Stock CV Carburetor Modifications

(How to make your own "Yost/Dynojet" Kit)

Your Stock carburetor can be modified to make good horsepower and above all, better throttle response.
 The stock CV carburetor has a venturi diameter of 40mm (1.575 inch). Properly tuned, a CV carburetor is capable of making 80+ horsepower in Twincam engines.  An Evolution Engine can develop up to 64 horse power with a well tuned and modified CV carburetor.

With a few parts from your local Harley-Davidson Dealer and some tools, you can re-jet a CV carburetor and improve the performance of your bike.

CV Carburetor

Step 1: Remove the carburetor

Use your service manual instructions for your model bike to properly remove the carburetor. You can probably get away with leaving the throttle cables connected, but it is much easier to remove the carburetor and use a work bench or table (the kitchen table or coffee table usually serves well if you are trying to piss off your Ole Lady). Leave the choke cable connected to the carburetor and disconnect the knob end (left side of the bike). 

 Remove the carburetor and the choke cable from the bike together.

Helpful hint
If you take out the rear fuel tank bolts and loosen the front bolts, the fuel tank can be raised a few inches. The extra room gained by raising the tank can be the difference between struggling with the job and making it easy.

Step 2: Adjust the idle mixture

 The aluminum plug covering the idle mixture adjusting screw needs to be removed. Turn the carburetor over and locate the plug toward the rear of the carburetor, in back ofthe float bowl. Using about a 1/16" drill bit, carefully drill a hole through the center of the bottom of the small plug.

Be careful not to use too much pressure when the drill bit breaks through the plug, you don't want to damage the head of the adjusting screw.

If the plug does not fall out while drilling, remove the drill bit. Carefully  insert a small self-tapping sheet metal screw into the plug and tighten snuggly. This will allow enough grip to remove the plug by pulling on the self-tapping screw with a pair of pliers.

Underneath you will find the slotted adjusting screw. Turn this screw clockwise until it is GENTLY seated. Over tightening this screw can damage the carburetor and needle. Back the idle mixture screw 2 1/2 full turns. This provides a starting point for tuning.

Step 3: Modify the Slide

Remove the top of the carburetor (slide vacuum diaphragm cover) be sure to remove the throttle linkage stop plate screw. There is a spring under the top cover, so hold it with a finger until all screws are loose. Holding the carburetor upright, remove the cover and spring. Using metal snips or side cutters, remove about 4 coils from one end of the spring. Remove the slide/diaphragm assembly. Inside the slide you will see the plastic spring seat. Under the spring seat is the jet needle. These two items can be removed by turning the slide over and pouring the parts into you hand. These parts are needed for reassembly of the carburetor.

Slide Detail

On the bottom of the slide are two holes. The center hole is for the jet needle. The second hole is off center. This is the vacuum port. This hole needs to be drilled to 1/8" (#29). Make sure this hole is a clean straight hole. Keep the shavings away from the rest of the carburetor.  Remove any burrs that exist and clean  the slide. Set the slide aside for now.

Step 4: Re-jet

Turn the carburetor over so the bottom is facing upward. Remove the float bowl by removing the four screws. Carefully remove the fuel bowl. Using a flat blade screwdriver, replace the stock #42 slow jet with a #45 jet (Twincams should already have a #45 slow jet). If you use the OEM needle, the main jet will be replaced with a  jet 10 larger than the OEM. The jets are brass parts that are screwed into an aluminum body. Do not over-tighten the new jets when installing them. 
Replace the float bowl, carefully placing the accelerator pump rod into its rubber boot.

Jetting Note:

Always verify that jetting is correct by reading the spark plugs after a short test ride.

Step 5: Re-assemble the Carburetor

Place the jet needle into the slide. Replace the spring seat into the slide and over the jet needle. Place the slide into the carburetor body, aligning the needle into the main jet outlet. Carefully place the edges of the diaphragm into the grove around the top of the carburetor. Install the slide spring and the carburetor top being careful not to mis-align or pinch the diaphragm.
Replace the throttle linkage stop plate screw and give the throttle grip a twist to be sure nothing is binding.

Assembly tricks for the diaphragm

The diaphragm is easily mis-aligned or pinched. This is a common source of problems after carburetor modifications are performed. The diaphragm frequently has the appearance of being too large to fit into the groove. The repeated up and down movement of the diaphragm causes it to stretch, making re-assembly tricky.
Apply a small amount of grease to the lip of the diaphragm and use the cap to position the diaphragm. It will allow you to evenly push the diaphragm into the groove. You can "wiggle" the top and feel when the diaphragm is properly located. A damaged diaphragm will have the symptoms of the engine being able to idle, but will not accept any throttle. If you think the diaphragm is damaged, check for any pinholes with a bright light behind the rubber, pulling on the edges to stretch the diaphragm a bit. If you find any, the entire slide/diaphragm assembly must be replaced.

Step 6: Put the carburetor back on the bike

The carburetor is now ready to be installed back on the engine.
Place the fuel line on the carburetor before you place it on the bike. Apply a thin film of grease to the inside of the manifold seal and replace the carburetor, directing the fuel line and choke cable into position as you move the carburetor into place. Remember to replace the vacuum hose from the VOES to the top rear of the carburetor. Checking all the work to make sure screws and hoses are properly installed. 

Be sure that all throttle cables, fuel lines, vacuum lines are installed properly and the fuel tank is secured.

Step 7: Warm up the bike

No final settings will be correct if set with a cold engine.
Warm the engine up to full operating temperature. Prior to final adjustments the engine may not idle properly because the carburetor is no longer as lean as the factory settings. 
DO NOT follow the owner's manual directions for cold starting.

The new procedure is as follows: 
If the engine is cold, pull choke out all the way. If the engine is warm, use half or no choke at all. Start the bike and push the choke all the way in, use the throttle to keep the bike idling while warming up. The throttle lock can be used for this purpose during initial adjustments.

Letting the bike idle to warm up is not desirable. The modifications made to your bike should allow it to run well enough for a sedate test ride around the block. This speeds up the warm-up time and also allows a feel for the improvement to throttle response from the modifications. As the bike comes up to proper operating temperature, adjustments to the idle mixture and idle speed can be performed.

Step 8: Setting the Idle Mixture Adjustment

With engine warmed up and at idle, turn the idle mixture screw inward (clockwise) slowly until the engine starts to stumble. 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 outward (counter-clockwise) until the engine begins to run smoothly, adjusting the idle stop screw as necessary to maintain proper idle speed. Twist the throttle grip a time or two and observe the results. If the engine responds quickly with a gratifying 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 to 1/4 turn. 
Under normal circumstances, the idle mixture screw should be between 2 and 3 turns out. Adjusting the idle mixture screw out to far results in an overly rich, low RPM fuel mixture leading to poor gas mileage, fouled spark plugs and carbon buildup in the combustion chamber.

 Idle Speed

The proper idle speed is 950-1050 RPMs. The oiling system needs better than 900 RPM to work properly.
 Resist the temptation to lower the idle excessively. It may sound good, but improper oiling will contribute to engine overheating while idling in traffic and premature engine failures.

Step 9: Main jet tuning

The details of carburetor tweaking and plug reading can be a very involved subject, but not terribly complicated, here is a very basic guide that will get the adjustments close.

To test the main jetting, always start with a NEW set of spark plugs.
 You must test ride at a fairly high and constant RPM (3000+) for 3-4 miles without the choke and without using the accelerator pump (just a constant open throttle), then pull in the clutch and turn off the kill (run/stop) switch and coast to a stop. Pull one of the spark plugs (careful, they will be hot) and read the plug. If you are too lean or too rich, only change one jet size at a time.

Plug reading is considered by some as much of an art as a science, but then, anyone can be an Artist, that's why most of them are called "Starving Artists", because they aren't smart enough to get a job! 
It doesn't take years of experience to understand the basics of what the plugs are really telling us about the engine. For most street riders, it just takes a little practice. 
Checking the plugs will provide the most accurate reading. If your plugs are black you are too rich, which decreases your gas and performance but will not harm your engine. Too lean means too hot, and engine damage will soon follow if proper steps are not taken to richen the mixture.