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. That is, if you know what to do and what size jet to install, which brings us to the reason for this article.
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 detonationjust to name a few problems.
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. A ratio of about These numbers represent the ratio of air to fuel in the mixture. 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. A 45 slow jet has a. However, if you made a big alteration to the engine, like punching an Sportster out to cc, 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. Rejetting a CV Carb. American Iron March 15, Why change the jets? To read the full story on just how to jet and tune your CV carb—plus make your own home dyno!
Tuning a CV Carb Part 2
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Appaloosa v2. Custom Motorcycle Feature. Next Up. American Iron.H-D has sold almost two million CV equipped bikes since then and its only the fact that all new Harley models have fuel injection that they've stopped being produced. In stock form they are a decent carburettor, sporting a 40mm throttle bore and a 39mm venturi. In standard trim it was only the early California models that had any problems, with an off-idle hesitation caused by a lean slow-speed jet to beat emission requirements.
The problem was easily cured however by changing the slow jet to a The engine was a stock 88" Twin Cam with a set of Branch 4 heads installed, while everything else on the bike was stock.
The results clearly show that a properly tuned CV carburetor makes excellent power on mildly modified engines. A Twin Cam with a stock CV carburetor develops 60 to 63 hp.
With a free-flowing air filter, performance mufflers and a larger main jet the Twin Cam motor will easily produce 70 to 75 hp. On a hot-rodded Evo or a Twin-Cam motor higher compression pistons, performance cam, aftermarket ignition, high flow air filter and mufflersthe standard CV carburetor will become the limiting factor in the engine builders horsepower quest.
There is more power lurking inside the CV however and there are several ways to improve its potential; buy a pre-modified carburettor, have yours modified by a reputable performance shop or get down and dirty and modify it yourself. The shops who work a lot with the CV carburetor are boring out the throttle body to 42 mm, installing a larger butterfly, contouring the slide and drilling bleed holes in the butterfly.
These carburetors have the highest potential output but most of these operations are beyond the abilities of most blokes out in their garage.
The amount of power between a professionally modified carburetor and a re-jetted stock CV carburetor can be as much as Slide Spring 2. Slide Diaphragm 3. Vacuum Slide 4. Vacuum Port 5. Needle Jet 6. Main Air Jet 7. Main Jet 8. Slow Jet 9. Idle Mixture Adjustment Screw They've bored out the throat to 42 mm and blended the venturi for a high velocity flow.
A new, larger throttle butterfly will be installed before they're finished, as will a new fuel inlet that has a larger capacity and the ability to rotate through degrees.The item selected will only fit the motorcycles listed.
Please select your bike here to see all the parts that fit your motorcycle. In Stock. Free Shipping. Carb Jets: Main Jets Diameter: 2. Usually ships in business days. This item is temporarily unavailable. A carburetor jet is a key part of any fuel system as it directs fuel into combustion chambers of the engine.
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Hassle-free exchanges.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. 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.
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 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 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. 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. 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.
Harley-Davidson used this jet needle in early Sportsters. Tuning a CV Carb Part 2. American Iron June 1, Previous Article. Next Article. Recent Posts. Indian Announces Launch of Click. Ride Program News. Indian Supports Local Businesses News. Appaloosa v2. Custom Motorcycle Feature. Next Up. American Iron. Current issue Columns. Buzz Steve. Back issue. American Iron Garage. New Products Product Review. Install Parts. Facebook-f Twitter Instagram Youtube.
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Harley-Davidson Carburetor Jet Kits
As you can see, Harley carburetor jetting is a fairly simple procedure that under most conditions will yield greater performance when requiring a richer fuel mixture. Enjoy the Ride! Main jet replacement should be reserved until after the slow idle jet is replaced and mixture is tuned unless a lean condition is apparent during full throttle.
Harley carburetor jetting can be accomplished with minimal mechanical knowledge. Tools to perform this task include just a simple set of screwdrivers. Remove the 4 screws securing the bowl to the base of the carburetor and remove the bowl. The jet size is stamped into the top of the jet i. Be careful not to strip the head of the jet. Pilot Jet location. With a flat head screwdriver unscrew the Main Jet from the brass needle jet holder aka Emulsion Tube.
Note the jet size stamped into the top of the jet i. There is no need to remove the emulsion tube unless required for cleaning. Replace the Pilot Jet with one size larger. This of course assumes that you are starting out with the stock jet size. If your stock jet was a 42 the next size larger will be 44 or Only increase the jet sizing one size at a time to avoid an overly rich idle. Main Jets are sized incrementally by 5, so if your stock jet was a the next size larger will be Avoid installing jets that are too rich as this will create a sluggish feeling at full throttle as well as contribute to plug fouling.
Many Twin Cam models except California are already appropriately jetted with a main jet that will allow for a good starting point for tuning. As mentioned, proper mixture adjustment is key to proper jetting and should be performed first.
Reinstall the bowl making sure to align the accelerator pump shaft and rubber boot.