Forgive me if I get some of this wrong... it's been a while... but I used to
be a fairly good tuner on SU carb's...
There are three primary adjustments for air fuel ratio with an Su Carb...
and a couple of tweaking points...
First off the 12 flats Bob mentioned is where I start as well... this places
the Jet just below the bridge and leaves the needle slightly exposed ...
there is then a gap between the piston in the dash pot and the bridge as
well as a small opening between the Jet and the Needle. The Float level is
next because it decides where the fuel level sits in the Jet.... Then of
course I have the Dash pot oil which doesn't really effect idle but does
determine how fast the piston rises which determines the amount of air the
needle and jet are exposed to in relation to Vacuum....
Okay soooo if I lower the Jet down I expose more of the needle which is
tapered to increase the fuel flow as the dash pot's piston rises.... it
shows to go you that if I'm flowing lots of air and the needle is way up,
there is more fuel sitting just below the bridge to be sucked into the
engine....
the float level is really critical here.... picture this as two
containers... one the float bowl and the other the Jet... the higher the
float level the higher the level of fuel in the Jet.... they simply
balance....
So all this having been said when I'm adjusting the idle I seperate the
carb's keep the throttle blades just cracked open and play with the plunger
which rises the dash pot a tick... and should see a slight rise in the
engine Rpm just like the book says.... if it's idling at say 1000 rpm it
should go up to say 1075 and then stablize at 1050.... if it's perfect....
If the fuel air ratio is Rich ( fat ) I would raise the jet first (
assuming I'd set the float bowls exactly right ) and then I'd lower the Jet
just a little bit at a time.... I can also, if it comes down to it lower
the needle just a hair...
Lastly.... if the dang thing has leaky throttle shaft's this is all junk....
you will never tune it correctly... the way I tell that if they feel tight
is to spray a bit of WD 40 on them while it's running and listen for a
change in Rpm... if it changes they leak..... ( by the way neat way to find
Vacuum leaks as well )
Okay I'm done.... LOL.... best I could do on short notice...
Keith
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