CV carbs have a strong reputation in the industry. Years ago, I did an article with Trock on his mods. I finally ran his modified CV on my Mudflap Girl FXR and it was terrific. I experienced sharp throttle response and almost 50 miles to the gallon out of the box.
Then I tore the Mudflap baby down and painted and powdered some of the components, but not the carb… Once back on the road it started to act up, popping on the low end. I took it to Bennett’s Performance and Eric adjusted it, but no go.
First, I want to tell you what’s been done to the engine. It’s a box stock 80-inch H-D Evo with modified and magnificent Branch heads, an Andrews cam and a few S&S touches. My compression may be slightly higher than stock, but it’s still 80 inches. I’m running a custom set of D&D exhausts and a shapely stainless steel Roger Goldammer air cleaner. It kicks ass.
I tried everything with my Trock carb, but nothing seemed to matter. Then the next option surfaced, replace the carb body. I had another one, so I replaced everything in the new carb from the old one, to maintain the Trock formula, but I didn’t drill the slide. The bike seemed to run okay, but the snappy throttle response was gone. It had softened.
There are a couple of delicate idle circuits surfacing near the butterfly. I could only surmise that a clog in one or the other was causing my dilemma.
After the switch, it ran next to perfect, but after several months, I noticed a puff of black smoke at idle. I checked the plugs and they seemed fine at high end but black around the ring, for the rich low-end results.
We have been working with CV Performance, and I noticed a recent kit for tuning CV carbs. I decided to give it a shot and run through the carb once more. One more thing caused me to enter into this delicate process. When I adjusted the idle mixture screw, I could never get it to stumble. Most carbs cut off the fuel to a point where the bike almost dies during tuning, and then you back off for the sweet spot. Mine never did. But we will discuss this issue as we move through the process.
First, they asked that I remove the carb. In my case the carb was easily accessible, but I removed it anyway. I needed to loosen the throttle cable first, and be careful with the choke knob. If you over tighten that bastard the cap breaks and the whole system needs to be replaced, so be careful. I removed Roger’s hand-shaped air cleaner cover, then the filter sock, the cage and then the vent bracket holding the carburetor in place. I washed the air-filter sock with a spray degreaser, let it set, then rinsed it with water, and let it dry.
I made sure the petcock was turned off, and I was reasonably careful to remove the line so I didn’t shower myself with gas. I use stock-like crimped hose clamps. They are cleaner than standard hose clamps, but unfortunately permanent. If I mess up then one clamp is destroyed. I often slip them in place but try not to crimp them until I’m sure we’re good to go. Removing them can be a trick.
I use a strong set of side cutters. Sometimes you can cut through the clamp, spread it, and ditch it, but be careful. Things break or get scratched fighting with these. Sometimes a small twist will pop the band loose, and then you can simply spread it, like taking a belt off, from around your waist.
Also, after awhile, a hose can stick, dry, and become a bastard to remove. A very small pick or flat-blade screwdriver and a squirt of WD-40 can save me from a dented gas tank. The directions were designed for a stock bike owner and mentioned labeling the throttle cables for accurate replacement. It’s also a good idea to take photos as you creep through this process. I do it for you and me. I can always go back and look at a shot to confirm placement, cable angle, etc.
I placed the carb on my bench upside down and checked the mixture screw. If there is a stock plug in place to keep you away from adjusting, you can remove it and adjust the mixture. Leave the float bowl in place for now. This tower is plugged with a soft metal insert covering the idle mixture screw. Gaining access to this screw is key to fine-tuning and it must be removed to expose the adjustment screw. Secure the carburetor and drill a hole into the plug carefully with the supplied 7/16-in. drill bit.
Always wear eye protection. Make sure not to punch through, as you don’t want to damage the mixture screw just below the plug. Allow the drill to slowly cut into the plug, rather than pushing. Pry the plug out using a pick or sheet metal screw and clean the area above the screw to remove any metal fragments.
I didn’t need to remove the screw. Under the mixture screw is a very small spring, washer, and O-ring. Use caution as these parts fall out easily and are not available from your dealers, although CV Performance has them in stock. They saved my ass once before.
Since I didn’t need to mess with it, I preset the adjustment be screwing it in with a very small tapered flat screwdriver until it gently seated. Do not over-tighten or you will damage the seat. Then I backed out the screw 1 ¾ turns.
I spoke to Ken Mendelson from CV Performance and I was able to obtain a much clearer understanding of this adjustment. Although you adjust this needle at idle, it is not an idle circuit, but an air and fuel adjustment between the pilot jet and the main jet. Ken also pointed out that during adjustment the slide is constantly moving to counter any changes. Consequently, it may take more than one adjustment period, but we will touch on this later.
Okay, now it was time to remove the four screws holding the float bowl in place. Be careful with these Phillips head screws. It’s easy to mangle these bastards. Three of mine came loose with a solid grasp of the screwdriver handle. The last took a set of very narrow needle-nose pliers. I worked hard to get just the right grip strength and just freed the screw from its locked position. Maybe just ¼ turn. I was free!
There is an L-shaped rod operating the accelerator pump. It slips into a rubber boot. I noted the position and made a point not to lose it. I was careful not to disturb the float system or put pressure on it, which would change its adjustment.
Pilot jet
Using a narrow 1/8-inch flat head screwdriver, I unscrewed the pilot jet from within the orifice shown. Make sure the driver fits the jet firmly to avoid damaging it. Giving the screwdriver a couple of light taps with a hammer will loosen the jet threads that may be seized by old lacquered fuel. Using the supplied table I picked the correct set of jets for my application: Big Twin Evolution 80-incher, Stage 1 Filter/Exhaust: #46 Idle, #185 Main.
With a wider flat head screwdriver I unscrewed the main jet from the brass needle jet holder (aka emulsion tube). At this point, I was also supposed to replace the emulsion tube, but I failed do so the first time around. I’ve done it now. Their emulsion tube is designed to deliver the fuel more efficiently. Plus, if one of the tiny holes, and there are over six in the brass tube, is clogged, it will affect performance.
Under the list of jets, there was a CV note: The above jetting specs have been extensively tested. When used along with the components in this kit, this will provide optimal performance under most conditions. Stage 1 refers to using a free breathing air cleaner and exhaust.
The CV Performance team cautions: There is a small stainless-steel sleeve (jet nozzle) inside this passage held in above the emulsion tube. This piece protrudes into the venturi where the tip of the needle meters the fuel spray. Do not remove or allow it to fall out. If removed, the wider opened end faces up into the carburetor. The engine will not run correctly unless installed. Install the new emulsion tube and HAND TIGHTEN ONLY.
I replaced the main jet with the jet size recommended in the table. I threaded it into the emulsion tube, allowing the jet to tighten the emulsion tube into the threaded cavity. DO NOT OVER TIGHTEN as it will distort or crack the tube. It is normal for a couple of threads to remain visible.
Reinstall the bowl, making sure to align the L-shaped accelerator pump shaft and rubber boot.
Tip: Use a very thin layer of clear spark plug boot lubricant (clear silicone grease) on the bowl gasket. Never use any sealants. If the gasket is cracked it will need to be replaced with a new gasket. Firmly tighten the four new socket head bowl screws.
Slide Needle Replacement and Reassembly
Remove the Phillips-head screw holding the side throttle cable bracket and pull it to the side so you can reach all four screws holding the black plastic cover on top of the carburetor. Be sure to hold the cap in place, as there is a spring under the top cover.
Remove the stock spring from the inside slide, followed by the plastic thimble spring retainer. Pull the needle up through the slide. Remember the order and orientation in which these items were installed, as the needle is always installed first.
Optional Slide Modification:
Gently secure the slide in a vice. Then using the provided 7/64-inch drill bit, and clean out the bore size of the off-center vacuum porthole. Do not make this hole any larger than 7/64. It’s the magic number. Drilling is optional and may be skipped. If the slide port was drilled larger than 1/8-inch it must be replaced.
Place the slide back in the carb body and insert the new needle first, followed by the original plastic spring retainer down into the slide cavity and slide the new spring over the retainer. Position the rubber edge of the diaphragm in the top groove.
CV Performance Tip:
If the rubber diaphragm edge will not stay in place, apply a few dabs of clear spark plug boot lube or Vaseline in the grove to hold the diaphragm lip in place while positioning the top cap.
Avoid pinching or damaging the diaphragm. If the diaphragm is not seated in its groove it may become pinched by the cap and damage the delicate rubber. Tighten the top cap screws and then the side cable bracket screw. Be sure that the bracket is aligned and the throttle operates without binding.
The directions mention the EZ mixture screw from CV Performance, and I tried one once before but didn’t feel comfortable with the loose fitment and replaced it with the stock unit. It’s cool and allows easy adjustment. You might have better luck. The instructions mention wrapping the threads with Teflon tape if it feels loose.
Reinstall the carburetor back onto the rubber manifold boot by reversing the steps taken during removal. Be sure to replace the fuel hose clamps and vacuum lines. It may be a matter of dexterity, but some folks prefer to install the cables first. Make certain to firmly seat the carb back into the manifold boot. Using a small amount of Vaseline or clear spark plug boot lube will assist with a proper seal. Refer to your photos if you run across any installation issues.
Carburetor Tuning:
Test the throttle for binding and smooth operation. If you haven’t lubed your throttle cable, this might be the time. I installed the Roger Goldammer stainless steel air cleaner and warmed up the bike. Don’t let your bike overheat in the process. Sometimes an external fan helps.
The instructions called for acquainting myself with the mixture adjustment screw or EZ-Just location at the bottom rear of the carb behind the float bowl before running the engine. It gets hot back there. I modified a short screwdriver specifically for this adjustment. I was working with the screw upside down, so clockwise may appear opposite. Never remove the mixture screw while the carb is installed. I think I did this once and lost some of the components, but CV Performance came to my rescue. There is a tiny spring, washer and O-ring stuffed into that hole.
With the engine warm and at a slow idle (900-950 rpms), turn the mixture screw inward (clockwise) very slowly until the engine starts to stumble. Turn the mixture screw outward until the engine begins to run smoothly plus an additional 1/8th turn. This is an area where I have problems. Most carbs will die out if the needle or adjustment is run in too far. It’s obvious, but not with this puppy.
I could close the jet or adjuster and the bike ran fine. So I backed it out two turns to where they said it generally ran the best, and it did. But it was just slightly smoother and faster, so I prayed for surf.
Here’s what the boss said: If unable to find this stumble point, start around 1 to 1 ¼ turns out, then start over. Maintain the low idle speed and repeat the adjustment anytime you change the idle speed. Blip the throttle a couple of times and observe the results. If the engine responds quickly without backfiring through the carb, you have your idle mixture set correctly. If hesitation (cough) occurs, then adjust the idle mixture screw out another 1/8 turn. Adding an extra 1/8th turn outward is often desired once the sweet spot is found.
I find this interesting. How the hell am I ever going to find a sweet spot at 2 turns if I start at 1.75 turns and turn it inward?
Get this from CV: Normally the idle mixture screw should only be between 2 and 3.5 turns out. Most Twin Cam and Evolution engines have a mixture screw sweet spot at approximately 2 1/8 turns out from the seat. Adjusting the mixture screw out too far will result in an overly rich fuel mixture at low RPMs. Avoid tuning the idle too rich, thus leading to poor gas mileage and foulded plugs. Only set mixture just beyond the point of hesitation (cough).
Carburetor Testing:
Here’s the fun part. I took it for a ride. They want to know how it responds off idle. This is another interesting point. This adjustment meters fuel and air between the idle jet and the main jet, which is perfect. Many carbs cannot be adjusted except at idle, which doesn’t give you much flexibility.
This also explains why the CV bosses want to deal with the stumble. It will idle no problem, but we are messing with the area between idle and the main jet. So if it stumbles or is sluggish off idle, we have an issue. Let’s get back to the CV instructions.
Take your newly tuned bike for a ride and note how it responds off idle. To smooth hesitation, adjust the mixture out another 1/8 of a turn. Smoke from the exhaust or a feeling of sluggishness off idle indicates the mixture is too rich. Smooth throttle response from a start indicates tuning is complete.
If your engine still runs lean, repeat the jetting instructions and install a pilot jet one size larger. Jetting beyond the recommended size (in the table) is an indication of a vacuum leak at the manifold. Testing the main jet is slightly different, since this jet does not operate below ¾ throttle and requires an open area to allow the engine to be run at full throttle in 3rd or 4th gear.
Most riders are satisfied with the recommended jetting and don’t need the full throttle tuning run.
Main Jet Tuning Using the full-Throttle method.
With the engine warm, run the bike up to 45 plus mph and maintain a steady speed as close to full throttle as possible. Back of the throttle by only 1/8 turn and feel how the engine reacts.
1. If the engine reacts with a slight pull or boost, the main jet is too small (lean). 2. If the engine emits any black smoke, feels sluggish at full throttle, or backfires through the exhaust, the main jet is likely too large.
3. A soft steady high RPM deceleration indicates the main jet is correctly sized.
If in question, a plug test will need to be performed to determine if the main jet is lean or rich. This requires up to 100 miles of riding to witness substantial evidence of plug change. The jetting table should provide correct jetting for almost any situation when combined with the other components of the kit.
Ken Mendelson
CV Performance
Roger Goldammer
http://www.goldammercycle.com/contact
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