A No-Budget Fix For CV Carbs

Written by Stick Fentzlaff

Taken from American Iron Magazine #100


Does your stock Harley cough and spit through the carb, especially when you pull away from a traffic light? Do you have to wait forever for it to warm up?
That can be fixed. Your CV carb will start easier, warm up faster, run smoother and go like hell when you grab a handful of throttle. And the best part of all is that it will cost you nothing.
The first problem to be addressed is the lean condition as set at the factory. Take a look at your spark plugs and you'll see what I mean. Unless you've been using your choke too much, they'll probably be very white. What we will is get at the air mixture screw and open it up a little. The screw is sealed from the factory but it's no big deal to gain access.
Second, we're going to drill out the air hole in the slide. The will make the carb more responsive and run richer. The air lets venturi pressure up above the rubber diaphragm in the top of the carb. By making this hole bigger, air will get up there faster, making the slide move and down faster.
Sounds easy, doesn't it? Well it is...and this simple operation works. I've done it countless CV carbs and every rider loved it.
Before we get started, be aware of 1988 XL carbs. 1988 was the first year Harley used the CV carb and it wasn't so hot. The problem with them that year was that they didn't have an accelerator pump, so the carbs are not as responsive as they should be. The Modifications will greatly improve the '88 XL's carb but not as much as it will improve the carbs with the pump. After 1988, Harley had the pump and these work very nicely.
First, remove the air cleaner cover and air filter. Then remove the backing plate from the carb and the heads. Be careful when you back out the three Allen head screws that go into the carb. Loosened them a little at a time, alternating among the three. They are designed to stay attached to the backing plate in case they ever come loose.
Now pull the carb loose from the manifold but be careful of the choke cable and vacuum advance hose. Remove the vacuum hose to give you a little more room to move the carb around. On Sportsters, you have plenty of room; on Big Twins, you'll have to watch out for the gas tank. You don't have to remove the throttle cables or the choke cable, but if you want a little more room to work, remove the choke cable from its mount on the left side of the bike. Note; the brass colored nut is 17mm. Be careful. It threads onto plastic.
Look underneath the carb behind the float bowl and you will see a little tower with a plug in the end. This is where the air mixture screw is hidden. There are a couple of ways to get at the screw. You can drill a hole in the plug, screw a sheet metal screw in and pull it out with the pliers or you cut the end of the tower off about 1/8" from the end. DO NOT try to cut it with a chisel. The may have worked on the old carbs with the screw on tip, but it will not work on CV carbs. Be very careful no metal shavings get in the carb or manifold.
Once you've gained access to the air mixture screw, turn it out counterclockwise 1/4 turn to make it richer. This is a significant adjustment and it's a good starting point.
Now it's time to remove the top of the carb and get at the vacuum piston (slide). Remover the Phillips head screw on the right side of the carb that holds the throttle cable bracket. This screw will be very tight and nearly impossible to break loose with a screwdriver. I favor a small pair of vise grips to break it loose before using a screwdriver. Newt, remove the four screws that hold the top on the carb. Move the cable bracket out of the way and lift off the cover. Remove the spring and lift out the diaphragm. You need to turn it upside down to remove the spring seat and needle. You will see a small hole in the bottom of the slide next to hole the needle goes in. This is the vacuum port. Drill this hole out to a #30 drill size (.128"). Be very careful not to tear the rubber diaphragm during this operation. After drilling the slide, blow off any metal shavings from the slide and diaphragm.
Now it's time to put everything back together. Put the needle back in the slide and install the spring seat. Lower the slide into the carb using your free hand to guide the needle into the emulsion tube and keep the slide from free falling the carb. Don't worry, the slide will go in only one way, so you can't get it backwards.
To get the diaphragm to stay in place while you put the cover back on, take on hand and hold the slide up, like full throttle, then fold the rubber down toward the carb. Now it will be easier to get the lip of the diaphragm in its groove in the top of the carb. Install the spring and top cover and be very careful. It's important to get the lip of the diaphragm in the groove in the top of the carb. Sometimes it helps to wiggle the cover when you get it almost all the way down. Once on, you let the slide go, install 3 of the 4 cover screws (leave the cable bracket screws for last), and install the bracket screw on side of the carb next.
Sometimes the idle screw hangs up and you have to move three throttle partially open. Only after you get this side screw in should you install the last top cover screw. Move the slide up and down, insuring it moves freely.
Install the carb, vacuum advance hose and backing plate. Make sure the choke cable wasn't pulled out during the procedure and check its operation. If this cable isn't all the way in, you see white smoke often only cylinder, That'll drive you nuts.
Twist the throttle a couple of times to check the accelerator pump and throttle return. Install the air filter and cover. Start it up, but don't use any choke. One or two twists of the throttle will be fine, unless you have an '88 XL, then use 1/2 choke. The less you use the choke, the less plugs will get fouled, so warm up the engine with the throttle. The engine might still pop through the carb a little during warm-up but don't panic. The motor will warm up much faster than before. Once you feel the rocker boxes getting warm, it's time to adjust the idle.
Take the bike out for a test ride and get it up to operating temperature. Check for popping through the carb during your ride. If it still pops, open the air mixture screw another 1/8 turn and try it again. Don't forget to recheck the idle after the motor is at operating temperature.
Jetting will depend on your exhaust system and type of air cleaner. Keep in mind, when making changes in jetting, change only one thing at a time. If the change is better, or worse, you will know what caused it. If you change two or more things at the same time, you won't know which changed helped, or hurt. You almost never have to change the slow speed jet. If you have drag pipes, you'll probably have to go up one size in the slow speed jet. Otherwise the air mixture screw will do the trick. If you have to open the screw up more than two turns from seated, go up one size on the slow speed jet and return the mixture screw to one turn from seated. Adjust the air mixture screw accordingly.
Once you think the carb is correctly set up, put at least 100 miles on it and take a look at the plugs. They should be fairly white or off-white. You don't want your plugs to be tan. If the spitting has stopped, you probably have it right. Remember, Evo's like to run lean.

If you've enjoyed this article or know of others like it,
Email me.

Back to Henry's Home Page

Back to Homepage