Tuning a CV Carb Part 2
By Chris Maida
As stated in Part I of this carb tuning series, though all modern Harleys are fuel-injected, not so long ago Harleys came fitted with a CV carburetor, the best carb H-D ever put on its bikes. In Part I, we covered how to dial in the jetting by adjusting the idle mixture and speed settings and swapping out the slow and main jets. We also covered why you need to rejet a carb whenever you’ve changed out the stock air cleaner, pipes, or installed performance cams, etc., as well as how to tell if you’ve got the air/fuel mixture settings correct, so we won’t cover all that again. This time around, as we promised, we’re going to show you how to adjust the jet needle, which is located on the carb’s vacuum piston (slide).
The Jet Needle
The jet needle is the straight metal needle that’s on the bottom of the carb’s vacuum piston located in the throat of the carb. You’ll see the jet needle when you lift up the slide. The jet needle controls the flow of fuel from the float bowl through the main jet, needle jet holder, and the needle jet that the jet needle moves in and out of, and into the throat of the carburetor. As we showed you in the last issue, the main jet is located at the bottom of the carb body, in the bottom of the needle jet holder, and is immersed in the fuel in the float bowl.
When you look closely at the jet needle you’ll see that it has a straight section at the top and a tapered section on the bottom. The diameter of the straight portion of the jet needle controls the flow of fuel from just after idle to a bit past one-quarter throttle. At this point, the tapered section of the needle comes into play and controls fuel flow to about three-quarter throttle. After that, the jet needle is so far out of the needle jet that it no longer affects fuel flow. The size of the hole in the main jet is now the only thing regulating the flow of fuel into the carb’s throat, into the air stream flowing through the carburetor, and into the engine.
The standard way to make the fuel mixture richer (add fuel) is to raise the jet needle a little higher than stock so the tapered section of the jet needle comes out of its hole in the needle jet sooner. This allows more fuel to come out of the needle jet when less air than normal is flowing through the carb throat, which richens the mixture. If you lower the jet needle, the tapered section will come out of the needle jet’s hole later when more air than normal is flowing through the carb throat, which leans out the mixture.
If the jet needle you pull out of your carb has five slots in its upper end with a C-clip in one of the slots, this is an aftermarket jet needle. One of the previous owners swapped out the stock jet needle for this one. Moving the location of the C-clip into a higher slot will lower the needle and lean out the fuel mixture. Putting the C-clip in a lower slot will raise the jet needle and richen the mixture.
If the jet needle you pull out of your carb doesn’t have any slots in its upper end, you have the stock jet needle. When you carefully remove the jet needle from your carb look down onto the top of the slide where the jet needle was, and see if there’s a small flat washer lying on the top of the slide. Also check the jet needle to see if there’s a small flat washer stuck to the stock jet needle’s flat head. This extra flat washer is not a stock part. It was put there by a previous owner. Putting a washer under the head of the stock jet needle, between the jet needle and slide, is the way to raise a stock nonadjustable jet needle and richen the fuel mixture.
There’s another way of making the air/fuel mixture richer, and I call it Joe Minton’s way. I never heard of this method until Joe wrote about it in a How It Works he did for us in the May 2011 issue of American Iron on tuning a CV carb. Instead of raising the jet needle, be it a stock or aftermarket one, to add more fuel sooner than normal to the incoming air, Joe changes the jet needle to an earlier, narrower diameter H-D jet needle. By installing a narrower jet needle, more fuel is allowed to pass by the jet needle’s straight section, which will richen the mixture at all throttle settings that the straight section of the jet needle controls. Harley-Davidson used this jet needle (#27094-88) in early 1200 Sportsters. And, yes, it’s still available from the dealership, but it usually has to be ordered. This needle is only a few thousandths of an inch thinner than all the other needles used by Harley, but that’s enough to richen the air/fuel mixture for most performance air cleaner and exhaust upgrades. According to Joe, if you make this jet needle change, there’s no need to mess with the main or slow jet. And although I have yet to try his method, he’s done it many times and says it works perfectly.
To read the full story on How to adjust the jet needle on an Evo or Twin Cam Sportster or Big Twin’s stock CV carburetor—you can purchase Issue #318 of American Iron Garage RIGHT NOW online or on your local newsstands.
Subscribe to American Iron Garage for premium all-tech and DIY content just like this every two month.