JRC Engineering offers a variant of the popular and efficient Keihin “PWK” carburetor as a replacement for Amal Concentric. The PWK is a much more technically advanced design than the Mikuni VM round slide, and decades ahead of any Amal design including the Concentric Mk II. This is not a “knock-off”, but a

legal and licensed product approved by Keihin. Click on the logo for their site.
    At present, 2 body sizes are available: 26mm, suitable for smaller singles and 500cc twins and 750cc triples (as well as 6T and other low-performance larger engines), and 30mm, suitable for all larger singles and 650cc & larger twins (and modified 500cc twins and 750cc triples).
    The most significant difference between the JRC and the PWK is that the original PWK is only offered in spigot (clip) mount, and requires an adapter for use on any engine with a flanged manifold, whereas the JRC has the Amal generic 2-bolt flange pattern, making it a suitable replacement for all Concentric Mk I (and potentially 1955-* Monobloc & *-1954 pre-Monobloc 275, 276, 289, &c.).
    The stock Triumph throttle cables for 1969-72 T120 650 and 1973-78 T140 750 fit. The free length of the cable required is the same as that of a Concentric. JRC offers carburetor tops with or without the threaded adjuster.
    The JRC body size is much smaller than the common Mikuni VM (round slide) replacement and therefore a better fit, especially on single-carburetor installations such as TR6, T100C &c. where clearance to the frame is limited. The length is greater than the Concentric (3.31” vs. 2.74”), which may create an issue on oil-in-frame Triumph 650 1971-2 with the stock air box.
    The Volks air filter used a threaded insert that the filter body clamped onto. The end of the JRC carburetor is a straight surface that the filter can clamp to. The later center mount threaded filter will not fit. The size on the end is 1.90”. The JRC replica filter # 83-1537T or the stock offset filter fits perfectly.
    Although the internal tuning parts are interchangeable with the PWK the JRC parts are less expensive. The JRC is as simple to tune as the PWK, but buying a pre-jetted JRC from one of their dealers moves you forward many hours on the learning curve, whereas beginning with a generic PWK is not only more expensive to purchase, and requires adaptation, but to sort out the jetting you’ll have to experiment.
    L. D. Bennett has tested this carburetor and says: ”This evaluation was probably one of the easiest mods to a Brit twin I have ever done! The JRC carb is almost a bolt on thing (stud on the left side of carb manifold has to be shortened and some 500cc manifolds need to have the inlet diameter enlarged a millimeter or two. JRC’s jetting was absolutely spot on for both my 500cc unit twins (1970 T100C engine and 1973 TR5T motor) from my home’s 3000 ft. elevation to well over 6000 ft.”
    Read his full review and comments on the Piled Arms Pub site here: , and pictures here: .
    Shown below are views of the JRC carburetor, click each for a larger version.

Size

Main jet

Slow jet

Jet needle

Throttle valve

28mm 29S

#135

#38

46JJH

C.A. 3.5

33mm S46

#165

#52

48CGL

C.A. 6.0

35mm 70R

#160

#68

48DGK

C.A. 6.0

38mm 71R

#172

#58

48DEG

C.A. 6.0

39mm 72R

#160

#60

48DGH

C.A. 6.0

What’s related?

Keihin PWK tuning components

bgm, OKO & KOSO replicas

Concentric tuning

Monobloc tuning

Mikuni VM tuning

Triumph Performance