air screw adjustment

[rodbolt17]

they would.
however LONG before the internet I went to varios schools.
some gave me enough information to go to the LIBRARY and actually us the DEWEY decimal system.
I actually like to read.
its funny but if I read it I can memorize it, if I watch it I am sunk.
just wish I could type,spell AND punctuate.

my sisters can.
however I can rebuild any holley you wish.
any quardra jet you wish.
any autolite 2 or 4 bbl you wish.
any mercarb,edlebrock or weber.

if ya really wanna play I can do the SU carbs on Austin healys and MGs.

they cant.
I haven't done an SU in many many years.
just wish I can figure out how to make my Winchester model 92 feed .357 reliably.
but I will get that too.
Boscoe is jacking with me posting about how carbs work, I have seen more than one design carb and more than one model.
I will call BOSCOE out on the ford VV carb .
kid aint got a clue.

[TownsendsFJR1300]

a manometer is a simple device that measures air pressurevs air pressure changes.
been around in use as a barometer for several hundred years.
next time you look at an old barometer you will notice 2 needle,one is manually settable the other moves by the manometer fluid pressure.
that's why when you check the baro pressure you set the needle to line up with the other.
next time you check it you can see did baro pressure fall, bad weather comeing,rise, good weather, or remain steady.

some are calibrated in "Hg,some in millibars and some in PSI units.

but in any carbed engine with a throttle plate you will have aa low pressure on one side and atmospheric on the other.
as the throttle plate is moved to the WOT position both sides of the throttle plate are now equal.
as ,in a 4 stroke, that piston moves down you now have barometric pressure at 14.7 psi or so trying to fill that void left behind.
in most two strokes, its as the piston moves up but at or near WOT there is no,or negligable intake vaacum.
both sides of the throttle plate are at the same pressure.

as EFI does NOT require any air flow to move fuel the engineers used manifold absolute pressure instead of intake vacuum to determine engine load.

on the typical carbed 4 stroke you will see at say 32"Hg baro pressure, about 18-20"Hg of intake vacuum.
now say subtract the 20"Hg from 32"Hg and you get about 12"Hg.
that is the pressure differential between the air on the airfilter side of the throttle and the intake side of the throttle.

now on EFI, at key on, most marine EFI system lock in barometric(atmospheric pressure) and retain this as a standard for the rest of that key on cycle.
boats typically don't change altitude much during a single key on cycle.

now we start the engine, look at the laptop and our manifold absolute pressure(MAP) is roughly 12"Hg.
whaaaaaat.

do the same math,subtract 12 from 32 and you typically get about 20.

measuring VACUM in the intake is the inverse of measuring pressure in the intake.

as the throttle plate is opened the vacuum measurement will go from about 20"Hg to about 0"Hg.

on most laptops you will see the map go from about 12"Hg to about 32"hg as the plate nears its wide open position.

confusing aint it .
then toss in each manufacturer likes to change the units its scaled in and this gets fun.

but both units whether measuring vacuum or pressure work, they are inversely proportional.

this is why most carbs utilize whats called the venturi principle.
that's that funky necked down area that the emulsion or main jet tube sits in.

mow we have established that intake vacuum is the same on BOTH sides of the throttle at WOT.
we have established that barometric pressure is the SAME on BOTH sides of the plate at WOT.
now why does the gasoline jump out of the fuel bowl.
its called the venturi principle again.
its also why older non efi motors used an ex crossover to heat the carb.
its also why various outboard manufacturers offered carb heater kits.

as the air passes through that venture its sped up and compressed, as it exits the venture a couple funny things happen due to some gas laws.
one is the pressure drops aas the air expands,this draws fuel from the bowl,still at baro pressure,into the air stream.
the other is a rapid temperature decrease.
when running in cold weather and relatively high humidity this can and does lead to carb throat icing.
on the humidity think of it as duck hunting when the ambient air temp is 25*F yet the water temp is in the 40's.
means the air,at or near the waters surface is cold yet still contains a lot of water.

now look at the throttle body of an EFI motor.
note the lack of a venture?
that's why EFI cylinder heads contain no ex gas crossover.
not needed nothing to ice.
fuel is moved with a pressure pump and injectors.

any clearer ?
other than I type slow and sometimes cant spell.

Clear as mud!