Help Needed on Amal Concentric MK1

I am restoring a '74 MK2 Roadster. It has sat for 5 years or so.

I am endeavouring to make sure the idle circuits are clear on both of the MK1 concentrics (32mm).

So far when I squirt carb cleaner in through the mixture screw aperture it sprays out the two tiny pilot holes near where the front edge of the throttle slide closes. I gather this is ok and proves the air passage to the idle circuit is alright.

When I squirt carb cleaner in through the hole in the bottom of the carb body (the one that draws the fuel up from the float bowl) and cover the mixture screw hole and the air intake hole where the air cleaner connects I cannot get any fluid to spray out of the two tiny pilot holes as per the previous test.

My questions are:

1. Should the carb cleaner spray out of the two tiny pilot holes at the front of the slide?

2. If fluid should spray out of the two tint holes and does not as in my case - where is the blockage most likely to be and how best can I clear it.

So far I have used a fine wire strand from a wire brush mounted on a match stick and put this through the mixture screw aperture and cleaned out that small brass jet (0.016" I believe). I know on both carbs the wire is going through till it stops at the fuel drilling on the far side.

What could I soak the carbs in to loosen any scale or gum on the insides of the carb passages.

Any suggestions appreciated.

Thanks Don

Good job, testing with spray. Amal’s have a CHAMBER with an expansion (welsh/freeze) plug.

I clean those tiny holes (the drillings into the carb throat) with a single bristle plucked from a wire brush, held with needle nose pliers. Works splendidly.

i use #78 drill bit epoxied into carb spray can straw twist the bit-tool as you put it in which helps draw the crud out rather than push it in

It means air part of the circuit is clear, gas part is clogged.

As above, the bit that is blocked is behind the jet in the 90 deg turn, I.e the route the fuel takes from the float bowl.
You need a bit of wire that has the right combination of stiffness and bend ability. I have used the thin wires of a gas torch nozzle cleaner. They are long enough and stiff enough, but you can still work them down the hole where the float bowl fuel supply mates to the carb body. Keep doing that and alternate with compressed air from the other end. You can use the pilot air screw as a blocker and blow down from the inlet mouth end. Poking through the jet will probably help as well, but you have come at it from float bowl supply end really. I have seen people saying drilling out the blank, then inserting a 2BA helicoil is the last option.
I did this exact thing about 2 months ago (not helicoil, wire poking), bike will idle at 600 (indicated!) now, though I set it at 900.

Thanks for the replies guys.
Does anyone know the thread type where the jet holder screws into the bottom of the carb body?
Thanks Don

If LAB can’t help drop the guy at Surrey Cycles a line, he would probably know.

@thunderbolt think it’s 5/16 inch BSF

thunderbolt said:

Does anyone know the thread type where the jet holder screws into the bottom of the carb body?

Click to expand...

I don't know the actual thread type (although it appears to be Whitworth thread form) but it measures a little under 7/16" and is 19 tpi.

Thanks Guys