Larry Quirk wrote:
> The carbs I have started out as 1977 tag # AVD665R with ABT stock needles.
> I have a stock motor except for 4 into 1 header and the twin SU - K&N
> setup. I run AAA needles with yellow springs. The AAA are too fat in the
> midrange and on the top end and I will be switching to AAQ or AAT (leaner)
> to lean out the mid range and top end. If you get yourself a SU manual,
> they usually contain heedle charts in the back. You can enter the data
> into a spread sheet and work out the flow area around the main jet as the
> needle moves up. These are constant velocity (depression) carbs so the
> vacuum drawing the fuel past the jet remains fairly constant as throttle
> position changes.
> You may not be noticing too much difference between your needles because
> you are not pulling the needle to full extension when you are wide open at
> near max revs. On CV type carbs I have found the greatest effect on
> jetting to be changes in restriction prior to the carb (i.e. air filters
> and air inlet temp etc.)
> Jetting carbs almost becomes unique to each engine, where and how it has
> been modified and where and how it is driven. (ps I live in cool dry air
> at about 2800 ft.)
I think I'll get that SU manual. I have noticed that after changing to
#5 needles, I have a smooth power range untio I reach about 4500 RPM.
The engine suddenly bogs. If I lift up slightly on the accelerator, it
picks back up and continues to accelerate. This "flat spot" can
probably be corrected by changing to a slightly different set of
needles.. But Which ones, he asks????
The saga continues.
Joe Curry '63 Spit (1500 high compression power)