Technical Articles


This is under construction – more as time allows!

AMAL Concentric Carburetors

Getting Started:

It’s the needle jet stoopid:
If there is one thing you come away with, from the time you spend reading this, is how import the needle jet is to every day performance of your motorcycle. To put this in perspective, a change in size of the needle jet’s metering orifice equal to 1/3rd the diameter of the average human hair, (one thousandths of an inch  .001″) , will make the needle jet too rich or too lean. In practice the typical 4 stroke needle jet’s orifice is .106″. A needle jet that has an orifice of .105″ will be too lean to a point where it could introduce unwanted detonation. A needle jet that has an orifice of .107″ will be too rich where it will significantly reduce fuel mileage and could foul the spark plug. The needle jet is in play in the throttle positions where we ride these bikes the most: closed to half throttle.

Caution: There are about ten people manufacturing needle jets around the world. When you see reference to using plug gauges to verify the needle jets size we are referencing checking new needle jets. Not all needle jets on the market have the jet orifice that is marked on the jet. The new .106 could have an orifice anything but .106″. While you can get an idea if a needle jet is worn with an smaller than standard orifice plug gauge, it is better to routinely change them from a know supplier like Burlen.

Don’t underestimate the importance of the proper grade of  the spark plug:
The grade of spark plug recommended by the motorcycle manufacturer is an important base-line for carburetor tuning. If you are fouling the recommended spark plug you should never change to a hotter grade plug in order to correct the problem.

With Amal, and most Mikuni carburetors, you are dealing with instruments that have multiple tuning stages. Typically it will be only one of these stages that is causing the problem leaving the rest having to deal with the wrong grade plug. For example let’s consider a worn needle jet.

The needle jet’s influence on the fuel mixture will be below half throttle. Yes, that hotter plug will give you that plug color you are looking for below half throttle, but when you take it out on the highway, and running above half throttle, the fuel mixture will be too lean for that new hotter grade plug. You are one step closer to destructive detonation. Add a few other factors like an abnormal increase in ambient air temperature, or you roll the throttle on without shifting down to move up even a slight grade (effectively lugging the engine), and you are ever closer to that hotter plug causing pre-ignition. Get a tank of fuel where the octane rating is not up to snuff and you better hope your cell phone’s batteries are charged, or you will be walking.

With a standard engine, tune to the manufacturer’s recommended plug. Of course if you change the compression this must be taken into consideration when selecting the new plug you will be using, and that’s another story.

Do your home work before you start thinking about tuning a carburetor Be sure of the following:
All of these things will effect manifold vacuum. Manifold vacuum directly effects how a carburetor works.
• New points, plugs and condensers are installed, adjusted and timing set.
• Valves are adjusted and bike passes a compression test.
• If one is installed, the air cleaner is clean.
• Factory recommended jets are installed, and are not worn, corroded or damaged.
• The fuel cap vent is clear, and there are no air leaks in the intake manifold.
• The fuel taps, fuel line, fuel filter and fuel connections will flow enough gasoline.
• You know how to operate the choke. Yes, I am serious as this has been a problem in the past.
You have installed  new needle and main jets.
• The fuel tank is free of rust! Very fine rust from the tank can block the fuel filter (causing a lean condition) and get under the float needle causing it not to seat (causing a very rich condition). There should be absolutely no signs of microscopic rust in the fuel!!

Next you must mark the throttle.
While riding you have to be able to quickly determine which one of the fuel  circuits you are using at any specific time. Is the slide:
closed to 1/4,
• 1/4 to 1/2,
•1/2 to 3/4 or
3/4 to full throttle.
There are specific fuels circuits working at each of these throttle positions. RPM or speed have nothing to do with tuning a carburetor! For Example: If the problem you are experiencing is under 1/4 throttle it will not be the needle clip position or main jet (as long as the main jet is the recommended size for the bike you are working on). If the problem is between 1/2 and 3/4 throttle it is not going to be the position of the pilot air screw or slide cutaway. Knowing the throttle position, and which fuel circuit services the carburetor at that point, will put you way ahead of someone who is using Wild Ass guesses. I cringe when I hear someone say, “My bike is running lean I must need a larger main jet.” You should always ask yourself at what throttle opening is it running lean and plan your moves accordingly.

Two Carburetors in One:

To better understand the Amal carburetor it is best to look at it as two carburetors. Idle and Main. The idle carburetor handles STARTING, idle and very low throttle opening. It is made up of the Pilot jet and Idle Air Mixture screw.  And the Main carburetor which takes you from very low throttle openings right up through wide open throttle (WOT). The two carburetors each have a main jet and variable air supply while they share a common fuel supply.

The Idle Carburetor’s pilot jet is its “main” jet. The pilot air screw controls air flow drawing fuel from a shared float bowl. The fuel air mixture is delivered to a small mixing chamber under two transfer ports. The transfer ports are located each side of the back edge of the slide. They are the primary, the smaller of the two, located just behind the back edge of the slide and the secondary, larger than the primary, and located under the back edge of the slide.

• The typical Pilot jet in a 4 stroke version of the Concentric is pressed into the body. It is located behind the Pilot Air Screw and has a diameter of 0.017″ i.d.. A lot of people with Amal Premiers (especially MKIII 850 Norton’s) with removable pilot jets are finding a .019″ pilot jet (622/502-19) works best.
• The Pilot Jet in a 2 stroke version is screwed into a threaded hole at the base of the main body. When the pressed in Pilot Jet has been enlarged by accident or on purpose, on some 4 stroke motorcycles, you can use the 2 stroke jet screwed into the body.
• There are two small idle circuit transfer ports adjacent to the back edge of the slide. The smaller, Primary, transfer hole is 0.027″ while the Secondary is 0.040″.
• It is the Primary transfer port that supports idle and transition between idle and main carburetors.
• The Secondary has two functions. At idle it feeds additional air into the mixing chamber. Lifting the slide just off idle reverses the flow through the Secondary port. It now supplies additional fuel into the venturi to help make the transfer from the idle carburetor to the main carburetor smoother.
• The Slide Cutaway is an important adjunct to the Idle Carburetor and helps make the transfer from the Idle Carburetor to the Main Carburetor possible. If the carburetor will not take throttle from idle as you rotate the twist grip the slide is too lean. If it runs lumpy for a short time only to clean out the slide is too rich.
• If the bike is hard, or will not start, or won’t idle it is quite possible there is something wrong with the Idle Carburetor. A blocked pilot jet is typical, but don’t overlook a blockage of the fuel passages or an air leak at the float bol gasket because of a warped float bowl. Even very fine rust from a fuel tank can fill the fuel filter and block the flow of fuel.
• It is important that all jets and orifices are clean and on size. These are jets, not holes.


The slide is in the idle position. The secondary transfer port under the back edge of the slide is taking in air into the small mixing chamber just under the two transfer ports. The fuel/air mixture that supports idle is ONLY coming out the small primary port just behind the back edge of the slide.



Slide is lifted: The Secondary transfer port reverses and is now delivering fuel adding to what is coming out of the main carburetor’s spray tube.

17-CurvedFloatBowl-1webA warped float bowl from over tightening the float bowl screws. The fuel supply passage for the pilot jet passes fuel from the float bowl to the main carburetor body through the float bowl gasket. If the bowl is warped the gasket will leak air into the pilot fuel passage. No fuel will be delivered to the pilot circuit. If the bowl is warped it must be flattened, or replaced.




Float Bowls:

pilot track color The pilot jet is pressed into the body. The arrow is pointing to the location of the .017″ pilot jet’s orifice. You can see the two air passages. One is in the center and leads to the needle jet. The other is at the top and leads to the pilot air screw and the fuel supplied by the pilot jet. The two transfer orifices, primary and secondary, are located the slot between the pilot jet and the tip of the pilot air screw.

Tip: If the pilot jet is blocked the bike will not start. If it is not on size because of a partial blockage the carburetor will not respond to changes to the pilot air screw and idle will be erratic.

The pilot jet can be cleaned using a #78 drill. It will have to be securely mounted in in a brass tube or equivalent. It is a confounded nuisance when the drill get stuck in the pilot jet and comes out of the brass tube! Twirl it between your fingers and it will draw any “swarf” down stream of the fuel flow.

Yes, a guitar string can clean the pilot jet orifice, but in the process the “swarf” in the jet is pushed up stream of the flow of fuel. This allows the “swarf” to flow back into the jet and potentially partially block the orifice. The guitar string technique works only if you modify the carburetor where the body is drilled and tapped to accept a modified pilot air screw opposite the working air screw. This way you can push the “swarf”  down stream of the orifice. See Amal’s web site to see this modification.

IMHO acetylene torch cleaning tips should NEVER be used to clean the pilot jet. In an exercise to look into this I measured several that I had in house, and bought several new ones from “reliable” torch makers. Not one of them would clean a jet orifice and leave it anywhere near the original size. Most cut the orifice several thousandths oversize.  Being such a small orifice small changes are significant to fuel flow.

You can clearly see the two pilot transfer holes. The Primary is the smallest, and closest to the reader. The Secondary is larger, and just behind the groove in the body for the slide. You can also see the brass “Straight Cut” spray tube.

This chart is not drawn in stone. It is a representation of the different fuel circuits, and at which throttle opening,  they are working. A version of this chart is often found on web sites. The usual problem with them is they have been copied directly out of the  Mikuni VM manual. One of the main differences between the Amal and the Mikuni VM is with an Amal, the tip of the jet needle never leaves the needle jet orifice.
• With the Mikuni VM the jet needle is moving up and down and never leaves the needle jet’s orifice all the way through wide open throttle. Thus with the VM the needle is in play through wide open throttle.
• With the Amal, depending upon which groove the needle clip is in, the jet needle leaves the needle jet at about 3/4 throttle.
Starting and ending throttle positions for each fuel circuit represented in this drawing are general representations and the actual ones can, and will, vary.



You will never understand how to tune an Amal, or a Mikuni for that matter, until you under stand the different fuel/air circuits and at what throttle opening the are in play. We often hear someone say, “My Amal (or Mikuni) is lean. I need a bigger main jet.” If you remove the main jet from the carburetor, the bike will still run normally until it gets above 1/2 throttle. As you see from the chart above the main jet doesn’t start to really come into play until well above 1/2 throttle.

There are five individual steps to tuning:
• 1st in sequence- Full Throttle:
At full throttle you are working with the main jet.

If it is a stock bike use the main jet recommended by the manufacturer. If you have raised the compression start about three jet sizes larger than stock and work back down.
• 2nd in sequence- Idle:
At idle you are working with the pilot jet, pilot air screw. With a Concentric start with the screw out 1 1/2 turns from lightly seated.
• 3rd in sequence- 0 to 1/4 throttle:
At 0 to 1/4 throttle you are working with the slide cutaway, needle jet i.d. and pilot air screw. Typically a dual carburetor bike I will start with a # 3 and a single carburetor with a # 3 1/2 slide.
• 4th in sequence – 1/4 to 3/4 throttle:
At 1/4 to 3/4 throttle you are working with the needle jet just as the taper starts on the needle, taper itself and needle clip position. Without any information to work with start in the middle notch.
• 5th in sequence – Slide Cutaway:
Adjust the slide cutaway for a smooth transition from the idle to the main carburetor. You should be able to turn the throttle briskly from closed and have the engine respond in kind.

Note: Amal’s web site makes it easy to find information on where to start with standard jetting for your year, make and model:
Click on Vehicle Search. Select your Brand, then Model and Year. Click View and scroll down to Parts List. There the Main Jet, Needle Jet and Slide Cutaway are listed. This is where you want to start. If you changed compression, cams or exhaust system you would want to start with a couple of sizes large main jet and work back down. You can always come back to the main jet after getting the lower fuel/air circuits tuned. The only problem would happen when the main jet is too small to supply enough fuel for the needle jet. If you start with the factory main jet, or larger, this will not happen.

Concentric 4 stroke main jet starting size by venturi:
622      120
626      140
627      170
928      180
930      200
932      220
1034*   360
1036*  370
1038*   38
* Out of production
TIP: These are starting points and changes to the motorcycle will require attention to the main jet size. The wrong main jet can cause serious damage to the motorcycle and you! Check the manual that comes with the bike.

Tuning The Carburetor:
Too Rich Condition
• Float Level Too High — Raise float needle seat in bowl. From experience, I am not a fan of bending the tab on a Stay-up float to set the fuel level in the bowl.
• Old style white plastic float “Sunk” full of gas — Replace Float.
• Main Jet Too Large— Install lower number main jet.
• Slide Cutaway Too Low— Install slide with higher cutaway (# 2 is Rich while a # 4 is Lean).
• Needle Jet Too Large— Measure needle jet replace if worn, or if not worn, install needle jet with lower number.
• Wrong Needle Jet Type— Install correct needle jet.
• Needle Clip Too Low— Raise Needle Clip to higher groove.
• Wrong Needle— Check motorcycle specifications.
• Wrong Spray Tube— Replace spray tube with leaner type.
• Pilot Jet Too Large or Missing— Install smaller number jet.
• Pilot Air Screw too far In— Turn pilot air screw out.
• Choke On— Turn choke off.
• Fuel Leaking Past Float Needle— Clean fuel tank, filter and/or replace float needle.

Too Lean Condition
• Float Level too Low— Lower float needle seat in bowl. From experience, I am not a fan of bending the tab on a Stay-up float to set the fuel level in the bowl.
• Float Bowl Gasket Surface Warped— Replace or flatten surface.
• Main Jet Too Small— Install higher number main jet.
• Slide Cutaway Too High— Install slide with lower number.
• Needle Jet Too Small— Install needle jet with larger number.
• Needle Jet Cross Drilled Hole too Large— It should measure .035″ if larger replace needle jet.
• Wrong Needle jet type— Replace needle jet check spec’s.
• Needle Clip Too High— Lower needle clip to lower groove.
• Wrong Spray Tube— Replace spray tube with richer type: Stepped or slanted.
• Pilot Jet Too Small or Blocked— Clean or replace jet.
• Idle Fuel Circuit Blocked— Clean.
• Idle Transfer Ports Blocked— Clean.
• Pilot Air Screw Too Far Out— Turn screw in.
• Fuel Tank Cap Vent Blocked— Clean.
• Fuel Petcocks Too Small or Blocked— Clean or replace as necessary.
• Float Bowl Vent Blocked or Restricted— Open vent.
• Orifice In Float Needle Brass Seat Too Small— Replace with larger orifice.

TIP: If you have a stock bike kitted out with Concentric carburetor of the proper size with a 0.016″ (standard) pilot jet, 0.106″ needle jet, standard 622/124 2 ring needle in the #2 (middle) position, 3 1/2 slide if a single carburetor model, or 3  if a two carburetor model and main jet from the workshop manual you will be very close to perfect fuel air mixture in all 4 fuel air circuits. All you should have to do is some fine tuning like raising or lowering the needle or a slightly leaner or richer main jet.

Odd Stuff:

The tapered needle seldom wears to a point where it needs replacing. It can be easily measured on the part that will wear the most, the major diameter. You will find the diameter to be .0985″ (2.5mm). The needle grooves are numbered from the top down, with #1 being nearest the top. If the needle is bent it is a candidate to be replaced.

    • 375 models     375/063 stamped  “B” on top 5 circlip grooves
    • 376 models     376/063 stamped “C” on top 5 circlip grooves
    • 376 BSA A10   376/096 stamped “C1” on top 5 circlip grooves
    • 376 models     RK6/116 stamped “C2” on to 5 circlip grooves
    • 389 models    389/063 stamped “D” on top 5 circlip grooves
    • 389 models    RK9/088 stamped “D2” on top 5 circlip grooves
    • 689 models    as 389/063
    600 Series:
    • 622/063          1 id groove- 2 stroke used with 622/079 needle jet, and 622/080 jet holder (600 series)
    • 622/099          “Y” stamped at top – Alcohol needle
    • 622/124           2 id grooves- 4 stroke use with 622/122 needle jet, 622/128  jet holder
    900 Series:
    • 928/063           3 id grooves- 2 stroke use with 622/079 needle jet, 622/080 jet holder (900 series)
    • 928/099          “Z” stamped at top – Alcohol needle
    • 928/104          4 id grooves- 4 stroke  Developed for the 850 Norton
    • 622/278          5 id grooves- T160 part of Trident lean kit
    Mark II—
    • 2622/063          2B1 – 2 stroke
    • 2622/124          2A1 – 4 stroke – T140 use with 2928/122 .106″ needle jet
    • 2622/125         2A2 – alcohol
    • 2928/030         2C3 – 4 stroke- T140 EPA Tuning use with 2928/031 stainless .105″ needle jet
    • 2928/063          2D1 – 2 stroke
    • 2928/124          2D2 – 2 stroke
    • 2928/125          2C2 – alcohol
    • 2036/063         2F1 – 2 stroke
    • 2036/077          2E3 – alcohol rich
    • 2036/125          2E2 – alcohol


Checking new needle jets. Pictured is a minus .106″ plug gauge. It measures .1058″ and should be a slip fit in the needle jet orifice. A .106″ plug gauge would be a light press fit and a plus plug gauge (.1062″) would be a press fit. The small hole on the side of the hex is .035″.

This illustrates that the internal threads of the main/needle jet holder are counter bored.

The counter bore detail is especially important for the needle jet. If you look at the needle jet picture above you will see that the thread does not extend all the way to the shoulder. If it wasn’t counter bored the jet’s orifice would close up as you tighten the jet. Thus your .106″ jet would end up .105″ or smaller. I mention this because a run of jet holders were run without the counter bore. When it was found out, instead of replacing the holders with proper ones, needle jets were supplied where the end of the thread was undercut. The thinking was the undercut would solve the problem. Well it didn’t ,and as you tightened the jet in place it either broke off or the orifice just below the undercut closed up as before.

The needle jet is the only jet that ISN’T identified by how much fuel it will flow. It’s orifice is measured in inches. The most common for four stroke engines is a 0.106″. The smallest needle jet is .105″. Below that the carburetor will not work. A few 4 stroke engines require a .107″, but they are few. Two strokes use larger needle jets as does carburetors set up for alcohol. An engine that runs well with a .106″needle jet will run lean with a .105″ and rich with a .107″. The needle jet is the only jet that gets larger in normal use and can  wear to .107″, and beyond, in as little as 10,000 miles. As it is the only jet in the carburetor subject to wear it should at the top of the list for replacement at annual service intervals. A worn needle jet is so common that it led me to coin the phrase “It’s the needle jet stoopid.”

A word of caution: There are some ten odd people making aftermarket parts for Amal carburetors. Some like Keyster in Japan make some real nice jets. Others, including ones that were made in USA and the Far East are poor copies, at best. Unless you are equipped to measure your new needle jet the most practical way to insure your new needle jet measures 0.106″ is to buy one in an Amal package. 

Needle Jet Part Numbers:
• 376/072
  –  state size: .105″, .106″, .107″ and up
• 622/079
  2 stroke –  state size: .106″, .107″, .108″ not crossed drilled .714″ o.a. length counter bored .138″ at bottom – jet orifice at top
• 622/100          alcohol      state size: .120″ and up
• 622/122          4 stroke     state size: .105″, .106″, .107″, .108″ cross drilled .035″ hole – jet orifice at bottom –  .803″ o.a. length – counter bored .158″ deep at top
• 2928/031       4 stroke     stainless .105″ special T140 EPA bikes
• 2928/079      2 stroke     state size: .105″, .106″, .108″, .109″ not crossed drilled – .770″ o.a. length – jet orifice at top – counter bored .140″ deep at bottom
• 2928/100      alcohol      state size: .120″ and up
• 2928/122       4 stroke   state size: .105″, .106″, .107″, .108″ – crossed drilled with .035″ hole .860 o.a. length – counter bored .158″ deep at top. annular identifying groove at top.

  • Needle Jet Notes:
    Needle jets are the only Amal jet where  the number refers to an actual inch dimension. The rest of the jets refer to flow in cc’s per minute.
  • Gasoline carburetors, both two and four stroke,  are commonly fitted with needle jets ranging from .105″ to .107″. A .106″ needle is by far the most common one used.
  • Alcohol carburetors are commonly fitted with needle jets .120″ or larger.
  •  Changing one size either way from the proper needle jet has a strong influence on just off idle to mid range running.
  • A slightly worn, or oversize, needle jet will cause the motorcycle to run very rich at throttle openings up to approximately 1/3rd throttle. Worn needle jets
  • The crossed drilled hole in the 4 stroke needle jets can be made larger to lean out the mixture at low throttle openings. Enlarging this hole has the greatest effect while the straight portion of the needle is in the needle jets metering orifice. As little as .001″ increase in the size has an effect upon the amount of fuel the needle jet will deliver. The largest I have ever used is increasing the hole to .039″. As the hole approaches .050″ the carburetor stops working properly.  To save embarrassment later, be sure to mark the needle jet as oversize.
  • You can use a reamer to enlarge a needle jet. If a .106″ is too small, and a .107″ large you can ream the jet .1065″.
  • If you work on these carburetors as a part of your daily routine a set of Plus and minus .105″, .106″ and .107″ are handy.
 Slide Positions:

Slide Closed (idle): (Red line on needle beginning of
taper. From the slide closed up to approx. 1/3rd throttle
the fuel metering is controlled by the i.d. of the needle
jet and the straight portion of the needle.


Slide at 1/4 Throttle: At 1/4 throttle the needle taper is
just beginning to leave the needle jet (as adjusted by the needle clip position).


Slide at 1/2 Throttle: At 1/2 throttle the fuel metering is
clearly being done by the needle’s taper.

Slide at Full Throttle: At full throttle the tip of the needle
is clear of the Needle Jet orifice. Depending upon which
needle groove the clip is in the Needle stops having any
effect around 3/4 throttle.

Float Bowls (with needle orifice by size):
• 622/052      .062″ float needle orifice – fuel pump – 2 stroke
• 622/054      .156″ float needle orifice – 2 stroke
• 622/055      .100″ 4 stroke – standard – drain plug type
• 622/056      .125″ 4 stroke – racing – drain plug type
• 622/057      .062″ drain plug type
• 622/058      .052″ 4 stroke – fuel pump
• 2622/055    .100″
• 2622/056    .062″ 2 stroke – fuel pump
• 2622/057    .125″ 4 stroke – standard
• 2622/058    .156″ alcohol
• 2622/059    .052″ 4 stroke – fuel pump

  • Pilot Jets:
    2 stroke Concentric and MKII (also used as choke jets)
    • 124/026      in sizes 15-up in increments of 5cc flow (15 would flow 15cc per minute)
    • 622/026-15     .014″
    • 622/026-20    .015″
    • 622/026-25    .017″  (standard 600 and 900 series Concentric come with a #25)
    • 622/026-30    .018″
    • 622/026-35    .019″
    • 622/026-40   .021″
    622/502 in sizes in increments of .002″
    • 622/502-15     .015″
    • 622/502-17     .017″ (standard 600 and 900 Premiers come with a 622/502-17 )
    • 622/502-19     .019″ (common to Norton 850 Commando)
    • 622/502-21     .021″
    • 622/502-23     .023″

Slide Cutaways:
slides are listed from 2 to 4 in 1/16″ increments. The #2 cutaway would be 1/8″. A #3 cutaway would be 3/16″.
#2    =  1/8″
#2.5 =  5/32″
#3    =  3/16″
#3.5 =  7/32″
#4    =  1/4″

Triumph 1979 T140 with MKII Carburetors update:
When using a MKII carburetors  on a Triumph for off-road, or racing, it is possible to use European jetting. This jetting is for stock pistons and mufflers. Any modifications to compression or exhaust will require different jetting.

Original EPA Jetting:
124/026    #25 Pilot Jet
124/0026  #50 Choke Jet
2928/031  .105″ Needle  Jet
2928/060  #3 Slide
2928/030  2C3 Needle
Suggested Non-EPA Jetting:
124/026     #15 Pilot Jet
124/026     #35 Choke Jet
2928/122    .106″ Needle Jet
2928/060   # 3 1/2 Slide
2622/124     2A1 Needle

Part Numbers that are hard to find:
Welsh plug in bottom of Concentric body covering the pilot air/fuel mixing chamber   622/085
Pressed in pilot jet   622/107
Replacement float needle brass seat .100″ seating  622/099
T160 lean Kit: 622/278 Needle, 622/060 #4 slide

Tips for Racing Carburetors
Plug Gage Sizes as they relate to Amal jets that are measured by flow

There are occasions when you what to check an Amal pilot or main jet’s number against a standard.
Use these ONLY as a guide, as the final jet orifice is adjusted by hand during manufacture and checked with a manometer.
Flow in cc’s     Jet Diameter
per minute          in inches

15                        –
20                        .015″
25                        –
30                        .018″
35                        –
40                        .021″
45                        –
50                        .024″
55                        –
60                        .026″
65                        –
70                        .028″
75                        –
80                        .030″
85                        –
90                        .032″
95                        –
100                    .034″
110                    .035″
120                    .037″
130                    .038″
140                    .040″
150                    .041″
160                    .043″
170                    .044″
180                    .045″
200                    .048″
220                    .050″
240                    .052″
260                    .055″
280                    .057″
300                    .059″
Reference: – Vintage Motorcyclists Workshop by Radco page 129

Slide Cutaway and Straight vs Megaphone Exhaust Pipes:
Megaphone exhaust usually requires a main jet 40cc larger.
• Straight pipes typically use Leaner slide (higher #) slide like a #4.
• Megaphones typically use Richer slide (lower #) like a #2.
reference Tuning For Speed by P.E. Irving

GP Carburetor notes:
• GP limited to 20° down draft.
• GP pilot screw turns out to richen and in to weaken mixture.
• GP2  introduced to allow greater down draft angles.
To do this they moved the pilot air screw and pilot jet to the atmospheric side of the mixing chamber.
Tuning is the same as the GP except the pilot adjustment screws in to strengthen and out to weaken the mixture. This also cures pilot-jet weep at steep down-draft angles.
• Pilot jets are the same as used in the Monobloc (376/076).
• Fuel level in float is set to the bottom of the circle cut in the side of the air jet cover.
• The float bowl should be located in front of the main jet. This prevents fuel
starvation on acceleration.
• The GP relies upon an air-jet to correct fuel/air mixture while running on the main jet  at a constant throttle opening (typically wide open throttle) subject to varying engine loads. As the engine load changes going over varying terrain, the air velocity through the venturi will change. As the air velocity changes so does the vacuum signal on the main jet. Without the air-jet, while climbing a grade, the air velocity will slow drawing less fuel and you will have a lean mixture. Going down the grade the air velocity will increase and draw more fuel giving you a rich mixture. The air-jet compensates for this change in air velocity at WOT providing a steady fuel/air mixture.
• The air-slide cab be helpful when tuning the GP, or TT. If at WOT the speed increases when the lever is closed slightly, the main jet is too small. If the speed falls off, the main jet is either correct or too large. The air-jet can be used to change the mixture to compensate for large changes in elevation or ambient air temperature during a race. This type of carburetor was designed for use in races venues like the Isle of Man.
• The air-slide, which controls the amount of air reaching the air-jet, is located on the side of the carburetor body. The air-slide cavity is easily recognized by the long slot on its side.  Many people think the air-slide is a choke. The air-slide is opened and closed to change the amount of air reaching the air-jet.
• The purpose of the air jet is to compensate  the mixture strength for varying air speed through the venturi.
• The air-jet for carburetor up to 1 1/16″ is .100″ over that it is .125″. The proper air-jet is installed at the factory and seldom needs to be changed.
Reference above:- Tuning for Speed by P.E. Irving

To read more about carburetors used on these motorcycles:
Tuning for Speed. by P.E. Irving
The Vintage Motorcyclist’s Workshop  by Radco