• All data compiled from HD Sportster Parts Catalogs unless otherwise noted
• All CV models come equipped with accelerator pump unless noted.
• The Serial Number on CV carbs is labeled on the adjacent side to the accelerator pump linkage

• Used in the Screaming Eagle Package line from HD as an Evolution XLH HI-FLO Carburetor & Air Cleaner.
• Adjustable Accelerator Pump stroke
• Incorporated a special intake manifold to carb spacer that had to be drilled and tapped (1/4 x 28) in the side for included Voes connection adapter
• Required a 1987 replacement style intake manifold for 1988 and up models
• Required possible grinding of the right rear corner of the front cylinder head, due to interference with the carburetor throttle pulley and cable guides, for proper clearance ²⁶⁾

²⁷⁾

*in carburetor

• Accelerator pump rod length: A number of Screaming Eagle carb kits (91731-85A), packaged during March of 1988 (3/88 date code), contained carburetors with accelerator pump rods that were too short. The incorrect pump rods were only 2.35“ long, end to end, and completely disabled the accelerator pump and caused the engine to stumble upon fast acceleration. The correct length pump rod (27374-88) is 2.55”. ²⁸⁾
• Dealers were to check their parts stock and replace any pump rods that measured short and also check the pump rod length on any motorcycles that exhibited a stumble upon acceleration.

HSR Series:

• Basic Mikuni HSR Information
• Rebuilding a Mikuni HSR42
• Mikuni HSR Function and Tuning
• Choke Relocation on a Mikuni HSR

VM Series:

• Basic Mikuni VM Information
• Rebuilding a Mikuni VM-38
• Mikuni VM-38-9 Function and Tuning
• Mounting a Mikuni VM-38

HS-40:

• Basic Mikuni HS-40 Information

Other than exact replacement OEM carburetors, all Mikuni aftermarket carburetors and tuning components are sold “As Is” for off road / racing use only. ³¹⁾
They are not intended or approved for use on vehicles operated on public roads or in locations where applicable engine tampering and anti-pollution laws apply.

There are plenty of Mikuni and Keihin carburetors in the market that are not authentic. ³²⁾
They're not safe to use and will cause many problems to your bike.
If you're not sure if it's a genuine carburetor, the Sudco International site has a web page with pics of genuine and fake carbs.
Follow this link or you can call them directly to verify a seller. ³³⁾

• Mikuni website (HSR page)
• HSR Installation Guide
• Performance Guide includes:
  • Air Cleaner Basics, Exhaust System Basics, Cam Basics and Ignition Basics

• Sportster / Buell Installation Instructions PDF
• HSR42/45/48 Tuning Manual PDF
• HSR Choke Cable Installation Tips

• HS40 Tuning Manual & Parts List PDF
• HS40 Tuning Manual PDF

• Mikuni website (VM page)
• VM "Round Slide" Tuning Manual
• Rebuild Kit VM26-606 PDF

• PAT Upgrade Kit Installation Instructions PDF

• Genuine Mikuni jets for HSR series carbs

Video:

• IronMick's VM-38 Startup Video

• Testing an S&S Intake Manifold Spacer
• Installing a ThunderJet® on a Super E

³⁴⁾

Throttle shaft leak repair.
Counter-bore each side a few thousandths less than the O-ring thickness so it sits a little proud. ³⁵⁾
These are 1/4“ x 3/8” O-rings from a HF&T kit.
Then use a couple 1/4“ ID SS washers with a good synthetic grease like Super-Lube.
The return spring is on one side and you position the throttle lever on the other side.
So the washers just pinch the O-rings but still allow proper throttle movement. It should solve a shaft leak problem and can be easily maintained.
This was done on a milling machine with a mold ejection pin that fit perfectly.
To keep center, the pin was put in the spindle collet with an indicator as a confirmation of position after having moved the Z axis.
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There are 4 screws to take out to remove the float bowl.
Three obvious ones and the fourth one is one of the accelerator pump screws as in below.

Removing the accelerator pump.
The spring inside holds the diaphragm up until the rod pushes it down like a bellows.
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The metering circuits of these carbs are configured for the best 1/4 mile performance. No consideration at all was given for street operation or kick starting. ⁴¹⁾
These were race only carbs for HD dragsters only, not for street bikes.
That didn't stop anybody from putting them on the street bikes which may be where the bad rap these carbs carry came from.

The one below has the 'street kit' that was made later to address the missing street manners.
The kit is the bolt on choke and the 1” spacer between the carb and manifold.
After a while these 'G's were replaced by the next model (“GA” & “GB”).
They were a slightly re-metered idle/intermediate circuit to give some street-ability.
The 'GA' was identical to the 'G' except for the re-metering.
The 'GB' was a smaller throat (1-3/4“ at throttle butterfly) and more importantly a smaller venturi than the 'G'-'GA' (1-7/8” throat, 1-11/16 venturi).
The 'GA' was just passable as a street carb but it needed at least 70 ci to feed (if not it sucked).
The 'GB' actually was street-able on stock displacement bikes. It wasn't 'tame' though.
You had to cater to its narrow ''sweet spot“ that changed with air temp, humidity, barometer, and most of all float level (that constantly creeped higher).
The floats & inlets were still race parts, made to be high flow at small float drops, combined with the metal to metal inlet valve, again a race thing.
The low gas consumptions of cruising on the street, barely opened the valve.
So the mile after mile of street riding vibration wore all the stuff and the float level crept higher.
So if you were OK with constant tinkering with float levels and jetting that needed to be tailored to small atmospheric changes;
(still drag race main metering circuit) you had a carb that lit your bike up like no other. A redhead girlfriend of a carb.
This amount of tinkering and the devotion it took to be on top of it was too much for all but the most race oriented owners.
So, enter the 3rd models. The infamous “L series” (“GAL” & “GBL”).
These are actually totally different carbs from the predecessors as the 1st “street carbs” with real drag tendencies.
All metering circuits are made to be totally street-able as 1st consideration while leaning towards being as drag like as full street-ability will allow.
Boy did they work as intended.
But to the untrained eye, they look the same as the 1st two versions, so they get the same bad rep.
They also have the same fitments. 'GAL' needs 70” min. or they ain't street-able enough.
The 'GBL' is the product of all the previous lessons and it's unsurpassed for 900 or 1000 bikes.
So if the 'GBL' is so great how come it's so obscure? Maybe because, of the 5 versions of side bowls, S&S carbs are usually grouped as only one version.
(if you really knew there were 5 different carbs before the rare “Super A” , you are in a small group).
And 4 of the 5 work like crap on a stock street bike. So those 4 get taken off and sold a lot (adding to the rep).
The last 2 versions, the real L series units were sold from 72 to e75, and bikers being bikers, “if I'm gonna get a S&S, I'm getting the big one.
So not as many “GBL” got sold and the guys who had one didn't ever want to sell it. The one size fits all Super A & B was the response to guys buying the wrong size.
Little known fact: The venturi on a “GBL” is smaller than the venturi on a Tillotson. The Super B is the same as a Tillotson.

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These were 1-7/8” 'drag only' carbs. The 'G' is the very first S&S carb model ever sold to the public. ^{45)
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* How to adjust the Lake Injector for all throttle positions

• Originally made for airplanes, the Lake injector was a popular fuel delivery system in the 70s era. It was designed for the ultimate in performance with a minimum of tuning effort. Don't confuse the word “injector” with fuel injection, as these series of carburetors are not a fuel injector or a fuel injector body⁵¹⁾. The body design, plus the velocity stack in unison with the throttle slide, forms a venture which enables the injector to achieve a good fuel/ air mixture. ⁵²⁾
  • The manufacturer claimed a 15% - 25% or more increase in horsepower, unequalled acceleration, immediate starts and instant tuning.
  • Installation was a simple bolt on design with no modifications needed.
  • Suitable for 883s as well as 1000 model Sportsters although could be installed on Shovels, Pans, Triumphs and Hondas.
  • Body options for an extra price include, show chrome, bright anodized and 24K Gold.
• A true low head, nil pressure injector as does not depend on venture suction like a carb but just pure air mass flow, so it self corrects for great altitude manifold pressure changes.⁵³⁾
• You wont fine tune this nail in a tube, they run rich when the tanks full, and get lean as the fuel drops.⁵⁴⁾
• Many people have to plumb air supply to the tank with a 'pop off' to keep pressure head constant below 1 PSI.⁵⁵⁾
• The unit is gravity feed instead of having a fuel pump for delivery and it was prone to leaking if you left the petcock open.⁵⁶⁾
• Posa and Star are all very similar and found their way from small aircraft and onto bikes.⁵⁷⁾
• POSA, Lake Injector, Revflow and Aero Carb are all float-less, diaphragm-less carburetors.
• These carbs do not operate well on more than .5 lbs of fuel pressure yes that is less than half a pound of fuel pressure. These carbs do not vent any fuel vapor that may form in the body to the venturi as there is not float bowl well or diaphragm vent.⁵⁸⁾

• Bendix Stromberg used a unique identification scheme where a series of letters and numbers were assembled to identify and describe how a carburetor was constructed. The first two letters described the carburetor design, followed by a number that identified the size, and numbers and letters to identify specific design details and modifications.
• The first (two) letter(s) comprise a Basic Design Code that places the Bendix Stromberg carburetor in a design category, where:
  • A – Pressure Injection Carburetors
  • E – Pressure Injection Carburetors
  • NA – Float-Type Carburetors
  • P – Pressure Injection Carburetors
  • Q – Pressure Injection Carburetors
  • R – Fuel Injection Systems
  • S – Speed Density Metering Systems
• Example: NA-R9G
• Prefix- “NA” – for float-type carburetors followed by a “-”(dash)
• The next letter indicates type, where
  • S – Single venturi updraft designed for two, three and four cylinder engines in the 25~95 hp class. NOT fitted with economizer, acceleration pump or mixture controls.
  • R – Single venturi updraft, designed for engines of all types in the 50-400 hp class. All are similar in design and incorporate needle valve type mixture control, accelerator pump and economizer. Some bodies are interchangeable in this series and deferent design economizers are used.
  • D – Double updraft venturi, with the float chamber to the rear, was considered obsolete c.1930
  • U – Double updraft venturi, with the float chamber between barrels. See specific models for details.
  • Y – Double vertical venturi (updraft), with two float chambers fore/aft of barrels. See specific models for details.
  • L – Inverted, down draft venturi.
  • T – Triple venturi, double float chamber fore/aft of barrels. The NA-T4 series is the only known model. Designed for use on Wright J-5 and other 9-cylinder radials, where each venturi feeds three cylinders. It has float similar to Y series, but with back suction mixture control. No economizer or accelerator pump.
  • F – Four-venturi with two separate float chambers. The last letter and numbers indicate a specific model.

⁵⁹⁾

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