The Keihin CV 40 carburetor used on Harley's from 1989-2006 is an excellent performing carburetor once it is set up correctly. One of Cyber Dave's pet peeves was getting his arm burned on the cross-over pipe when he was setting up CV carbs on the Dyno at Direct Parts. He tried using thick rags, welders cloth and even some exhaust matting but it he was always burning his forearms. Another pet peeve was the linearity of the adjustment. A seemingly small turn of the idle screw often changed the mixture considerably. Several counties in Arizona have an EPA mandated idle exhaust "sniff" every year. When customers were running EVO's with free flowing exhaust, intakes and performance cams, adjusting the idle mixture prior to having the bike sniffed was important. in order to pass the emissions test you have to lean out the mixture significantly.
While the engines have changed significantly he says the tuning basics remain the same. When you have the jetting set properly, you will get snappy throttle response, the best mileage possible, easy starting and your engine will sound much "crisper" than it does with just using the "guess a jet" method. A properly sized pilot jet will do more to increase off idle throttle response and part throttle mileage more than either the needle or main jet will.
I can do a pretty good job setting up a CV but, CD was a wiz at it. Here are a few tips he impressed upon me.
- No leaks in the intake tract. An intake leak makes adjusting the idle mixture properly impossible.
- The ignition system must be in good shape, new or nearly new plugs, good wires, coil and properly timed.
- An engine with poor sealing piston rings will be difficult or impossible to adjust properly.
- Jetting must at least be in the ball park to start with. Too often, "tuners" throw a large pilot jet into the CV to overcome the "lean pop". This large pilot makes leaning the idle out enough to pass emissions difficult to say the least. If you are not sure of your jetting, here is a revised article on tuning the Keihin CV and other carburetors.
- Setting up a CV properly requires some basic mods. Sanding and smoothing the radius on the leading edge of the slide, drilling the slide vent hole to 1/8" speeds slide response to throttle changes. Using a lighter slide spring and needle from a Sportster and installing the Hi-Flow brass fuel inlet elbow and the Dakota kid low turbulence accelerator pump pedestal were all standard mods he made.
After burning his arm one to many times, CD had enough and invented the EZ-Just needle. Since the Keihin butterfly carburetor was still pretty abundant, he designed an EZ-Just for it also.