Carb Mods for Amal Mk1 Concentrics

When Used on Vincent Twin and Singles

Fuel Nozzle Mod:

The fuel nozzle is a small, and short tube that is on top of the needle jet. The needle can be seen going into it when viewed from either end of the carbs bore. These carbs are mostly used on 650cc motorcycles that usually have higher slide openings at lower RPMs. Only large bore motors will benefit from this mod. The nozzle is partialy covered by the bottom of the slide at slower speeds. Air has to be able to pass over the open top of the fuel nozzle before it can discharge fuel. Large bore motors suffer, because the motor is getting enough air, but not enough fuel at slight slide openings, causing a lean mixture, and detonation. This can be cured by cutting the entire back half of the nozzle away behind the needle. Take a long fine pointed scribe, and from the down-stream throttle opening, make a scratch tracing on the entire exposed back half of the fuel nozzle. Remove it by removing the bowl, and needle jet, then gently press it down, and out. Now take a Dremmell cut-off tool to carefully cut away the material along the scribed line. Now the air entering the cut-away opening in the slide doesn't have to rise, and go above the nozzle, it can go around the sides of the nozzle, and the resulting vacuum will pull fuel up, and around the needle at lower slide openings. I noticed that the factory did this on the Concentrics on my '75 Norton 850 Mk3. Do this carefully! You don't want to be looking for new nozzles.

Float Setting:

The Concentrics are normally non-adjustable, but with a gentle touch, they can be re-adjusted. The needle dangles from the float, and the brass seat is pressed into the float bowl. The seat can be tapped in either direction by first heating the bowl in running hot water for about 5 minutes. There is a small hole in the bowl at one end of the seat where a small drill bit can be used as a drift. a larger drill bit in the end of the seat will move it the other way. Make a seat position change, and then take a measurement. It will take several attempts to get it just right. Test the fit by holding the bowl upright with the float, needle, and shaft held lightly in place (as though it was installed), and note how far the drop of the edge of the float is below the edge of the bowl opposite of the needle. Do not use the gasket for a measurement. The drop should be .080" for Vincents, BSA, and Triumph, and use .090" for Nortons. Take a measurement first to see how much, and in which direction to make a change. They are never correct, and this will vastly improve the performance of the carbs. I use a .080" drill bit, and I hold it against the bowl edge near the float edge to estimate the drop.

Needle Jets:

The Concentrics almost always come with the #106 (.106") needle jet. Large bore motors can usually benefit from an increase to a #107. Most people aren't aware that the needle jet is always feeding some fuel up by the needle along the needles straight shank section, which is always passing some fuel. It also influences the pilot jet choice. The right choice needle jet will help the pilot jet before, and after the the pilot jet delivers fuel. Modern fuel seems to require a step-up in needle jet size these days. I run 107s in my big-bore Vincent, as well as my Norton 850. At my high elevation of 7,000', I have found that 105 needle jets work best for Mk1 cams, and low compression motors.

Pilot Jet Mod:

Only the very early Concentrics came with pilot jets in various sizes. At some time, they instead installed a restrictor jet (.015") in the pilot jet passage, and did away with the removable pilot jets. These are not big enough for large bore motors. To correct this, first remove the mixture needle, and spring from the side of the carb. Look into the hole, and you will see a little brass restrictor with a tiny hole. Take a .040" drill bit, and on a Dremmel tool with a chuck, carefully open up the hole to .040". Now you can install pilot jets up to a #40 (.040").


Vincents seem to require hi-flow petcocks when using Concentrics. The import Norton style don't work very well, while the european made Norton style petcocks seem to be just big enough. Don't let your petcock become your main jet!


Most Concentrics come standard with 3.5 cut-away slides. The 3.0 slides work better for large bore motors. They give quicker revs, and act like an accelerator pump to improve throttle response.

Air Cleaners:

Always use a high quality fine mesh filter with the Concentrics. The smallest grit particals will scratch the slide, and carb bores.

Fuel Line Banjos:

Try not to use the plastic 1/4" fuel banjo connections that come with Concentrics. Try to find the pot-metal 5/16" sized banjos. They can be found at the larger british supply houses.


James R. Mosher

(505) 466-7870