Tuning a Harley Carb

CoolMaker · #1 (**permalink**) 04-11-2007, 08:52 AM

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 carb's from 1989 to present that use the CV style Harley Davidson carburetor . Even newer model bikes with a factory stock harley carb can benefit from proper tuning. Earlier models equipped with the older butterfly style carbs (pre-89 Evo's, Shovelheads, and Ironheads) are excellent candidates for upgrading to a modern Harley carburetor .

Note: Before proceeding to rejet your carburetor it is recommended that you perform the following tuning procedure. Once the the mixture has been properly tuned there is often no need to rejet the carb.

Preparing the carburetor for tuning will require removal from your Harley-Davidson's engine but this is easily accomplished. Begin by shutting off the petcock fuel valve and starting the engine to allow all fuel within the carburetor bowl to be emptied. Remove the aircleaner assembly including the backing plate which is attached to each head with a banjo bolt. This is a good time to inspect these bolts for obstructions in their passages. Remove the choke cable from it's mounting bracket on the opposite side of the bike.

The cable will stay attached to the carburetor during this procedure. Disconnect the fuel line from the fuel inlet on the carb or the opposite end connected to the fuel petcock, whichever is simpler to access. The hose is likely fastened using a special crimped clamp. This may be cut or pried off to remove since you won't be using it again. Be sure to have a new hose clamp available.

Next loosen both throttle cables from their adjusters located just beyond where they exit the throttle grip. A couple turns is usually all it takes to give you enough slack. If you count how many turns each adjuster is loosened then you can return them to the exact adjustment when reinstalling your carb. This is a good time to label each cable to avoid any confusion when reconnecting them to the carb's throttle cam. A simple "Top" and "Bottom" should suffice when tagging each cable. Now that you have enough slack in the cables you can pull the carburetor away from the manifold. A Harley carb is only held to the manifold with a slip fit rubber boot. Gently rock or twist the carb back and forth as you pull it away from the engine. Remove the cables that you tagged and remove any vacuum hoses. If your model has multiple vacuum hoses it would be a good idea to label these as well.

With the carb removed place on a sturdy work surface upside down.
The CV style Harley carb has a small cylindrical tower protruding from one 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 our carburetor and must be drilled to remove. The metal plug is very soft and only requires a household drill and 1/8" bit. Secure the carb in either a vise or by other means that will allow the carb to remain steady. 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. Allow the drill to slowly cut into the plug rather than push.

Tip: I found to keep from drilling too far into the plug is to wind electrical tape around your drill bit quite a few times about 3/16" from the tip. This will create a stop to keep the bit from going too deep.

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.

Tip: Drill the hole off center where the plug edge meets the tower edge. This will weaken the plug for easier removal.

Now that the plug is removed clean the area around the mixture screw so no metal fragments remain. At this point there are two methods for adjusting the mixture.

Adjustment Method: Using a small flat head screwdriver turn the screw clockwise until it gently seats. DO NOT OVER TIGHTEN AS THIS WILL DAMAGE THE NEEDLE SCREW. Count how many turns it takes to reach the closed position. Mark the screwdriver if needed to properly count each turn. Now turn the screw out counter clockwise stopping at a 1/4 turn beyond that which you originally counted. For example, it you turned the screw in 1-1/2 turns then unscrew it 1-3/4 turns. This is your base starting point and alone will allow your idle mixture to be slightly richer than the factory's EPA setting. In many cases this will be the ideal setting.

If the mixture screw has been reset by the dealer or previous owner (evident by the plug already being removed), turn the screw clockwise until it seats. Now turn the screw outward 2 turns to establish a starting point.

Reinstall the carburetor back on your bike by reversing the steps taken during removal. Be 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. Make certain the carb firmly seats back onto the manifold boot. Test the throttle for binding and smooth operation. Double check each hose and connection.

The air cleaner assembly MUST be installed prior to starting the engine, not only to hold the carb in place but to prevent having the carb backfire in your face while tuning. With everything back in place start the engine as normal and bring up to operating temperature prior to fine tuning. Let the bike idle for no more than 5 minutes. The modified carburetor should allow your bike to run well enough for a mild test run around the block to speed up the warm-up process.

With the engine warmed up and at idle you may now fine tune the idle mixture screw for optimal performance. You may wish to acquaint yourself with the adjustment screw location at the bottom rear of the carb prior to running the engine to avoid burning your hands. 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 clockwise may appear counter-clockwise.

If the engine will not idle on its own during this procedure, raise the idle by adjusting the idle set screw until it does. Now turn the idle mixture screw outwards 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 right. 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.

Twin Cam are the exception as they tend to self adjust their mixture due to readings from the MAP sensor. Most Twin Cam Harley engines have a mixture screw sweet spot approximately 2 to 2-1/8 turns out from seat. Adjusting the mixture screw out to 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 there is a spring, o-ring, and washer that 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 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 there's no reason why you can't have some of the same performance gains advertised by the major racing carb manufacturers. Stage 1 modification kits are available for those who wish to take their carb to the next level of performance.