Understanding How a Carburetor Works - Carburetion for any automotive application is fairly complex, getting at least a basic understanding of how it works can go a long way. My aim with this article is to give people that basic understanding, you could spend days, even weeks discussing everything that is involved with carburetion so a basic understanding is more than enough for one day.
Jetting Basics
There are 4 main circuits used to optimize gasoline delivery and therefore engine performance. You might adjust the air screw, adjust the jet needle's clip position or exchanging the pilot (slow) jet, main jet, throttle valve (slide) or jet needle for one of an appropriate size to achieve the correct stoic metric ratio. The stoic metric ratio for 4 stroke engines is 14.7:1 (14.7 parts air, to 1 part fuel) although some people believe going rich or lean at certain points can aid performance. Ie. going as rich as 12:1 can be beneficial to acceleration, and going down to 15/16:1 can aid fuel economy at idle revs. This is all dependent on the engine, even 2 identical engines could require different settings to achieve there relative best performance
The different jets in a carburetor are targeted at specific throttle openings. But none act idependently, this helps to achieve a smooth power delivery, but plays havoc on the tuner in some instances. See below for a list what the jets are targeted at.
The air screw is most effective between idle through 1/8 throttle. The pilot (slow) jet is most effective between 1/8 through 1/4 throttle. The slide valve is most effective between 1/8 through 1/2 throttle The jet needle is most effective between 1/4 through 3/4 throttle. The main jet is most effective between 3/4 through wide-open throttle. Pilot Jet/Air Screw
The pilot jet, or slow circuit, can be adjusted by either swapping the pilot jet for a smaller or larger jet, or by adjusting the air screw, the air screw controls the flow of air into the circuit. Usually turning the air screw in (clockwise) will richen the mixture (reduce the amount of air) and vice versa.
The air screw is perfect for fine tuning the pilot circuit. The pilot jet determines the flow of gasoline entering the cylinder at idle/low revs. Pilot jets have a precisely machined hole running through their center which gasoline passes through. With all jets we need to remember the bigger numbered jets richen the mixture, while reducing the number leans out the mixture.
Slide/Throttle Valve
The slide/throttle valve has the most effect between 1/8 and 1/4 throttle and does affect the mixture up until 1/2 throttle, but on a lesser scale. The throttle valve can be exchanged for one with a bigger or smaller cutaway. The larger the cutaway the more air flows, the smaller the cutaway the less airflow.
The jet needle
Has the greatest effect between 1/4 and 3/4 throttle. It's attached directly to the throttle valve. As the throttle is rolled open or closed the jet needle moves through the needle jet's bore exposing different sections of the jet needle's profile to the needle jet's inner bore. There are many different jet needles available all of which are specifically designed to a certain application. generally replacing the needle is not necessary. Because most needles have a number of clip settings allowing you to adjust how far in or out the needle is in the needle jet. Replacement is usually used when massive adjustments to airflow have been made. I'll talk about jet needles in greater detail in the Tuning section.
The Main Jet
Regulates the flow of gasoline from ¾ to Wide Open Throttle. Again this jet has a precise hole in its center allowing for specific metering of gasoline. Increasing the size of the main jet ( size of the hole ) richens the circuit by supplying more gasoline, and reducing leans the mixture out at these throttle settings.
Tuning
Always before tuning your carburetor you must insure there is a fresh air filter, new tank of gas, and i also recommend new spark plugs are installed, this should insure your are jetting you carbs for the correct air/fuel requirements. Old airfilters could cause you to tune your carb slightly lean (because of less airflow) so when you did replace the air filter your jetting would be off. You should also ensure there are no leaks in the system, and everything else is running sound. engine problems could lead you to believe there is a jetting problem and tuning them in would just mask your problem
When making alterations to your jetting test the result one by one. It's very helpful to keep a log book for your carburetor where you can log changes to the jetting, the temperature, altitude etc. This way you if you make a wrong move you can easily correct it.
The easiest method of verifying your results is by means of a plug chop. It is great for beginners and a pros alike (although pros tend to rely on how the jetting "feels", that's a result of being a pro) a plug chop is relatively simple to do, but requires a few spare spark plugs. you would make your alteration, then install new spark plugs. get you engine up to temperature. Then you are ready. As already mentioned jetting a about throttle position not speed.
i like to set my throttle at the place i am tuning (ie wide open for main jet, 1/2 way for needle setting, etc) use 4-5 gear and ride/drive up a slight incline to put load on the engine. stay at this throttle setting for a at least 15 seconds (if you thinks the settings are lean do the chop after 15 seconds as to reduce the risk on your engine, and then in one move.
Apply the clutch, release throttle, stop ignition, coast to a stop.
This is a plug chop.
Once on the side of the road you will need to remove the spark plug(s) and check the colour around the porcalin area of the plug.
Good jetting will give you a milk chocolate colour reading on the spark plug.
Light brown will be to lean.
White is BAD increase fuel delivery or engine damage is likley.
Dark brown is too rich.
Dark brown deposits is WAY TOO RICH.
And black suit is oil fouling (engine problems)
When you begin jetting i would advise you use this method for all throttle settings. As you become more experienced you wont need to, you will also "feel" the changes. I still do plug chops, only for WOT (wide open throttle) as this is the first jet that must be tuned (all the others are reliant on the main jet setting) Even now getting a nice milk chocolate coloured spark plugs for the main jet give you great satisfaction and fills you with confidence to tackle the other settings.
Setting the main jet
This a the most critical stage of your tuning, all other jetting should be done after the main jet to ensure linear delivery. to select you main jet you should try at least 3 different jets ball park jet, 1 higher and one lower, after you have a done a plug chop to get in the ballpark you should do a few speed runs with the different jets, choosing your main should usually be a case of top speed, with the jest you have selected find the one that gives you the best top speed and "feels" like its giving you the best power. This will be your main jet, although just to note, in climates that can change considerably select the jet number above your top speed, to allow for temperature change if jetting is done in the hotter months, if done in the colder months select the best top speed jet.
Selecting jet needle clip position
once your main jet is selected you will need to move onto your needle clip position. as a rule of thumb i usually start with the clip in the middle positions, and doing a plug chop can remove the need to test every clip position. (ie, if you are set in the middle clip and the plug comes out lean, you only need check the lower (richer) clip positions and vice versa. Again check the ball park clip position, one above and one below. Check for best transition from 1/2 to 3/4 throttle and quick throttle response. usually bad throttle response is down to a lean condition, and sloppy power delivery is due to rich conditions.
note: doing a plug chop for the needle clip position require you to hold the throttle a 1/2 throttle for longer periods (at least 30 seconds, more if feasible) all other aspects of the plug chop stay the same.
Tuning the pilot circuit, idle to 1/4 throttle
Tuning the idle circuit is probably the easiest jet to tune. This is because it work in conjunction with a air screw. to start get your engine nice an warm. Then with the ignition off turn the air screw in (clockwise) until it seats, not too tight now, remember its brass. now get a note pad out, you'll need it. unscrew the air screw 1/4 turn at a time, trying to start the engine and noting how many turns so far, when it does fire set the idle as low as possible without stalling the engine. now continue to turn the airscrew out 1/4 turn at a time noting how many turns out you are. your idle revs should increase turn by turn. when they stop increasing as you turn out the air screw stop. check how many turns out you are. if you are more than 1 3/4 turn out install a smaller pilot jet, if you are less than 1 turn out install a bigger pilot jet. repeat procedure until you are within 1-13/4 turns out. You now have the correct pilot jet, and fine tuning will be easy. you will now repeat this procedure again only using 1/8 turns to achieve the best off idle throttle response. test your throttle response on the road and find a setting that is the quickest. plug readings arent important here. there are 2 benefits to this:
1. you get the best throttle response, which is great.
2. you also get the best fuel economy, due to a slightly lean mixture (not dangerous at all)
Once your done, give the engine a good run to check all the parts are working together well. If you approach the tuning process in order you should have no trouble achieving a good linear response.
Some people fear carburetor tuning, they think its black magic, done by mechanic come wizard folk. I hope this clears up a few things.
