A customer pulls in. "I cleaned the carbs and the bike still will not idle right." Translation: they sprayed Berryman B12 in the top of the carb and put it back together. The visible parts are clean. The actual problem — the pilot circuit — has been untouched.

The pilot circuit is the hardest part of an Amal to clean because it is the smallest. The pilot jet, the air screw passage, the small drilling that connects them, the angle drilling that emerges into the carb throat. Tiny passages, dried fuel residue at every corner, sometimes a chunk of debris from a varnished tank. Spray cleaner does not reach into the middle of those passages.

What you need

  • Carb cleaner — Berryman B12, Gunk Carb Cleaner, anything in a spray can.
  • A small ultrasonic cleaner. The $50 Harbor Freight unit is enough for a single carb body. The $120 unit is enough for two.
  • Very thin wire for poking through passages. A guitar string (high E, .010") or stranded copper from a 22-gauge wire.
  • Compressed air with a blow gun.
  • A small parts tray, magnetic if possible. Pilot jets are easy to lose.

Teardown

Remove the carb from the bike. Not "loosen the float bowl." Actually unbolt it from the manifold and take it to the bench. Cleaning a carb on the bike is a half-job that the bike notices.

Remove the float bowl, float, float pin, and needle valve. Set aside in your parts tray.

Unscrew the pilot jet — it is the small brass jet near the pilot air screw, not the main jet which is in the bottom of the float bowl. On a Monobloc the pilot jet is screwed into a recess in the body. On a Concentric it is in a pocket near the air screw.

Remove the pilot air screw. Before you do, count the turns out from fully seated. Write the count down. The default starting position is 1.5 turns; the bike may have been set differently.

The slide and needle stay together as a unit. Pull them out the top and set aside in the parts tray.

Cleaning

Brass parts (pilot jet, main jet, needle valve, float pin) go in the ultrasonic for 30 minutes in a 50/50 carb cleaner and water mix. Run the ultrasonic warm if your unit has that setting.

Carb body goes in the ultrasonic for 20 minutes. Do not put a complete body with butterflies still attached — the linkage and any rubber parts will be damaged. Strip the body down to just the casting.

Pilot jet inspection: hold the jet up to a strong light. You should see a clean circular hole through the center. If the hole is pinhole-blocked or you cannot see daylight, the jet is replaced rather than cleaned. New pilot jets are $6 each. The hour you would spend trying to clean a stubborn one is worth more than $6.

Passages: spray carb cleaner through every drilling in the body, in both directions. Run the thin wire through the pilot circuit drilling — gently, without scoring the brass. Spray again. Then blow with compressed air. You should hear cleaner spraying out the other end of the passage.

The verification trick

When you think you are done, here is how to check.

  • Hold the carb body in your hand, throat facing down.
  • Place a finger over the main jet hole in the bottom of the float bowl pocket.
  • Spray carb cleaner into the float bowl recess.
  • The cleaner should spray out the pilot circuit orifice in the carb throat (a tiny hole on the engine side of the butterfly).

If cleaner sprays out cleanly, the pilot circuit is clear. If it does not, the passage is still blocked, and you go back to ultrasonic with more time.

On an Amal Monobloc the pilot circuit orifice is on the carb throat side, about a quarter inch up from the butterfly cutout on the engine side. On a Concentric it is in roughly the same spot but the drilling is slightly different. Both are visible to the eye and you can see the cleaner spray when the passage is open.

If you cannot see daylight through the pilot jet, no amount of spray can save it.

Reassembly and setup

Reassemble in reverse. Pilot jet snug, not torqued — these are small brass jets in soft alloy threads. Air screw back to the position you wrote down, or 1.5 turns out as a starting point.

On the bike: warm the engine. With the idle stable, turn the pilot air screw in quarter-turn increments toward seated. The engine should rise to a peak and then start to fall as you go too far in. Back off a quarter turn from peak. Set the throttle stop screw for 900 rpm or whatever idle speed your bike likes.

A correctly cleaned and set pilot circuit gives you a stable idle that does not hunt, a clean response from idle to the first crack of throttle, and a bike that does not require you to keep the revs up to keep it running. It is one of the cheapest large improvements you can make to a vintage Triumph, and the difference between fussy and not fussy.