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Turn a HDA 120 into a HDA 87

super3

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Many yrs back I ported my first 262. On the other place I asked if any one had bored a carb. I only got one reply that "it had been done". OK it must be a secret how far you can go b4 you ruin the carb. I thought #### it, I gotta find out.

I had an extra 120 and no 87 to get a hint of how much to bore and only the venturi spec for reference.

I bored both sides to 19mm, made a set of butterfly's, put it all together and thought I was ####in tall cotton. (several yrs later I finally got a hold of a couple 87's and the bore is 19mm, lucky guess on the boring)

Put it on the saw and my idle was over 4800. #### Took it back off and then seen that I hadn't payed close enough attention that the butterfly's were not closing right. Took them back out and miked the old ones and seen that they are not round.

Few hrs with a needle file and the mike I had them fitting proper.
Back on the saw for a test run. :campeon: I'm likin what I'm seein.

I had my son on the stopwatch and seen a 7/10's gain in the same wood as the 120 was tested in.


Quite a while back someone asked for a video of how to do it. I checked to see if I could but the butterfly's so as to not have to spend half a day making them and found out they are available for cheap although they took several months on back order to arrive.

Being that I'm slower than even Wiggs at getting around to my own projects, I'm just now getting it under way.


I made a dowel to speed up the alignment of the bore on the mill.

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Bore to 19mm. Turn it over and the same on the other side.

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Ready for the next step to open up the venturi.

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If the butterfly's are on back order and you are in a hurry, make em.

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Hog out the venturi. Roughed in.

P1020795 (Small).JPG
 

Stihlbro

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Most butterflies are kinda oblong and are tapered on what contacts the bore. I'm curious how you did it.
 

super3

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Not yet. I have 3 more doners.
 

super3

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Most butterflies are kinda oblong and are tapered on what contacts the bore. I'm curious how you did it.

Neddle file a few strokes at a time with more presure where the shaft will be and measure. .003 smaller at the throttle shaft and tapered off to nothing at 2:00 and 4:00. same for the other side.
 

Mattyo

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if you get good at it I'm sure someone could stamp out butterflys for ya :)
 
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