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