> Well, okay. A copy is just a copy. This would be easier if
> I had it on my
> web site, but it's not there (yet). This is the simple
> version, nothing
> fancy required.
>
> First order of business: Be sure the shoulder on the main
> jet needle is
> flush with the bottom of the air float.
>
> Tuning twin SU's is simple. Because there's a balance tube
> in the intake
> manifold, the engine will idle well running on only one carb.
> Loosen the
> clamp fastening the throttle shafts between the two carbs so they can
> operate independently. Back off the idle screw (also fast
> idle screw) on
> the front carb until it is completely closed, set the idle on
> the rear carb
> and do the mixture adjustment there. Then close off the rear carb and
> adjust the front one. Then open each idle screw about 3/4
> turn from dead
> closed and retighten the shaft clamp. If you want to measure
> it you'll
> find the balance to be pretty close. A little off doesn't
> matter because
> of the balance tube. Just be sure that when you start to pull on the
> throttle cable, both carbs start to open at the same time.
>
> When you get the mixture too rich it will gallop (the British
> call this
> "hunting"). It's a combination of periodic misfire and the resulting
> variation of engine speed at about 2 cycles per second. When
> you get the
> mixture too lean it just starts to misfire on one or two
> cylinders and the
> engine shakes. The mixture adjusting nut on the bottom of
> the carburetor
> has right handed threads. Turning clockwise (looking from the bottom)
> raises the jet and leans out the mixture. Counterclockwise
> lowers the jet
> and richens the mixture.
>
> Start with it rich and proceed towards lean, very slowly, one
> flat at a
> time on the adjusting nut. When it goes lean and starts to
> misfire, back
> off the nut towards richer mixture, just enough to quell the
> misfire. Then
> give it another two flats towards rich to avoid that
> borderline case of
> misfire when the operating temperature changes. You have
> then set it to
> the most lean condition it can tolerate at idle without being
> finicky. The
> taper of the main jet needle controls the mixture the rest of
> the way up
> the scale.
>
> Last order of business: Turn the idle screw on both carbs
> the same amount
> to fine tune the idle, then reset the fast idle screw within
> 1/4 turn of
> the fast idle cam, but not quite touching it.
>
> Now I expect I'll get the same flames and addendums that went
> around the
> last time.
>
No flames, just a question from one of the semi-clueless - in
the terms of your excellent procedure above, what is a "flat"?
Started the B last night and confirmed your suspicion - there is
a large intake leak somewhere. I'll pull the SU's and shield plate
tonight and see what the deal is. Thanks!
Dan Pockrus
'74 CBB
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