FXR Carburetors, Graded

Fixed 38mm venturi, butterfly throttle, accelerator pump. FXR part numbers include 27470-80, 27029-86 and 27029-88.

A cable, a butterfly and a fixed venturi. The accelerator pump covers the lean spot when you crack the throttle. Units from 1979 carry the emissions changes: leaner jetting, and from 1980 a sealed idle mixture screw (Sportsterpedia, Keihin butterfly ID). Fine on a stock Shovel or an early Evo. A built motor outgrows the venturi fast. If yours runs right, rebuild it and ride it. If you are adding cam and pipes, this is the carb to replace.

40mm bore, butterfly with a vacuum operated slide and needle, accelerator pump. CVO part 27492-96B, 1.70mm main.

The cable turns a butterfly. Engine vacuum lifts the slide. That makes it smooth and forgiving, and it makes it lazy. Harley shipped it lean in the middle and rich on top for emissions (Hot Rod, May 1995). Sportsters got the CV in 1988 without a pump and with one from 1989 (Sportsterpedia, carburetor history). On the bench it flowed 159 CFM, the least of any carb in the test (Carb Shoot-out flow test). On a stock motor that commutes it is hard to beat for mileage and manners. On anything else it is the first thing to fix.

Larger slow and main jets, a different needle, and the slide damping port opened up.

The cheapest real improvement on an FXR with an air cleaner and pipes (Hot Rod, May 1995). It cures the midrange stumble and wakes up the slide. It does not add airflow. The body still flows 159 CFM, so a built engine will outrun it (Carb Shoot-out flow test). The Thunderslide kit is sold for closed course use or bikes that predate emissions rules (Dynojet Thunderslide kit, Evo). For a stock or mild FXR, start here before you buy anything.

44mm CV, same layout as the CV40. Needs SE intake manifold 29636-99. Tuner kit 27419-99.

A bigger CV with the same parts logic. Harley calibrated it out of the box for an engine with an SE cam, air cleaner and exhaust. Pre-1990 Evos also need 1990 and later throttle cables, it does not work with cruise control, and Harley labeled it for race use (H-D instruction J01695; H-D instruction J01771). We found no bench number for it. On a cammed 80 inch Evo it is the low drama upgrade: CV manners with more bore. It was never factory equipment on an FXR.

42mm smoothbore flat slide on roller bearings, adjustable accelerator pump. Uses stock throttle cables.

The throttle lifts the slide directly and the slide rolls on bearings, so response is instant and the return is light. The Easy Kit keeps the stock manifold and cables. The pump is tuned at the nozzle, #70 standard, and Mikuni wants the pump stroke finished by 2/3 to 3/4 throttle (Mikuni HSR catalog; Mikuni HSR42 to 48 tuning manual). Pilots run 15 to 50 and mains 140 to 240, so there is a jet for every engine (Mikuni HSR42 to 48 tuning manual). The flow test author found the HSR the easiest carb of the group to tune (Carb Shoot-out flow test). For most Evo FXRs, this is our pick.

45mm smoothbore flat slide on roller bearings, adjustable accelerator pump.

Same carb, more bore (Mikuni HSR carburetors). It flowed 215 CFM bare and 224 with its backplate, more than a Super E and close to a stock Super G (Carb Shoot-out flow test). It wants an engine that can use it: cam, heads, more inches. On a stock 80 inch motor the 42 is the better carb.

47.6mm bore, 39.6mm venturi, two fuel circuits, adjustable accelerator pump, enrichener in place of a choke plate.

No slide and no diaphragm. A butterfly, a venturi and jets, which is why they last. It flowed 197 CFM, 24% more than the CV40 through a venturi 4% larger (Carb Shoot-out flow test). S&S points anyone unsure toward the E over the G (S&S instruction 510-0250). Jetting changes take some reading. In our judgment it is less crisp off idle than a flat slide. On a stock or mild Evo FXR it is the carb that runs and keeps running.

52.3mm bore, 44.5mm venturi, two fuel circuits, adjustable accelerator pump.

The big S&S. It flowed 225 CFM stock and 240 modified on the bench, and the tester found it harder to tune and a little worse on fuel than the smaller carbs (Carb Shoot-out flow test). S&S says it is for modified engines of 100 inches or more and not for small, low compression engines (S&S instruction 510-0250). On a big stroker it is a proven carb. On a stock FXR it is the wrong one.

47.6mm throat, 39.6mm venturi. A "B" is cast on the throttle return spring side.

The older S&S. On paper it has the same throat and venturi as the Super E (S&S instruction 51-1006; S&S instruction 510-0250). We found no bench numbers for it, so these grades lean on its size and layout and carry less weight than the others. If one is on your bike and it runs, there is no reason to swap it for an E.

Metering rod design, not a CV.

unverified These grades rest on owner reports only.

Owners describe it as hard to get right across the range, with heavy throttle return springs, and parts are scarce now (HDForums, tuning QwikSilver II; HDForums, Qwik Silver II help; V-Twin Forum, Quicksilver 2). Some owners like it on fuel. If one came on the bike and it runs, ride it. We would not buy one.

Flat slide with a single metering rod in place of the needle and jet set, adjustable idle, power jet.

unverified No independent test found; maker information only.

Lectron's case is that one rod does the work of a needle and main jet (Lectron HD-X). Flat slide response is the draw. We found no independent bench or dyno test, so these grades rest on the maker's information and the design. Tuning parts come from one source.