Tuning your Harley Davidson carburetor is simpler than most think and can be performed with a few common tools. This simple procedure is a great Harley tech tip that applies to all Harley models from 1989 to 2006 that use the CV (constant velocity) style carburetor. Earlier models equipped with the older butterfly style carburetors (pre-89 Evo's, Shovelheads, and Ironheads) are excellent candidates for upgrading to a newer Harley CV carburetor.
Before you rejet your carburetor it is recommended that you perform the following tuning procedure first. Once the mixture has been properly tuned as described here you may not need to rejet the carburetor.
Preparing the carburetor for tuning will require removal from your Harley-Davidson's engine but this is easily accomplished. The entire removal and installation procedure can be found in your service manual. With the carburetor removed, place it on a sturdy work surface upside down. The carburetor has a small cylindrical tower protruding from the base of the rear throat against the back side of the bowl. The tower is plugged with a soft metal insert covering the idle mixture screw. Gaining access to this screw is key to fine tuning and must be drilled out. The metal plug is very soft and only requires a household drill and a 1/8" bit. Secure the carb in either a vise or clamp and drill a hole into the plug making sure not to "punch" through too fast. You don't want to damaged the mixture screw just below the plug.
Tip: I found to keep from drilling too far into the plug is to wind electrical tape around your drill bit to form a thick band about 3/16" from the tip creating a stop point.
With a small hole now drilled into the plug, pry the plug out using a pick or awl. You can also thread a sheet metal screw into the drilled hole and use this to pull the plug out. With the plug removed, clean the area around the mixture screw so no metal fragments remain.
Adjustment Method: Using a small flat head screwdriver, turn the screw clockwise until it gently seats. Do no tighten as this will damage the screw or seat. Now turn the screw outward 2 turns to establish a starting point.
Reinstall the carburetor back on your bike making sure to replace the fuel hose clamp and vacuum lines. It may be a matter of dexterity but I prefer to install the cables before pushing the carb back onto the manifold. Test the throttle for any binding and double check all connections.
The air cleaner assembly MUST be installed prior to starting the engine, not only to hold the carb in place but to prevent any backfire while tuning. With everything back in place start the engine as normal and bring up to operating temperature. Do not overuse the enrichener (choke) and don't let the bike idle for more than 5 minutes.
With the engine warmed up and at idle you may now fine tune the idle mixture screw for optimal performance. With the engine idling slow (no more than 900 rpm) turn the screw inward (clockwise) SLOWLY until the engine starts to stumble. Note: you are working with the screw upside down so think about the orientation.
Raise the idle speed if necessary. Now turn the idle mixture screw outward until the engine begins to run smoothly + 1/8 of a turn. Maintain proper idle speed and repeat the adjustment each time you adjust the idle speed. Blip the throttle a couple of times and observe the results. If the engine responds quickly with a smooth blast and no backfiring through the carburetor, you have your idle mixture set correctly. If backfiring occurs through the carburetor then adjust the idle mixture screw out another 1/8 turn. Normally, the idle mixture screw should only be between 2 and 3 turns out from it's seated position.
Adjusting the mixture screw out too far will result in an overly rich, low RPM fuel mixture. Avoid tuning the idle too rich, thus leading to poor gas mileage and fouled plugs. Set this just beyond the point of backfiring (coughing) through the carburetor. Never remove the mixture screw with the carburetor upright as the spring, o-ring, and washer will fall out.
Take your newly tuned bike for a ride and note how it idles and responds off idle. If you experience any coughing through the carburetor not experienced before, adjust the mixture out another 1/8 of a turn. Black smoke seen from the exhaust at idle or a feeling of sluggishness off idle indicates you may have set the mixture too rich. If your bike is now idling steady and responds well from a start then you are all set. If your engine still runs lean (coughs or hesitates) you should move on to rejetting your carburetor. The same stock Harley Davidson carburetor has been used on all production bikes since 1989 due to it's reliability and ability to adapt to different conditions. With just the right amount of tuning the CV carburetor will support any stock to mildly modified Harley.
Enjoy the ride!
Ken Mendelson is the author of many articles on Harley-Davidson motorcycles and performance tuning. In addition, Ken runs the popular Harley web site Harley-Performance.com
