To add onto what knuckehead said, every twist of the throttle is a squirt of fuel into the cylinder. Below is a write up from the Big Dog dealer here in St. Louis. It is a primer of how your Super G carb works:
Carburetor Jetting
One of our motto’s here at Gateway Big Dog, (and we have several), is that Information is King. When we have a little more insight into what a customer may be experiencing on their Big Dog, it helps us diagnois and correct the issue. So, what I want to accomplish with this short page is to help you understand a little more about your carb. What it’s doing at differing throttle positions and rpms. Here we go.
Before start-up, turn the fuel on. Yep, that fuel should’ve been off when she got parked. It helps extend the life of the needle and seat in the float bowl.
Every twist of the throttle on an S&S Super G and E and Mikuni shoots a shot of fuel into the intake whether the engine is running or not. So, when you buddy is cranking on the throttle when the motor isn’t running, remember, raw fuel is going into the cylinders.
Choke Lever. I like using the choke on a cold engine, no matter what the outside temperature. If the engine is cold, it always seems, (at least for me), that if fires faster. Now, in warm weather, the choke gets shut off immediately, cold weather, maybe leave it on for no more than 45 seconds. The choke circuit puts large amounts of fuel to the engine and leaving it on any longer than 45 to 60 seconds over-fuels the engine, washing precious oil from the cylinders and causing damage. A warm or hot engine does not need the choke for start-up.
At an idle, the aptly named, Idle Circuit is feeding your engine with fuel. The S&S carbs have a large brass Idle Mixture Screw near the top center of the carb. This is what meters fuel to the idle circuit and is generally the one that get’s twisted on the most when somebody thinks his bike is not jetted properly. The kicker is, the Idle Mixture screw can change the amount of fuel the engine gets at all rpms, but is designed to really only meter fuel for the idle circuit. So, if the bike feels lean at part throttle on the highway and the idle mixture screw is turned out to richen things up, when the engine is returned back to idle, it’s now going to have poor idle quality if it idle’s at all. A good baseline for the idle mixture screw is about a turn and a half out from being seated. Careful though, turning the idle mixture all the way in with a screwdriver can damage beyond repair the idle mixture circuit. If you think the idle mixture is off, shut the bike off, gently seat the idle mixture screw into the carb, and back it off 1-1/2 turns. This is the preliminary setting we use for tuning. Absolute razor sharp tuning is accomplished with the exhaust gas sensors we have on our dyno, but tuning by engine sound and idle quality is perfectly acceptable. Final suggestion, if it ain’t broke don’t fix it.
Off idle and cruising speeds/throttle positions are handled by the intermediate jet. This jet feeds your engine fuel at off idle and up to about 3000 rpm. Access to this jet requires the float bowl to be removed. Free flowing exhaust (see – drag pipes) and or air cleaners nearly always require re-jetting in this area. A lean fuel mixture here will generally cause the engine to feel a little rough running. A rich mixture generally makes the engine feel like it’s a little lazy on accelleration. Again, this is where dyno tuning with exhaust gas sensors really come into it own. We can load the bike just as if you were running down the highway and see exactly what your engine wants.
The Main Fuel Circuit. This is where the fun is. The Main Jet begins to start to feed fuel around 3000 rpm to Redline. This fuel circuit, like to others, needs to be jetted properly inorder to extract premium performance and efficientcy from the engine. Too lean, and the engine runs hotter than it should and might even pop out the exhaust under hard excelleration. Too rich, and again, the motor will seem lazy or surge under that same hard excelleration. I’m certain like most all other Dyno facilities, we have seen bikes come in with less than optimum jetting and leave a completely new motorcycle once the jetting is corrected.
The Main Jet Air Bleed. This is a jet inside the float bowl on later models of the S&S Shorty Series Carbs (Super E & G). This allows us to change the rpm at which the Main Jet begins to feed fuel to the motor. Think of it like a small hole in a straw that you are drinking out of. The larger the hole, the more it takes to get that beverage up through the straw. These engine, like people, like things (fuel) differently and S&S really helped the tuner when making these carbs with adjustable Air Bleeds.
If you’ve labored though this brief description, I really hoped this helps in your understanding of what is going on inside that carb of yours. This is only a very brief overview of how things work in there, so keep an eye out on the bike magazines as they generally have very indepth articles covering these very things.
Happy Motoring!
