It wasn't an adjustment screw, but a jet of some type, with a notch to allow for removal with a standard screwdriver. If you removed the top of the carb and turned it over, it was at the top, right hand corner of the opening of the primary barrel and assembled, it would point down into the barrel. It is inline with the middle of the three tubes that vacuum hoses connect to on the firewall side of the carb, when assembled. It has a very small opening, less than 1 mm, probably about .5 mm.

 

The jet (or jets because most carbs have several) is a tube with small hole through which fuel is metered as required by the engine depending upon the quantity of air passing through the venturi (at its most basic description, the hole through which the carby breaths). The jet hole is very carefully sized to allow a specific flow rate of fuel through it, though the amount of flow will vary with the air flow through the venturi.

There will be more than one jet, each jet performing a specific task depending upon air flow through the carb, e.g. there is the idle jet that meters fuel at idle speed, being the only jet that is adjustable in order to tune the idle mixture. There is the main jet that will meter fuel for most conditions above idle (though there may be other similar jets as well), and there will be the 'acceleration' jet, which is more like a nozzle at the end of a pump that squirts extra fuel into the carb to account for the additional fuel required for hard acceleration.

This 'acceleration' jet is unique in that its the only one directly connected to the drivers foot, i.e. you actually cause a plunger to push fuel out the jet as you open the throttle. All the other jets rely upon atmosperic pressure to push the fuel through the jets. This is achieved by differences ibn air pressure in different parts of the carb. If you look into the venturi you'll see thatnear the main jet it's diameter decreases, and this causes air passing through to speed up as it passes through this narrower section.

If you understand the basics of downforce / lift and wings etcyou'll know that when air flow speeds up the result is a drop in air pressure in the faster moving air (e.g. the air beneath the wings on a Formula 1 car etc). It's the same in the venturi, the airflow speeds up, the pressure drops at this point in the venturi. So what? Well the fuel in the float chamber is exposed to atmospheric pressure, so is exposed to a hjigher pressure than in the venturi. The ends of the jets are in or near narrow section of theventuri, so atmospheric pressure acting in the float chamber 'pushes' fuel out of the jets into the low pressure / high speed airflow in the venturi, and from there it atomises and passes into the cylinders.

Thats the dead basics of carb function, if you want / need to know more then I'm sure it's not that hard to find on the net.

 

Hello, I'm new here and I came across with this subject about the carr getting flood it. I have a 1985 honda accord that is getting floot it, it keeps driping gas inside until it makes a poodle inside. Davidgirlswold, is this is something that happen to you, please advise.. and did you came to fix it and eliminated the problem, because I'm stock with this car not runing and I want to get it fix asap...thanks...

 

"honda1985"- I've replaced the float needle and seat on a 3rd gen (86-89) before because of this same issue.

When it "floods", can you see raw gas dripping into the carb, ref one of the pic's in this thread. Also as prev posted, what is the fuel level in the bowl when this is going on.