At 09:46 AM 8/27/98 -0400, Matt Kulka wrote:
>> Barney Gaylord <barneymg@ntsource.com> 08/27/98 04:05am:
>>It means that this information has been posted to the open list before,
so now it's a private response.
>
>.... This is a car oriented list and the information applies to the
majority of us. Please share your knowledge.
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.
Cheers,
Barney Gaylord
1958 MGA with an attitude
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