From the Boutique:

Rejetting the CV Carburetor

contributed by L. Maurice Riggins (1998)
updated by L. Maurice Riggins (2001)
updated by L. Maurice Riggins (2005)

Introduction

The slide vent hole is the size it is for a reason. Like a shock absorber, it damps the movement of the slide. If it is drilled to 1/8" as is commonly done, the slide will open too fast when the throttle is whacked open suddenly and it will overshoot where it should be, then as a result, fall back down, but do it too fast, overshooting where it should be again. This results in oscillation, and a huge variance in mixture. The diaphragm spring establishes the constant velocity. Changing to a lighter spring, or worse yet, cutting coils off the spring, lowers the constant velocity, and this is the velocity that creates the vacuum that sucks fuel up from the fuel bowl. Even if you increase the jet sizes to compensate, the mixing becomes erratic. You can see evidence of a drilled slide or lighter spring as a dip in the torque curve just after the throttle is opened.

Blindly shimming the needle up or replacing it is not recommended. Late model needles are richer, sometimes more so than aftermarket. The only time this is required is when dyno tuning. In the process of dynoing, starting with a small main jet and doing runs with progressively larger main jets, if going up one main jet size causes high rpm horsepower to fall off (i.e. if you've just gone past the optimum main jet size) but low rpm power improved, go back to the previous main jet size, and shim the needle up .030" or so with a small washers. This is not common, and seldom required.

And finally, although it may result in less than optimum idle mixture, you CAN temporarily skip the part about drilling the plug off the idle mixture and setting the idle, if you are concerned about warranty issues. By increasing the size of the slow jet, you are automatically making the idle mixture richer, and the factory setting is often pretty close. Idle performance is usually adequate. The sign that it isn't is an occasional stumble when the throttle is applied at idle. The author encourages you to "do it right" and perform this step if possible.

Rejetting

1. Turn the fuel valve off.

2. Find the fuel bowl drain hose end (front of the engine behind the oil filter and put it in something (non-glass) to catch the gasoline.

3. Looking at the backside of the carb between the cylinders from the left side of the bike, find the fuel bowl drain screw on the bowl and using a long skinny screwdriver open it counterclockwise about 3 turns which will let the fuel bowl drain, then close it back.

4. Using an 11/16" open end wrench, loosen the nut on the backside of the choke knob bracket and slide the choke knob/cable assembly out of the slot in the bracket. Don't rotate the plastic nut on the front... it adjusts the tension on the choke ("enrichener") cable.

5. You'll have to use an awl, skinny screwdriver, and perhaps needlenose pliers to remove the one-time-use hose clamp on the gas hose at the tank, destroying the clamp in the process, so have a replacement worm-drive hose clamp from the hardware store for later reassembly. Slide the hose off the tube on the fuel valve. You may have to grab the hose with pliers and rotate it back and forth to break it loose, but don't let the plier's teeth cut into the hose. Do not remove the vacuum hose (behind the fuel hose) from the fuel valve.

6. Back on the right side of the bike, find the vacuum hose that plugs into the top of the carb behind the black plastic cap and pull up on it until it slips off the tube coming out of the carb. Note here now the routing of the fuel hose going to the tank and the choke cable so you'll get them back in the right places during reinstall.

7. Find the fuel bowl drain hose and fish it out from behind the front cylinder pushrod tubes and behind the oil filter so it's loose.

8. Put a big thick towel on the crossover exhaust pipe and engine case below it to protect them from getting scratched by the carb or tools. (actually it's better to do this before you start pulling the old air cleaner off).

9. With one hand on the top of the carb and one on the bottom start rocking the carb in at the top and out at the bottom, then do the reverse, back and forth, all while pulling the whole thing toward you. Don't let the carb rotate side to side or it may scratch the rocker covers. The carb will POP out of the rubber gasket on the manifold with the throttle cables still attached to it, and it will be pulling the fuel hose and choke cable through the mess between the cylinders.

10. You can lay the carb hanging from the throttle cables on the towel if you need to grab tools, or use both hands for other things.

11. Rotate the carb up so you're looking at the bottom. On the back, behind the fuel bowl, you'll see what looks like about 1/4" silver plug. This covers the idle mixture adjustment screw and we are going to drill a hole in it, screw a sheet metal screw into that hole just until it bites good, then grab the screw with a pair of pliers and pull it and the plug out.

12. Prepare an 1/8" drill bit by using either a drill stop or several turns of electrical tape around the bit so that it can't drill into the plug any deeper than about 1/8". If it does, it may break through too hard and plunge into the screw. If that happens the screw may be gouged up a little, but even worse, it may spin closed (clockwise) hard, damaging the idle mixture needle and seat. So don't put a lot of pressure on the drill, use a sharp bit and let it do the work. Drill the hole in the center now.

13. Start a self-tapping sheet metal screw into the hole until it bites good (it may even start spinning the plug), grab it with a pair of pliers and pull the screw and plug out. You should see the idle MIXTURE screw.

14. Before removing the fuel bowl, note how there is a rod coming down from the throttle linkage into a bellows on the bowl. You'll have to put this rod back in the bellows when reinstalling the bowl. Also, note in the mouth of the carb there is a brass pipe standing up in it. This is attached to the bowl and slides up through the carb body during bowl reinstallation.

15. Find the four brass screws that hold the bowl on the carb. Using a phillips screwdriver that completely fills the slots in the screws remove them without letting the screwdriver slip and damage the screws. This may take considerable pressure. When removing the screws, hold the bowl in position until they are all out then pull the bowl off straight down. Be extremely careful... the floats are just inside the bowl. The rod will come out of the bellows and hit against the carb body... no you didn't drop it. Set the bowl (with dangling drain hose) aside some place clean. Note the brass tube on it that was sticking up in the carb mouth.

16. You will see the floats hanging from the bottom of the carb. Be extremely careful not to push on these in ANY direction. The metal holding them is thin and will bend easily, getting the floats out of adjustment. Also, if you push up on the floats hard, you may damage the needle and seat they control. If you lay the carb down on the towel make VERY sure the floats aren't touching anything.

17. Looking at the bottom of the carb, find the two jets. The main jet will be more or less in the middle, screwed into the emulsion tube and should say "160" on it for an 883, "170" on it for a 1200, or 190 on it for a Sport or International model. The emulsion tube will have flats on it for a 5/16" wrench. Hold the emulsion tube from turning with the wrench and using a common (flat) blade screwdriver that's wide enough to fill the slots on both sides of the main jet, unscrew the jet (counterclockwise).

18. Screw in the replacement main jet as follows:

                  SE air cleaner OR SE mufflers      BOTH
         883                 #165                    #170
        1200                 #175                    #180
        1200 Sport/Intn'l    #195                    #200
    

    Tighten firmly, but do not strip. Do not use LockTite.

    Note: These jet sizes are a good place to start, but realize that some tuning may still be needed. Some 1200's come stock with larger jet sizes and altitude can play a big role in jetting... if you live above 3,000' above sea level, you may not need to rejet! And if you live right down at sea level, you may need to go larger. Also, I have heard that CA bikes come from the factory with larger jets due to emissions equipment on the bike. The ONLY accurate way to get the main jet set correctly is on a dyno... PERIOD.

    WARNING! The above jet recommendations are for '03-earlier rigid engine Sportsters. It has come to the author's attention that Harley is jetting '04-later rubber-mount Sportsters even richer than these recommendations, which themselves tend to be slightly on the rich side. Some recommend not changing their main jet size, while others even recommend reducing it. Since I don't know what changes were made to other parts of the carburetor, and have no experience with these models, I'll refrain from making a recommendation.

19. Find the slow jet on the carb bottom. It should say "42" on it and you'll replace it with a #45. It will be down in a tube cast into the carb body. You must find a screwdriver with a blade wide enough to fill the slots in the jet that doesn't taper wider above its tip, or it won't fit down into the tube to reach the jet. You may have to have a machine shop grind a screwdriver down so that it's shaft is no wider than the blade at the tip. Do not try to force an inappropriate screwdriver in or you will crack the cast carb body. Unscrew (counterclockwise) the #42 slow jet and replace it with a #45 slow jet. Tighten firmly, but do not strip. Do not use LockTite.
    
    Note: The #45 slow jet is the stock slow jet in Twin Cam 88 engines. As with the XL, the stock slow jet is replaced with one size larger, so the dealer's service area, or a buddy with a Big Twin, may have this jet laying around, and you might be able to get it for free.

20. Reinstall the fuel bowl on the bottom of the carb, taking care to:
    1. Line up the brass tube that goes up into the carb mouth.
    2. Get the rod from the accelerator pump into the bellows, and
    3. BE CAREFUL NOT TO CATCH OR BEND THE FLOATS.

21. Reinstall the four screws holding the bowl on the carb bottom. Do not overtighten. Do not use LockTite.

22. Check the rubber seal on the manifold to make sure it is seated on it all the way around.

23. Push the carb back onto the manifold making sure the fuel hose and choke cable are going back where they came from. Rocking helps, but don't worry whether the carb is tilted just right at this time. It will be lined up when you reinstall the breather bolts on the air cleaner (or air cleaner support bracket for non-SE air cleaner).

24. Slip the vacuum hose back on the tube on the top of the carb.

25. Reroute the fuel bowl drain hose back where it came from.

26. Back on the left side of the bike, slip the new hose clamp over the fuel hose and slide the hose back on the tube from the fuel valve. Slide the clamp up near the valve and tighten, but do not overtighten.

27. Slide the choke knob/cable assembly back on it's slot on the bracket and tighten the nut on the back with fingers, then tighten a bit more with the 11/16" wrench. Do not overtighten or the plastic assembly will crack... just enough to keep it from sliding out of its slot. Pull the choke knob out, making sure it comes out as far as it did before and stays there. If it doesn't come out as far, loosen the nut on the back, check the routing of the cable, then try again. If the knob doesn't stay out, loosen the nut on the back and turn the plastic nut on the front clockwise a little and try again.

Setting Idle Mixture and Speed

1. With a flat blade screwdriver, turn the idle MIXTURE screw (the one you drilled the cap off) full in (clockwise) GENTLY until it seats... GENTLY (get the picture?), then turn it out (open, counterclockwise) exactly two full turns. Leave the idle SPEED screw (top right side of the carb looking at it above the air cleaner) where it was before.

2. Turn the fuel valve to on, pull the choke fully out, and start the engine. If you drained the fuel bowl, it will crank for as much as 20-30 seconds without starting while the bowl fills. Once the engine has started push the choke in about 1/2 way and then wait about 2 minutes. Then push the choke in all the way, and if you must do so to keep the engine running, open the throttle slightly. You can keep it there with the throttle lock, the star shaped screw below the right switch housing. Once the engine is fully warm (rear rocker cover is hot to the touch), fully release the throttle lock and make sure the throttle is fully closed. The engine should be idling, perhaps nicely, perhaps not.

3. With a flat blade or phillips screwdriver, turn the idle SPEED screw in (clockwise) about two full turns until the engine is idling fast.

   WARNING: the crossover pipe is HOT. You should adjust the idle MIXTURE with at least heavy cotton or leather work gloves (not your good ones) to protect from burns. Use a short screwdriver so the handle will not melt on the crossover pipe.

4. With a flat blade screwdriver turn the idle MIXTURE screw in (clockwise) GENTLY until it fully seats. The engine should not die. If it does, turn the idle MIXTURE screw back out the two full turns and go back to step 3, increasing the idle speed setting (in clockwise another turn).

5. Now turn the idle SPEED screw out (counterclockwise) just until the engine dies or is stumbling badly. The idle speed is now set correctly. This is an easy way to set it without a tach. It will result in the idle speed being a little high, which will give more oil to lubricate the engine. If the idle speed is too low, insufficient oil will be pumped.

6. Now turn the idle MIXTURE screw out (counterclockwise) exactly two full turns out (counterclocwise), restart the engine if it died, and let it idle a minute or so to stabilize.

7. The following takes a good ear for engine speed. As the idle MIXTURE screw is turned in clockwise, the mixture will become lean and the engine speed will start to decrease. As it is turned back out counterclockwise the engine speed will increase and stay there for awhile as you continue turning out, and then the engine speed will starts to drop as the mixture gets too rich. Note the points where the engine speed started to drop and set the idle MIXTURE screw in the middle of that higher speed range. Now the idle mixture is set correctly.