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Old 15th May 2009
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Default Carb basics and jetting by reading the plugs

In this day of fuel injection, air-fuel meters, dynos, and the internet, it seems more and more guys get their first carb experience with their Sportster. That leads to all sorts of problems and confusing questions related to carbs, how they work, and how to make them work.

I will try to summarize basic carb tuning information and add in short discussions of common problems related to carbs. Hopefully this will make it easier for a guy to get started understanding the basics and then he can do all the additional research he needs to.

If your bike was running good and then suddenly starts running poorly and you did not change anything, you must eliminate all other sources of problems before you even look at the jetting. Common problems on rubbermount Sportsters frequently confused as jetting problems are a bad crank position sensor, leaking intake manifold seals, and vacuum operated petcock valve leaking fuel into the carb through the vacuum hose, or a dirty air filter. If you bike was running good, and now runs poorly, jetting is NOT the problem. Jetting does not change, something else did. The only exception to this is if a jet gets clogged or something weird like a jet coming loose. If that is the case, clean the jet and put it back in.

The choke, or enrichener, and how to properly start a carbed bike is the next thing that baffles some guys. You should use only enough choke to get the bike started and keep the motor running as you ride away before it warms up. With a little experimentation, you can figure out the minimum amount of choke needed. I start my bike with ¾ choke, but ride off with ¼. Overuse of the choke can foul plugs very quickly, especially if you only ride short rides that never let the motor, and oil, completely heat up. Another frequent cause of fouled plugs is staying too low in the rpm range. Rev it to the rev limiter once or twice, you may be surprised how little rpm you are using.

One unique thing about Sportsters with vacuum operated petcock valves is you may need to crank the starter a bit longer than usual to fill the carb float bowl with gas. This is especially true if your bike has been sitting long enough for the gas to evaporate out of the carb bowl.

What do you do if you have the choke full on and you have been cranking and cranking and it still doesn’t start? Congratulations, you have just flooded your engine and spark plugs with gas. Push the choke off, twist the throttle full open and hold it there. Then hit the starter button again. This lets the engine suck in air only, and pump out the excessive gas. Be ready to let go of the throttle as soon as the engine fires to prevent the motor from over reving. The engine will likely quit as soon as the excessive gas is burned off. Pull the choke ½ way out and start it again. I usually try to strart it 3 times with the choke on. If it doesn’t start, then try 3 cranks with the choke off and throttle held wide open. If you have to go back and forth more than 3 times and the engine does not start, you probably have other problems. Once you learn how to use the minimum amount of choke needed, your flooded days will be over.

Another good way to flood the engine is to keep twisting the throttle when the motor is not running. Each twist squirts more gas through the accelerator pump into the carb. If the choke or enrichener is not working, twisting the carb may squirt enough gas into the manifold to keep the bike running. However, this is never needed with a properly tuned carb. In fact, anyone who has to keep twisting on the throttle to keep the motor running still has a lot of learning to do. Race carbs with no choke are an exception.

Starting a bike for the first time after it has been sitting all winter also causes problems. Put Sta-bile in the gas. Keep the tank full of gas to reduce condensation and rust formation. If the bike has been sitting longer than 3 months, might as well siphon out the gas and replace with new gas. The old gas can be diluted with fresh gas and burned in your car. If the plugs were worn and fouled when you parked it, new plugs will make the first start much easier. I like to open the float bowl drain to drain out all the old gas, and make sure new gas is flowing before I try to start a bike that has been sitting a long time.

So now you have a new bike, or a bunch of parts to put on the old bike and rejetting is needed. There are 3 ways to rejet. If you find someone with your exact set up who has properly rejetted his bike, you can duplicate his settings to start with. Or you can call the companies whose stuff you bought, and ask them what jetting changes are needed. I use that as a starting point. Then I read the spark plugs and make jetting changes accordingly. FosterUK, and others, are now using air/fuel mixture gauges to find the best air/fuel mixture at all throttle positions.

Before you start changing jets, you need to understand how a carb works. There are 4 basic fuel circuits in most carbs. The first is the choke or enrichener. It only is in effect when you have the choke pulled out. Other than making sure it works properly, the choke is not part of jetting a carb because you only adjust the jetting on a fully warmed engine when the choke is off. An enrichener and choke do the same thing, but with different parts. For this discussion, I use choke to also mean enrichener mainly because I can type choke easier. The other 3 circuits are the ones you end up tuning. Each circuit works at different throttle positions.

A “jet” is simply a small screw with a very specific size hole through the middle of it. The bigger the size of the hole, the more gas can flow through. A 180 jet has a bigger hole than a 170 jet. The 180 jet will let more gas through, and that makes the air-fuel mixture richer.

The "floats" actuallly float on the gas in the float bowl (the bottom of the carb). The "needle valve" is connected to the floats. When the gas level is high, the floats are high and close the needle valve which stops gas from entering the float bowl. As gas is sucked into the motor, the level drops, the floats move lower, which opens the needle valve and lets more gas in. If the floats stick in a high position and the needle valve is closed, the motor will starve for gas and quit. If the floats stick in the low position, the needle valve is open and gas overfills the float bowl and flows out the carb and overflow tube. That will make the motor run rich, rough, maybe quit, and cause the plugs to look rich no matter what else you do to the carb. Always check and set the float level before you start rejetting.

One of the reasons I avoided putting details in the original post is there is no way for me to know in any specification applies to all years and all different carbs. I have and 06 manual. This information is from it.

This is how to adjust the float. Remove carb from bike, remove float bowl cover, and place carb on a flat table with the intake manifold side down, tilt it 15 to 20 degrees. Tilt it in a direction that allows the float to close the needle valve. Do not press on the float, it should swing free as you tilt the carb, and close the needle valve with it's own weight. Measure the distance from the flat face of the carb body, where the carb bowl seats against, to the far face of the float. That dimension should be 0.413 to 0.453 inches, 10.49 to 11.51 mm. From the picture in the manual, it looks like the outer face of the float is parallel with the face of the carb body. There is a small metal tab that pushes on the needle valve, bend tab to set the float level properly. If you over bend the tab and break it, you get to buy a new float.

Use a fuel tube on the carb fuel inlet and blow through it as you move the float back and forth. If the float is properly adjusted and does not totally close of the air flow, the needle valve and/or seat is bad and must be replaced.

Here is a picture of different floats. If you look at the two on the left, you can see the tab located exactly in the center that pushes on the needle valve. That is the tab you bend to adjust the float level.

The idle circuit is composed of the idle jet, air mixture screw, and throttle screw. The throttle screw is called "idle screw" in the parts manual, but I use throttle screw to avoid confusion with the air mixture screw. The idle jet and air mixture screw directly regulates the air fuel mixture at idle. The throttle screw moves the throttle linkage in the same manner as twisting the hand throttle. This circuit keeps the engine idling when the choke is off and the throttle is at idle.

The second circuit is controlled by the needle at the mid throttle position. When the throttle is closed, the tip of the needle is in the main jet and blocking any fuel from getting through. The needle is mounted to the slide and as you twist the throttle, the slide opens to let in more air, and the needle is raised out of the main jet to let in more gas. The needle has a taper to it, as you twist the throttle, the tapered part gets pulled out of the main jet and that lets more gas pass through as the taper gets smaller. (This explanation is not technically correct, but is good enough to get the basic idea.)

The main jet is the third circuit that needs tuning. It provides fuel at wide open throttle, when the needle is completely out of the main jet.

This is how all 3 circuits work together work. At idle, the tip of the needle is in the hole of the main jet and that prevents any fuel from getting through the main jet. Therefore at idle, all the gas needed to keep the bike running is moving though the idle jet and fine tuned with the air mixture screw. As you twist the throttle, the needle is partially pulled out of the main jet, slowly increasing the amount of gas that flows through the main jet to match the increasing amount of air going past the slide as it opens. At full throttle, the needle is totally out of the main jet and has no effect because fuel is flowing through the main jet without restriction.

There is some debate about how much overlap occurs between the 3 main circuits. Here is a diagram that demonstrates the overlap. It is not for a CV carb, because I couldn’t find one for it. Be sure to click on the link at the bottom of each page to see the next page.

The next thing is learning to read the plugs correctly. Be careful about using pictures of plugs that may from the days of points and oil burning engines. Today’s plugs look a lot cleaner because of clean burning gas formulas, electronic ignitions and more efficient engine designs. You probably need to get several hundred miles on a set of plugs before you can use them for jetting purposes. Plug reading can get very complex and detailed. However, for most Stage 1 applications, this link is good enough.

I make all the changes I plan on making at one time so I have to go through the rejecting process only once. I start with whatever jetting changes were recommended for my modifications. I start with the air mixture screw 2 ½ turns out. When it starts, I try to keep it running long enough to heat the motor up and run with the choke off. If you push the choke off, and the engine dies, either it is not warmed up enough or the idle jet needs to be changed to one richer. If you turn in the air mixture screw all the way in, closed, and the engine keeps running, change the idle jet to one leaner. Do not over tighten the air mixture screw, you will damage the tip or taper.

Be careful to not overheat the engine. Using an infared thermometer aimed at the flat spot next to the spark plug, the temperature shouldn’t get much over 330 degrees F.

At this point, all you are trying to do is get he motor to idle with the choke off and engine warmed up. At first, you might have to increase the throttle screw to raise the idle up a bit more than normal. Then use the idle drop method to go back and forth between the air mixture screw and lowering the rpm with the throttle screw as needed to get the idle where you want it. The lower the throttle screw sets the idle, the easier it will be to tell what effect the air mixture screw is having.

If the air mixture screw does not change the idle as it is supposed to, try to lower the rpm’s with the throttle screw. If that doesn’t work, the idle jet is wrong. Put the air mixture screw back to 2 ½ tuns out, lower the throttle screw to get the idle as close to normal rpm as possible, let it idle for about 30 seconds and read the plugs. They will tell you if the idle jet needs to be changed to richer or leaner.

Some people say the best way is too jet is to start with the main jet (full throttle) because changes in the idle mixture are not likely to change the mixture at full throttle. That is true if you have a dyno and air fuel meter. But I tune my carb by riding the bike, and I need to get a good idle to be safe riding on the street.

Then I start riding around the block until the motor is warmed up enough to take the choke off and transition to mid throttle position. If it falls flat at mid throttle, pull the enrichener out a bit to see if it runs better at mid throttle. If it does, you need to raise the needle with one washer to richen up the mid throttle position. If it seems close, I ride to a long straight road with a turn off location where I can get off the road and pull the plugs. I keep the rpm’s and throttle position as low as possible because I want to avoid over heating due to a lean mixture.

When the road opens up, start in a lower gear, hold the throttle at mid position, let the revs build up as much as you can, and shift up for 2 or 3 gears. Pull the clutch in, hit the kill switch at the same time, but do not move the throttle until the engine is off. If you put anti-seize on the plugs, and let the motor cool off a bit, the plugs will come out easily. Keep a spare plug in your pocket in case you break one.

As long as the mid throttle plug reading is not too lean, I will then do a full throttle run and plug reading. Then I head home to make whatever jetting changes are needed and test again. If the plug readings are on the lean side, I will pull the enrichener out 1/4 of the way to avoid riding lean on the way home. I don't know if this will work with chokes.

Once the idle, mid throttle and full throttle plug readings are good, then I go back tune out any rough spots which may occur where the carb transitions from one fuel circuit to the next. Usually I will richen up the next higher circuit. If it stumbles between idle and mid throttle, raise the needle. If it stumbles between mid throttle and full throttle, go up one size on the main jet.

My last adjustment is to richen up the air mixture screw ½ turn beyond whatever worked earlier. Then I test ride and lean the air mixture screw 1/8 turn until the carb farts are gone.

Remember, when it comes to jetting, starting off too rich is always better than starting off too lean. Hard riding with a lean mixture, especially with a lean main jet, can melt pistons.

Here is a picture of my plugs, 12,500 miles, Amsoil (burns cleaner), Magnecor wires, and breathers vented to atmosphere. They look the same at all throttle positions. One thing I noticed about taking pictures of plugs is the light and camera settings can make big differences in how the plugs look on screen.

Former Ricor test rider for IAS Shocks, Intiminators and Vibranators. Works Dual Rate fork springs, fork brace, Avon Venom X tires, loosen drive belt, and set frame rails level to floor. Read the "7 Pages of Suspension" thread in the Suspension Sticky Index to learn how to fix your suspension.

Last edited by XLXR; 16th May 2009 at 22:17..
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