboristSite Member

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    Are the gains from doing this noticeable? And how much do you have to remove before you notice a gain in power? Is this something you’d recommend on all saws or just specific purpose saws?
     
  9. trappermike

    trappermike ArboristSite Guru

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    The gains are always good,anything you can do to increase airflow thru a much too small stock saw carb will give good results.
    It will work on any saw,but some carb venturis cannot be bored due to their design on some smaller saws.
     
  10. Beetlejuice

    Beetlejuice ArboristSite Guru

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    Just a dumb question.. How about the choke?
     
  11. Beetlejuice

    Beetlejuice ArboristSite Guru

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    Like on Tilly HL series
     
  12. trappermike

    trappermike ArboristSite Guru

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    Boring a carb has no effect on the choke,you are not modifying anything with it,so it still functions normally,do not attempt any mods in the choke area,all you are doing is reaming the venturi(smallest part of the carb bore) to a larger size and getting more airflow like a bigger carb.
    HS carbs respond well to boring.
     
  13. Beetlejuice

    Beetlejuice ArboristSite Guru

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    I think I got it.. Pick the biggest diameter and run the the length of the carb. Am I close?
     
  14. trappermike

    trappermike ArboristSite Guru

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    I gave the specs in the beginning,if you're boring an HS you need a 3/4" drill bit and a drill press vice,clamp it in the vise choke side up,with the drill off lower the bit into the carb until it touches down and turn it backwards by hand just to center the bit,then raise the bit,turn on the drill and slowly and carefully run the bit thru using lots of oil. I suggest you practice on an old carb first to get the hang of it.
    Of course you can give it to a machinist or buy an expanding reamer,but I never had a casualty doing it my way...
    For metric minded folks 3/4" is roughly 19mm, 49/64" is about 19.5 mm. I measure a stock HS venturi to be 17.4mm,boring it out gives almost a 25% increase in area and flow!
     
  15. boltonranger

    boltonranger ArboristSite Operative

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    This is interesting but don’t you also change the Venturi effect?
    I would love to see timed cuts with the same saw using stock and bored carb.

    Have you ever measured any of your saws performance this way?

    If you change the saws air volume you have to up the fuel, yes?
    If you greatly up the air you’re going to reach a point where the fuel increase can’t be balanced can it?
     
  16. Beetlejuice

    Beetlejuice ArboristSite Guru

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    One thing I do have is practice carbs.. More later and thanks
     
  17. trappermike

    trappermike ArboristSite Guru

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    The venturi effect is not adversely affected.
    Venturi definition- When you reduce to a smaller size the hole going thru a carburater "You increase the air velocity and lower the pressure.",which draws fuel into the airstream.
    On an HS carb for example the air inlet(choke side) is about 22.3 mm and the venturi is 17.4 stock,19mm modified.
    Remember,no matter how big you make all the ports you need a big enough carb to supply enough fuel/air mixture for them.
    Say on a 390 Husky for example,draw the actual size of your stock or modified intake port onto paper,then draw the stock venturi (an 18mm circle) beside it,wow a big port that can flow lotsa air,but a very small carb hole that cannot supply it,and is obviously a big restriction...
    I did lotsa dyno testing on saws and there is a good increase.
    Yes the bored carb will need more fuel at high speed,you may find that you need to open the HS screw about another 1/8 to 1/4 turn more,but the whole idea of building more power in any engine it to get more fuel/air mixture into it!
    As a comparison,94cc saws today have usually an 18mm venturi (WJ),a similar sized high performance 2-stroke like a cart or MX bike may have up to a 30mm carb.
    As a note- If you bore the carb with a drill you will not end up with a perfect machined smooth finish,,but the basic job is accomplished and will work fine,however if you want to smooth or polish the bore after, that will help even more.
    Heres some of the math comparing the venturi AREA (in square mm's) of a stock HS carb compared to a bored one-
    -Stock 17.5mm venturi = 238 sq. mm area
    -Bored 19mm venturi= 283 sq. mm area
    That's a 19% increase in area and flow!
    As for your last question,on stock or modded work saws the carbs can pass the extra fuel required ok,but on some applications like race saws a few fuel circuit mods may be needed.
     

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