Rejetting a CV Carb

Rejet and tune the stock CV carb on an Evo, TC, Sportster, or Big Twin

by Chris Maida

This story appears in American Iron Garage Issue #218, available online now or on newsstands 3/20. Subscribe today to get your hands on all our DIY tech stories every two months for less than half the cost of the cover price.

All modern Harleys are fuel-injected, but not so long ago Harleys came fitted with a CV carburetor, the best carb H-D would ever put on its bikes. Though some owners would swap them out, the CV, with the proper jetting, can handle most moderate performance modifications. And though some believe changing the jets on a CV is difficult, it’s really an easy process. That is, if you know what to do and what size jet to install, which brings us to the reason for this article. Since American Iron Garage readers work on their own bikes, we felt a How-To on CV carb jetting was in order for owners with CV carb-equipped bikes. Of course, the carb’s jet needle, be it adjustable or fixed, can also be a factor in proper tuning, so we’ll cover messing with the jet needle in the next issue (AIG #318).

Why change the jets?

Any time you make a modification that’s intended to improve the performance of your engine, you need to rejet the carb to correct the air/fuel mixture. That’s because the purpose of an engine performance mod is to get more air flowing through the engine, but the engine doesn’t run stronger because it’s getting more air. The increase in air allows you to add more fuel to that air and increase the amount of fuel and air (actually oxygen) that’s being burned over the pistons. That’s what gives you more power. When you only increase air flow in and out of the engine, you lean out the air/fuel mixture. When the mixture is too lean, the engine will take forever to warm up, cough out of the carb when you try to accelerate, surge when you try to hold a steady speed, and ping like hell on a hot day (detonation), just to name a few problems. Lean the mixture out a lot and you’ll burn a hole through the top of your piston. And if you have to hold the throttle open to keep the engine running while you watch the header pipes turn red where they attach to the heads, the engine is way too lean! As surprising as it may be for some owners, an air cleaner or exhaust change will usually lean out the mixture. In fact, just swapping out the stock filter element for a freer-flowing K&N one will sometimes give an engine a little case of the coughs off idle. In fact, if your engine runs fine other than coughing through the air cleaner/carb when idling or when accelerating from a stop, your problem may just be an idle mixture jet adjustment. You may not need to change the main and/or slow jet.

Harleys run stronger when they’re a bit on the rich side of the stock settings, which are 15:1 to 16:1 from idle to a little more than half throttle. They run best when there’s about a 14:1 air/ fuel ratio for throttle settings from idle to about three-quarters throttle. A ratio of about 12.6:1 or so is best for wide open throttle (WOT) performance.

 

 

 

 

 

These numbers represent the ratio of air to fuel in the mixture. A 14:1 ratio is when the air/fuel mixture consists of 14 parts air to 1 part fuel. A slow or main jet’s size is the number that’s stamped right onto the jet. The larger the number, the bigger the hole in the center of the jet, so more fuel is added to the air coming through the carb. For example, a 40 slow jet has a .40 square millimeter hole. A 45 slow jet has a .45 square millimeter hole, and it flows about 125 percent more fuel than the 40 jet, so make small changes when you’re trying to dial in the jetting. However, if you made a big alteration to the engine, like punching an 883 Sportster out to 1200cc, make the first main jet change two sizes larger (richer). Then, if needed, richen or lean out the jetting one jet size at a time until the engine runs correctly. This is safer than running the engine very lean during the jetting tests after a major engine alteration and causing detonation, overheating, and possible engine damage.

To read the full story on just how to jet and tune your CV carb—plus make your own home dyno!—you can purchase Issue #218 RIGHT NOW online or on newsstands 3/20.
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