The following articles and instructions are provided as
general information for enthusiasts working with AMAL
carbs on British Motorcycles, without the need for
expensive tools or paying for expensive shop labour. Many
British motorcycles are not properly enjoyed as the owners
have neglected the basics of carb maintenance that they
run so poorly and are frustrating to ride. A Norton
Commando should idle evenly at 700-800rpm and be able to
jump off the line at a traffic light with a fistful of
throttle, without hesitation or stalling. Use this
information with common sense and at your own risk. There
is a lot of technical information available now on the
internet about specific settings and components, I would
encourage readers to expand their knowledge base by
reading as much as possible in this subject. This article
is intended to help understand the whys and wherefores and
offer guidance to owners who are willing to take on tuning
their own motorcycles and enjoy them fully on the road.
Remove carb(s) from your bike. Completely dismantle them
to last screw and soak in carb cleaner. Use stiff bristle
brush, bottle cleaners, probes etc. to completely remove
all dirt and scale from all passages. Clean filters. Blow
dry with compressed air. Bead blast if required.
Number one, make sure that your carbs have been checked
out for factory settings and that all jets, slides,
needles etc are set the same on each carb. Check the
settings, do not assume anything.
It is advisable to purchase and install new gasket sets.
These are inexpensive and they come with gaskets, new
rubber O-rings and washers. If you are restoring an older
carburetor also consider a jet metering kit that replaces
the needle jet and the needle and a VITON tipped float
needle. Later style tickler conversion parts are also
available as a kit.
The pilot (or the slow-speed, idle) circuit has three
parts:
1. an air passage that is adjustable by the AIR SCREW on
the side (coloured in Blue)
2. an internal gas passage with the FIXED pilot jet.
This jet is a small brass bushing with a tiny 0.016" (16
thou) orifice that is a press fit in the passage
opposite the air adjusting screw. Gas is brought up from
the float bowl at the back and travels toward the front
(coloured in Red). When it reaches the pilot jet it is
metered by the jet orifice before it mixes with the air
in the pilot mixing chamber.
3. Pilot circuit mixing chamber (coloured in Yellow) –
this is where the metered amount of gas is mixed with
the metered amount of air and travels up into the engine
through two tiny holes
The pilot circuit provides a metered amount of gas and
air for idle and low speed running. Think of the gas
circuit as an artery - over time the gas passages slowly
clog up with scale and varnish from dried gasoline and
eventually clog right up. The only remedy is
to physically unclog the jet with a wire probe.
The pilot jet is drilled 16 thou dia. (0.016"), a very
tiny orifice indeed. I use an old guitar string wire
with a filed flat end to probe the jet from the air
side, after the air adjuster screw is removed (use a
flashlight to locate the orifice). Another useful tool
is a #78 drill, mounted on an end of small brass or
plastic tube, The drill will re-size the orifice, so be
careful and make sure all swarf is blown out by
compressed air.
Best to do this on the bench with the float bowl
removed so that any scale removed will exit the carb out
the orifice on the underside. Make sure the fuel passage
on the float bowl is also clear, crud and scale tend to
collect on this at the bottom (there are two on each
float bowls, one is not used)
Many people say they have soaked the carbs in carb
cleaner and blown them out, or have had them
ultrasonically cleaned - BUT this will not usually
unclog the pilot gas jet, it needs to be physically
probed and the scale scraped out.
NEW TIP - CLEANING
PILOT JETS WITH CARBS ON BIKE:
I have found recently that the pilot circuit needs
regular cleaning and I simply do it with the carbs on
the bike. You will need a can of carb cleaner, red straw
and rubber sleeve, screw driver and the wire probe. Turn
off the gas, drain the float bowls if you can, screw in
the air screw counting how many ½ turns you go (usually
1 ½ turns), then screw it all the way out and remove.
Use the wire probe, with some patience the wire will
find the jet, work the wire into the jet, and with
pressure push the wire through the jet. I have left my
wire long enough so that when the black cover on the
wire is at the air screw opening, then I know the wire
is all the way through the jet. You should be able to
the slide the wire back and forth until it meets no
resistance. Remove the wire and give the pilot a shot of
carb cleaner to blow the swarf back into the float bowl.
Reinstall the air screw, all the way in, then back out
the same number of turns you counted going in.
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MODIFICATIONS FOR BLANKING SCREW:
A useful modification is to actually drill out the
blank plug on the opposite side from the air screw and
physically probe the jet so that you can see with your
own eyes that the jet is clear. The drilled hole is
tapped 10/32 (in real life it should be 2BA) and install
a blanking plug (an old air adjuster screw cut to size)
with Loctite. This is a machinist operation and should
not be attempted without the proper tools. This process
is a must if you plan to reuse old carbs on a new
restoration
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These are the round thingies that slide up and down and
over time they wear in the carb body and let air past,
particularly noticeable at idle.
One simple test to see if the slides are contributing to
the rough running is to hold the throttle at idle and
then take up the slack on the cable by slowly turning
the throttle - if the idle speed drops, it indicates
that the slides are a loose fit in the carb body and
should be replaced. As a rule of thumb, slides can be
replaced at least once; maybe twice in the life of a
carb, they are expensive at around $30US each, so most
people don't think of changing them.
There are also new options with chrome plated brass
slides and hard anodised aluminum which are not supposed
to wear as fast.
If new slides don't fix the running problem, new bodies
can be purchased at a reduced cost rather than buying
new carbs. However one should consider that other parts
like needles, needle jets can be worn as well and that
new carbs maybe the way to go.
Re-sleeving is a viable solution, especially for one off
carbs that cannot be easily replaced (Monobloc 689's for
instance). Nowadays just about ALL AMAL carbs are
available newly manufactured, even the TT's and 276's.
There is now also the AMAL PREMIER range of carbs that
seem to be the modern updated version with many
upgrades.
BACK TO
TOP
Improper float level can be determined by several
factors
- fuel leaking out from the gasket joint (too high)
- fuel spilling out the intake holes at the rear into
the air cleaner (too high)
- rich running, erratic idle (too high)
- slow to tickle (too low)
- lean running (too low)
There is a very simple method to checking the float
height, without special tools or risking the chance of
spilling fuel and a fire. This is taken straight from
the Triumph Factory Service bulletins. This is how they
were set up in the factory and is good enough for
general running, including Norton Commandos and BSA's.
This setting is good for singles, twins and triples. For
down-draught carbs that slope down (i.e. on Atlas), the
setting will need to be adjusted higher
The factory setting is 080" (80thou) or
approx. 2mm BELOW the back edge of the float bowl.
This is the measurement of the TOP edge of the plastic
float to BELOW the top edge of the float bowl. Make a
scribe mark on the metal float bowl and eye sight the
plastic float when the float needle is seated by gently
pushing down with a flat blade screw driver in the area
next to the brass pivot pin and float needle.
To adjust the seat height, DO NOT try and bend the
plastic tab, it has memory and will not stay adjusted.
Heat the float bowl on hot water then use a 1/8" drift
to move the brass seat in the float bowl itself. It is a
tight fit, but can be moved with gentle tapping. Adjust
from underside to lower float height, from top side to
raise the float level. The Factory suggested heating
with a propane torch, but as a measure of safety, I do
not recommend this due to gasoline fumes and possibility
of explosion.
Remember to replace any of the old white nylon float
needles with the viton tipped version
NEW TIP:
STAY UP FLOATS
A recent product is the new stay up floats from AMAL –
these are the black ones with the metal mounting. They
are resistant to ethanol and can be adjusted to set
float heights by bending the metal tang rather than
moving the jet seat in the float bowl method used above.
Due to the age of the original while nylon floats and
their unknown performance with modern gasolines, it is
becoming common to see these floats discoloured, become
brittle and leak (fill with gas) due to pin holes. My
theory is new stay up floats are worth the extra cost
sooner than later.
NEW TIP: NEEDLE JETS
AND NEEDLES
The diagrams below demonstrate the actual operation of
the needle, needle jet, throttle slide and main jet
As carbs cover the miles in use, inevitably the needle
jet will wear. The needle jet is the last jet after the
main jet for the gas to enter the engine. Every time you
open the throttle, the needle which is attached to the
throttle slide goes up and down and due to the air
velocity of the air passing into the engine, there is
slight pressure on one side of the needle which causes
it to rub the needle jet. This is a constant activity
and the rubbing eventually wear the needle jet orifice
oval.
The result of a worn needle jet is evidenced in several
ways – the bike no longer runs crisp as the mixture is
now running richer than was original specified for, plus
the gas mileage increases – more fuel is added, over top
of the fuel that is required for proper combustion and
is wasted.
As part of the tune up process, or any time the carbs
are apart for servicing, it is important to check the
wear on the needle jet.
The needle jet specified in almost all of our British
singles, twins and triples is a .106 Needle Jet.
Thankfully AMAL in their infinite wisdom many years ago
translated the jet sizing to correspond with an imperial
measurement of the orifice diameter (not cross sectional
area as would have been more appropriate for flow). This
means simply that a 106 needle jet has a diameter of
0.106” – or if you have a number drill set on hand it
would be #36 Drill.
I went through a box of spare, “good” Monobloc needle
jets with the #36 drill blank end and only two of the
six jets were within spec. The drill is really a
go-no-go gauge – it will slide smoothly in a spec size
jet. In a worn jet you can feel the oversize wobble
quite easily.
Make a habit of buying new needle jets. Some claim the
needles wear as well, it is possible over a much greater
period – I have yet to see a needle showing signs of
wear, except in extremely high mileage motorcycles, they
get to spin freely when in use.
This is one area where there is a lot of misinformation
on the process, even in workshop manuals. This will
outline the simple methodical professional shop steps to
synchronize your twin (or triple) carbs, once all the
prep and rebuild work above is completed.
- Adjust pilot screw to 1 ½ turns out (turn all the
way IN till stops then count 1 ½ turns OUT).
This assumes that the pilot gas jet has been cleaned
as above)
- Open the throttle to full open and check that each
throttle slide is up the maximum amount (i.e.
disappears up into the carb body). Adjust the cable
adjusters until an equal amount of throttle slide is
seen, just peeking through on each carb. It is
surprising the number of bikes I have seen with a
major difference in the full open position.
- Once cables are adjusted for full open, fully turn
out the throttle adjusting screws (to past were it
touches the throttle slide)
- Insert two 3/16" dia drill bits (or 4" long piece of
round bar) into each carb (diameter size should be
adjusted to suit the throttle cut away), seating the
drill bit under the throttle slide cutaway, so that
they stick out the end of the carb evenly.
- Screw in each throttle stop screw until the drill
bit "dips" a little, find the happy spot, where the
screw just touches the slide. Repeat for the other
carb. With a felt pen mark the screw head flat-slot
position for reference on each carb body.
- With the drill bits still in place, perform the fine
cable adjustment - by gently opening the throttle and
seeing that each drill bit moves at precisely the same
moment. Adjust the cable adjusters on the carb or on
each cable so that movement is even (should be only ¼
- ½ turn each) – don’t adjust the throttle stop screws
in this step.
- Start the bike, it will likely idle too high, wait
for engine to warm up, then adjust each throttle stop
screw down (out) by EXACTLY the same amount, i.e a ¼
turn at a time on each, using the felt pen reference
mark to make sure the adjustments are same. The idle
should be even and by blipping the throttle the
pick-up should also be even.
- The pilot jet air screw may be adjusted at this
time, but by very little, no more than ½ to 3/4 turn
in either direction. The effective range of adjustment
for the pilot jet is 1 turn to 2 turns. This is where
a bit of skill & experience comes into play. By
turning one screw a little at a time with the engine
running (on both cylinders) you can gauge any
difference in running and find the sweet spot. When
you hear the engine revs race, you want to turn it
back a 1/8 to ¼ turn - the sweet spot is not at the
extreme. The idle may need to be dropped equally
again.
If you have to screw it in all the way or out all the way,
then the pilot circuit is still clogged, maybe not
completely, but enough to affect the running. Go back and
clean the jet
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TO TOP
Mikuni VM series carbs are a popular modern replacement
for the old AMALS. The advantages of VM Mikuni's over
Amal's are:
-fully tunable in fine increments
-made of lighter aluminum, not zinc pot metal
-have a lever cold start circuit (choke)
-pilot fuel jets are replaceable
-have long lasting anodized slides
-throttle slides come in smaller increments for fine
tuning (i.e. .019" compared to 0.03125")
-air jets are replaceable
-float height is fully adjustable
-assortment of needles with varying tapers available
for 4 stroke engines
-assortment of replaceable needle jets
-larger sizes up to 38mm
Setting up the VM Mikuni's for a British twin is not
difficult. The following list is a starting point, make
adjustments as required by road testing.
On a 650/750 Twin (Triumph &/or Norton) for a PAIR
of Mikuni VM's 32mm dia, use pilot 20-25, throttle slide
2.5, needle jet type 159, needle jet size P-2, P-4,
needle 6DH2, 3, or 4, Main 220-230, air jet 1.0.
On a 850 Norton for a SINGLE Mikuni VM 34mm dia, use
pilot 30-35, throttle slide 3, needle jet type 159,
needle jet size P-4, P-6, needle 6DH3 or 4, Main 260,
air jet 1.0.
These settings were provided by the Victory Library
"Mikuni Tuning for British Twins" author Jeffery
Diamond www.victorylibrary.com
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TO TOP
:
The following chart shows how the carb components
interact and the effective range of adjustments.
The process of road testing and fine tuning is simple.
Firstly get it running. Do the wide open throttle (WOT)
test in say 3rd-4th gear and plug chop - change main
jets to get desired plug colour (light tan, not
chocolate brown) and performance.
The pilot circuit (IMS, idle mixture screw and the air
screw) is next - make adjustments so that there is no
hesitation when the throttle is blipped or blubbering
when at low speed. Adjustments can also be made to
needle clip position to fine tune mid-range. Changing
throttle slide should not be necessary.
Return
Link to JB CYCLE MAIN WEBSITE
THIS ARTICLE MAY BE USED IN CLUB PUBLICATIONS WITH
AUTHOR ACKNOWLEDGEMENT
Jim Bush, Vancouver, BC CANADA, jim@jba.bc.ca
Last Updated on November 7, 2015