jrccarb

Purpose:

The purpose is to find a replacement for the Amal carb that came stock
on Brit bikes. That is necessary for several reasons. Even though the
new Amals are offered in several models as replacement carbs, Amal
managed to reproduce all the faults of the original designs and add
some too. Those faults are:

1. Slides made of the same material as carb body promoting galling and
fast wear leading to both sticking slides and loose slides that allow
air around them through excessive clearances which makes the idle
uneven.

2. Flanges from the carb body to the motor's intake manifold that
when over tightened permanently distort the carb body, causing the
slide to stick in its bore. Once the body is distorted the precision
of the carb is lost.

3. Some have come through the manufacturing process crudely made.
Are they Chinese or Indian copies or is Amal's quality control not that
good?

4. Pilot jets that are pressed in bushes not meant to be removed or
resized, stuck down in a passage way that is prone to clogging if the
bike is stored with gas in the carb. This passage way is not easy to
clean as there is access only to one end as the passage way is a right
angle turn off another main passage.

5. Idle and transition circuits, including small holes into the mouth
of the carb, that are prone to clogging and hard to thoroughly clean.

6. The latest one for me is the sinking float. My friend's relatively
new Concentric's float filled with gas while out on a ride, about 50
miles from the truck. We had to drill a hole, drain the gas from the
float, epoxy up the hole, and pray we would be able to get back to the
truck before the float filled again. We could find no hole for the
fuel to leak into the float in an in-field inspection of the float.
The float again filled to about 20% full in less than 20 miles of
operation. Add to that the comment from a Piled Arms poster that he
had to buy and test about five floats to get one that didn't leak.

Common fixes for these maladies is to have the slide bore of the carb
fitted with a chrome or brass thin liner or have the slide hard
chromed to start with. But if the body is already distorted these
fixes might work off the bike but once the carb is bolted to the
manifold the sticking slide returns. Brute force bending the carb
flanges back into place or machining the flanges flat still leaves the
body of the carb distorted. The idle bush is drilled out and a
separate idle jet is installed in the float area of the carb
(available because the first version Concentrics came with an idle jet
in the float bowl area, not underneath the air screw adjustment). Amal
moved the idling function years ago because idle quality suffered
initially with the jet so far from its source of suction, the idle air
passage.

A New Carb:

In a casual request for information on the JRC (Bill Getty) carb that
I only recently heard about, Bill Getty offered to allow me an
evaluation carb after hearing that I write TECH articles for the Piled
Arms web page. The carb arrived with a new throttle cable (which was
not required or used), a plastic tee for the fuel lines, several feet
of fuel line (again, I had my own which I used), a set of six hose
clamps (Sorry Bill, but they were sub standard and two broke while I
tightened them…used the name brand micro clamps from an auto parts
store), and several main jets (130, 132, 135, 138) all leaner than the
main jet installed (140) in the carb.

The carb is from the Keihin PWK family and is marked as a PWK 30 but
the bore actually is oval and measures 30mm vertically and 28mm
horizontally, perfect for my Triumph Trophy Trail as it originally
came with a 928 28mm Concentric. It is a modification of the standard
PWK 26 mm carb but consideration is being made to get Keihin to make a
new body for a full 30mm body. The carb was jetted as follows:

Main Jet…140. (may be a bit rich so will make adjustments on the first
field test)

Slow Jet…38 (same as pilot or idle jet)

Needle jet was a pressed in jet (unknown size) but only one size
appears in the Keihin parts catalog.

Needle unmarked and in middle clip position.
Slide (not marked but measures about a 2 1/2 cutaway (?) and is chrome
Plated, Yea!) The air jet size is fixed.

For specs the comparison should be made between the Concentric, the
JRC, and the Mikuni. The Concentric is a simple carb with only a few
choices for needle and needle jets and a fixed air bleed system for
the main and idle jetting. The JRC is also a simple carb with only one
needle jet but about 35 needles with various tapers and needle
diameters allowing a reasonable choice with only one variable for mid
throttle tuning, the needle itself. The JRC slide is a modern flat
slide with chrome plating to avoid galling of the carb's slide bore,
unlike the Concentric. The Mikuni has virtually an infinite choice of
needles and needle jets, and changeable main system air bleeds (air
jets). With a dyno and enough time and knowledge a person could get
the Mikuni jetted perfectly but I don't have the knowledge or the dyno
and with all those variables for tuning, the Mikuni can be a nightmare
to jet. Not so the JRC or the Concentric. The Mikuni's have a rubber
manifold while the JRC's flange mount is rubber isolated from the body
(no body distortion from flange over tightening as on the Concentric,
Yea!). The Mikuni and the JRC have starter systems that include a jet
and an air passage while the Concentric has a cable operated choke and
a tickler button to flood the carb for starting. The floats of the
Concentric are mounted to the removable float bowl while the Mikuni
and the JRC carbs have them attached to the carb body. The JRC carb
requires that the main jet and needle jet holder be removed before the
float bowl can be removed and even then you have to snake the float
bowl around the floats to get it off. Finally the JRC is sized
externally to fit in the space allotted for a Concentric, with an
intake bell that is the same size as the AMAL carb to allow the stock
pancake filter assembly to fit perfectly.

Specification wise the Concentric is simple to tune but very prone to
wear and damage from abusive maintenance. The Mikuni is complex to
tune but pretty good for wear. The JRC is a simple carb, simple to
tune, and very wear and maintenance abuse resistant by design.

Bill Getty (JRC) tells me he has successfully jetted a version of the
JRC carb for the following bikes: Triumph T140, TR7, TR6, T120, T160,
T110, T100 and TR5T, an Ariel 4 (same jetting as TR7). He has not yet
done any Norton, or BSA but has plans to do so. The jetting for the
BSA's or Norton's can not be too far off equivalent Triumph
displacements. He offers only the one slide but has yet to have anyone
request a slide different from the one provided. Bill is willing to
share tuning information and will stock jetting for the carb. He is
upgrading the hose clamps.

JRC Install:

All went well but I did have to shorten the bike's left side manifold
mounting stud as it bumped into the JRC carb body. The carb is
fractionally longer than the Concentric so fitting it to my Rickman
may be more of a challenge than the fitting I did to the TR5T Triumph
Trophy Trail (no air box…just a filter sock). But the JRC Carb is much
shorter than a Mikuni of the same bore size. My original cable worked
fine as did my air filter sock that was on the original Concentric,
formerly mounted to the bike. The bike started right up after the
install. I tweaked the idle speed screw and the air screw after the
bike warmed up a bit. It idled very smoothly, ran smoothly and had no
flat spots anywhere in the power band that I could determine running
up and down the street in front of my house. It certainly ran at least
as good as the Concentric, if not better. So far it looks like a
better choice for a Brit carb replacement than either a new Concentric
or the Mikuni.

Field Testing:

The test ride was a 60 mile loop from 3000 feet to over 6000 feet, on
both paved and dirt roads and some jeep roads. The bike starts more
readily from cold or hot. The enrichment system (pull the plunger up
and it detent locks up for starting) is adequately rich for starting
but not overly rich, so you can let it idle for a few minutes with the
enrichment system on. The bike ran well at all altitudes with no flat
spots or rich bogs. The needle jet and needle seem perfect with the
clip in the middle. The motor accelerates smoothly which tells me the
slide cutaway is correct as well as the slow jet (pilot jet) is
probably correct as well. The idle is smooth and can be made to be
very slow without the engine stalling. At altitude I tried many times
to induce a rich bog by giving full throttle at slow engine speed. No
rich bog. The carburetion was near ideal, to the best of my abilities
to tune a carb.

I pronounce this carb a viable alternate to the Amals and a better
choice than the hard to tune Mikunis or the problem prone Amals. JRC
has the jetting for my bike right on. As for pricing, what a deal! At
this time the price is $116 for the carb, only. All the extra piece
parts I got are to be extra. Except for the gas line, the tee, clamps,
and the heavy duty return spring there is really nothing else you
need. It is indeed a bolt on mod.

Conclusion:

The JRC carb is indeed a viable, good alternative to either the stock
Concentric or the Mikuni. See www.jrceng.com for the dealer list (JRC
is a distributor, only, and does not sell retail). Many thanks to Bill
Getty of JRC for all the help.