4a084 CARBURETOR FUEL FLOW

[Bart]

This not the best for semantics, but maybe it can help some understand a little more about how the fuel flows through the 084 carburetor from idle to full throttle. it is a simple set up, similar to many of the basic carbs that are used on many small engines today.
the 084 carb has been discussed and talked about on many of the posts on this forum, but i though it might be nice to add some pictures so that it may help understand more what has been talked about. and thanks to Corky for finding a print of the carb with all of its parts and pieces. it helps a lot.


there are of few metering holes on the screw and the fuel metering tube, i have came to the sizes of these holes by using some small drill bits that i have in a kit from kingtool.inc part # KTD01.
the screw has 6 holes total. they are all the same size. .0380 or a # 62 drill size.
2 are in the base, which sit in the float bowl and gas enters through them to feed the carb circuits.
the other 4 holes are just above the threads, this part of the (screw) sits in a cavity in the base of the carb, where a venturi feeds air to it.
these 4 holes meter the amount of air which siphons fuel out of the 2 holes, near the base of the fuel metering tube.

the Fuel Metering Tube is hollow, and near the base of the tube there are 2 holes that are .0420 or drill size #58, (which provide fuel for everything ABOVE the idle circuit.)
there are 2 holes near the top are .0225 or drill size # 74. (these feed ONLY the idle circuit
(NOTE IN THE PICTURES YOU WILL SEE ONLY ONE HOLE, REMEMBER THERE IS AN HOLE ON THE OPPOSITE SIDE OF THE TUBE ALSO)

i am starting with the idle circuit.
with the throttle in the closed position. Air is drawn up next to the fuel metering tube, there it draws out fuel out from the base of the (#1) screw, up to the top 2 metering holes on the tube.
and depending on how much the ( #27 idle speed jet) is turned out. determines how rich or lean the idle will be.

there is a exit port coming off the idle circuit just inside of the throttle plate. when the throttle is cracked open, it allows a little more fuel to come out of the idle circuit so it will not stumble, before the fuel mixture comes out of the top of the screw, which is at the bottom of the carb throat.

there is an venture port that funnels air to the cavity around the screw, which has four ports (.0420) that meter the amount of air that will draw fuel out of the 2 ports that are about 1/2" from the tip of the fuel metering tube.
from this set up, it appears the air/fuel mixture should remain the same, the amount depends on the volume of air passing over the top of the screw, through-out the throttle range.
these generators mainly were designed to either idle, or run at full throttle.
i can see where the hesitation or stumble can come from, for if you are running at 1/2 throttle, the air/fuel mixture should be running for that setting, but when you change the throttle setting, there is no accelerator pump, that give the engine that extra shot of fuel to compensate for the extra demand that is placed on the motor. before the air fuel mixture can adjust, for the change.

[Corky]

Nice illustrations Bart !!It was a good idea for Zenith to have the idle circuit feeding down from the top with the horizontal intake arrangement...But as we've seen by reading the plugs, the front intake tubes get the most atomized mix and the back tubes tend to get less and run leaner than the front...It's just the limitations of the system... I'm sure it was done to keep the engine contained in a smaller package for it's original intended use...Thanks for taking the time to document the circuits it's much easier for owners to make some modifications to the carb if it's completely understood...

[Bart]

just added some more information on one of the pictures, to help out with seeing what the fuel and air paths are, in the screw, and the metering tube.and how they are mixed

this is the stock, or as close as i know to stock measurements.
many other have made some minor modes, as to the size of the holes that they have modified.
if you want, you all are more than free to add your comments on what changes you have made, and if it had an positive or an negative outcome.

[Corky]

Papee brought up a good point...He enlarged the holes on the fuel metering tube...My factory setting on the idle speed jet was 4 turns out to get a richer idle mix and ease the transition as the rpms go up... But the factory wants a starting point of 1.5 turns out from seated... I would assume for our application that enlarging those holes would not only add more fuel for the transition between the idle circuit and the main well as Papee has stated, but also allows the idle speed jet to be set closer to the 1.5 turns out and may give a better response to the butterfly movement..
Judging by what 1 thousandth more hole diameter did for my afr on the main jet I bet it doesn't take much to make a big difference in that metering tube...