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[Datsun 1200 encyclopedia]

Bike Carbs

From Datsun 1200 Club

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Category: Fuel System

Motorcycle carbs, bike carbies. Flat-slide, constant vacuum or variable venturi carburetors.



TUNING is critical. Bike carbs as-is run poorly on automotive engines. But with the right jet or needle selection they can run very well.

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bike carbs on A12Y engine
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adapt carbs from your favorite motorcycle
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Suzuki GSX-R (aka GSXR)
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Yamaha YZF-R1 (aka R1) carbs are a fad in USA as of 2012.


Adapter Kits

* Tube & Flange adapters
* Cast/Machined adapters
* Welded adapters
* Twin Weber manifold

Tube & Flange
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Machined adapter
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Cast adapter
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MSR carbs fit any DCOE type manifold
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Fits on modified standard manifold
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Custom Manifolds

Fabricate your own manifold

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Keihin CV

The Keihin brand were used on nearly all brands of bike, Yamaha, Honda, Suzuki, Triumph etc.

CV (Constant Velocity) is an SU-type carburetor, with a vacuum slide raising and lowering a jet needle.

CV34 discussion
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Harley 41 (1988-up)
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Honda 28
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Keihin FCR

Keihin Flat CR (FCR)
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rallytwit made 73.22 RWHP on his A-series.

39mm FCR carbs with a near standard GX motor; ported head, headers 2 inch exhaust and 14lb flywheel. As far as reliable FCR carbs are stone axe reliable. For street (or racing) use you would have to do some tuning but it's not a big deal. I only needed an hour of dyno time to get the car 95% dialed. My only problems were because the carbs were on 175HP A-series powered sports racer so the jets were way off what my motor needed............lots of guys use these on street bikes and they are standard equipment on MX bikes. 32MM would likely be better for a street car, the 39's on my car border on over-carbed.

As fars as how the motor runs they are the bomb the motor sings......revs like mad and instant throttle response.
Flatslide FCR Sidedraft
Flatslide FCR Downdraft
CR-MX off road ruggedized
Roundslide CR

FCR41 kit for oval-port A-series
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Cast Adapter Kit

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Fumiya Kit

Datsun 1200 enthusiasts Fumiya sells these

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Matsuoka MSR

Matsuoka MSR carbs
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Matsuoka MSR are like FCR but about $1000 US cheaper. They use the same sliding throttle valve design. They're made by Mikuni. Manifold can be any DCOE (Weber/Dellorto/Solex) type.

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new, jetted to your spec: 96K yen (1230 AUD) or with manifold and linkage closer to 200K yen.

Feral said:

When I say just about as good as injection I mean the low speed response from the Mikuni flat slides is incredible. It will still not be as accurate as any well laid out injection system, but really do work very well
MSR Flat Valve 42 mm 238,000
MSR Flat Valve 45 mm 258,000
MSR Flat Valve 48 mm 298,000


Mikuni also made constant velocity (CV) carburetors. The venturi size is what sets the HP characteristics. CVs use the same bore size as venturi size except for a fixed obstruction to generate vacuum.

Mikuni BS series. For example 1980- Suzuki GS1000 came with BS34 carbs (34mm) for 90 HP. A 1981 Kawasaki KZ1000 fitted with BS34s made 102 HP.

Mikuni BS series
variable venturi
constant velocity design
Side draft
Mikuni BD series 

These being CV carburetors are similar to SU carburetors. They have a traditional butterfly throttle but the actual airflow is controlled by the round slide (piston) that floats up by vacuum action. It is opposed by the dome spring.

Flat Side
* Mikuni RS
* Mikuni HSR
* Mikuni TM

Flat Slide
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Mikuni VM - late model round-slide series

VM28 may be ideal for a stock A12 (see Carburetor Sizing) - and are only $100 each.

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CV constant velocity

BDST36 is the famous Honda CBR600F3 carburetor 85-100 hp (F1 used 32 mm carburetors)

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R=Radial slide

BDSR40 is the famous Yamaha R1 carburetor 150hp


Mikuni BS32 was chosen by Nissan to power the A-series engine in the 1970 Sunny Cab (model C20).

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16010-G0301 BS32-5A [from '69-7]
16010-G0302 BS32-5B (from '70-1)

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16189-G0320 JET-slow No. 45
16036-25522 JET-slow air bleed No. 210
16296-G0320 JET-air, 0.8 diameter
16033-G0340 JET-main R170
16295-G0320 JET-enrichment, No. 50
16346-G0320 SPRING-diaphragm
16353-G0320 SET-needle
16354-G0320 NEEDLE-jet

16546-G0300 Air Cleaner ELEMENT
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 90 hp 1980- Suzuki GS1000 
102 hp 1981 Kawasaki KZ1000
SX650 1989-1984
* 132.5 main jet
* 2.5 pilot jet
* Y-O needle jet
* 5HX12 needle

BS34s are rather forgiving and will at least run most engine set ups with stock jetting. W

The pilot air jet is removable, and the main air jet is fixed.

3 main circuits of the carb 
* slow speed or idle (mix screw and pilot jet)
* midrange (slide, slide needle, and needle jet)
* high speed or main (main jet) 

These all overlap the circuit next to them and changes made to one will slightly effect the other

Float height on the brass floats is 27.3mm. For later units with plastic floats, 22 mm. +/-1mm. BS34 are are fitted with drain valves opened by a screw, with a hose nipple on the outlet. To read fuel level, attach the sight tube to the outlet and set vacuum petcocks, if fitted, on "Prime," and open the screw until fuel flows into the sight tube. With the motorcycle level, hold the sight tube upright beside the carburetor body. Fuel should rise in the tube no higher than the lower lip of the carb body where it overlaps the float bowl, and no lower than 2 mm. below it.

The pilot circuit fuels the engine in low-speed operation, when the slide is lowered.

real Mikuni jets are availible . Get them from the stock size up to two over on the pilots and up to three over on the mains. Follow the guide and test them to see what your engine needs.


Tuning kit
* Pilot jets
* Needles
* Main jets

BS34 Jet Needle

5IX11 richer
5HX12 leaner

Needle Jet

Needle Jet

Yahama 1200

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Photo Index

fcr,msr,keihin,cycle carb,bike carb

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Photo Uploads

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