Tom----I'll take a shot at what could be at least part of the answer to
your "running rich." Pull up a chair, as it sounds like you're
serious...To start more from the bottom part of your message and working
up, I would say that your engine just coming out of "rebuild" is the
best time to get it right. You don't want what seems be extra fuel
washing down new cylinder walls, or even new rings, which could delay a
proper break-in of these parts.
While your spark plugs may be new, the fact that they are black AND you
have an intermittant misfire, suggests that they should be replaced, or
at least cleaned. Go with the Champion N-12YC, or its equivalent heat
range. You'll want to keep an eye on these if you decide to do what I'm
about to recommend, to follow the "color."
You said you had the S.U. carbs on your '74. These are sort of like the
original ZS, but their adjustments are different. The ZS, as you may
know, can be set rich or lean by adjusting the height of the needle, in
relation to the jet. The SU has a fixed needle (sort of) and the mixture
adjustment is made by raising or lowering the jet, by using the nut
underneath the carb body. It's not hard to do so let's go thru it,
starting with the setting of the floats in each carb.
Take the screws from the top of the float bowl, lift off the lid,
turn it upside-down and set the height of each float at 3/16". Do both
the same way. Put the "lids" back on. You're thru here.
Take the four screws from the top of suction chamber and lift off
this aluminum bell-shaped piece. Lift out the spring and the piston,
needle attached. Easy does it, as parts here are somewhat sensitive to
any abuse. The needle has no external height adjustment, but their is a
desired location for it. Loosen the set screw that holds it in place and
pull the needle down (out) far enough to be sure that the groove that
goes around the needle is flush with the bottom of the piston that holds
it. (If it was up too high inside the piston, this will cause richness.
Likewise it could be set too low, so just make the groove disappear, and
then tighten the set screw.) Do this to both carbs. You're thru here,
but do not put the tops back on the carbs yet.
The adjustable fuel mixture for these carbs come from rising or
lowering of the jet. Since the top(s) is off of the carb, look down
inside and you'll see the top of the jet. It's brass, and round it is
the aluminum bridge of the carburetor. The big nut underneath the carb
body sets the jet height. Turn the nut up (about two full turns) till
this circular jet is even with, or close to the top of the bridge. Then
turn this nut down two full turns, or approx. 12 "flats." Now you can
put the top of the carbs back on. All of the above settings, from the
float heights to the needle and jet locations should get your engine
running very close to where you want it. Further adjustments to the
mixture will be from raising or lowering the jet according to which way
makes the engine run smoother, or gain idling speed. It should not be
more than one-half turn in either direction! A vacuum gauge would be
helpful here, to get the highest reading off the intake manifold.
I know this was a long proceedure to grasp, but if you take it in steps,
you'll get the results you're looking for. A manual on the SU
carburetor is always nice to have, since your engine is now equiped with
them!
If any of this needs clarification, you can write me directly.
Dick Taylor
'73
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