This is how to change your main jet and needle on the stock Road Star 40mm Mikuni Carburetor.
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Written by Mr Tidy with many thanks to Thor
The jetting is not terribly complicated if you remember one thing:
Do not disconnect the throttle Position sensor from the Carburetor.
- Remove the nut that holds the choke cable on the left side, and remove the cable from the bracket.
- Remove the idle adjusting cable from its bracket.
- Disconnect the gas line at the carburetor.
- Completely loosen the throttle cables.
- Unplug the two electric leads to the carburetor heater on the bottom of the carb.
- Now loosen up the clamp that holds the carb in the rubber boot.
- With a pull and wiggle motion the carb will come out of the boot.
- Tilt it slightly to the left and remove the throttle cables. The cables align with a groove and slide out.
- Keep a rag under the carb as gas will come out when you rotate it.
- With the cables now off, rotate the carb to the right until it is upside down.
- You have to loosen the brass carburetor heater to remove the bowl. Caution, when you tighten the heater back up, be very careful not to tighten it too much, they break quite easily.
- You have to remove the four screws from the bowl to remove it. They are soft headed screws and strip quite easily. I used a good set of needle nose vise grips to break the tightness of the screws. Others have used a Dremel grinding tool with a cut-off wheel and cut a slot for a sloted screwdriver, or even cut the head off if worse comes to worse. Replace with Allen head screws.
- Remove the screws.
- When removing the bowl lift it straight up and be careful not to hit the floats.
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- Under the bowl you’ll see several jets. The one you are replacing is the jet that sits on a brass spacer. If you have a 1999 to 2003 1600 model Road Star it should have the number 165 on it, if you have a 2004 or higher 1700 Road Star then it will be 182.5.
- Remove the jet with a flat blade screw driver and put in your new main jet.
- Replace the float bowl being careful not to hit the floats.
- Use the Barons allen head screws to secure the bowl.
- Tighten the Heater back up….lightly.
- Now you have to remove the brass plug from the Pilot mixture screw housing, if it has not been removed already. This is a tubular housing that rises from behind the bowl in the middle. It should have a brass plug with a small hole in the middle. TRIPLE CHECK what you are about to drill, it is easy to make a mistake!
- The Baron gives you a drill bit to drill the hole larger. Be careful not to drill into it too far and hit the screw below.
- Use the metal screw the Baron gave you to screw into the brass plug.
- Use a set of vise grips to grab the screw and wiggle the plug out.
- You will see a small flat head screw inside.
- With the carb upside down, tighten the screw clockwise until it lightly seats.
- Now back the screw out three and one half turns.
- Now put the carb back into the boot, placing the throttle cables in place before you insert it into the rubber boot.
- Tighten the clamp.
- Now remove the two screws that hold the black plastic cap on the top.
- The cap will rise because of the slide spring under it.
- Remove the cap and remove the long spring. The slide is connected to the rubber diaphragm. It just lifts straight out the top.
- With the slide removed, look inside the slide. You’ll see a plastic tab. Grab the tab with a needle nose pliers and wiggle and pull straight outward. BE VERY CAREFUL not to pull out too hard or you will loose the very small spring that is under the tab. Also be careful as not to break the tab by pulling at an odd angle. It will pop out as it is held in with only an O ring. It should have a small spring connected to it. Now the needle is ready to come out.
- Push the needle up and grab the end. When you have a hold of the needle, turn the slide upside down and finish removing it so the washer and spacer don’t fall off the end.
- Now take your new needle and place the clip on the indicated groove the Baron has recommended.
- Always count down from the top.
- Place the white plastic spacer under the clip from the stock needle.
- Do not use the washer under the spacer as was on the stock needle.
- Put a washer on the top of the clip. Now insert it back into the slide and press the plastic tab with the small spring back in.
- Make sure the rubber diaphragm is seated properly with the O ring towards the rear.
- Put the slide spring back in and replace the cap and tighten the screws.
- Connect the two electrical leads to the heater
- Connect the gas line
- Put the choke cable back in the bracket
- Put the idle cable back in it’s bracket.
Your have finished with the carb.