There is a specific kind of frustration that comes from chasing the wrong setting on an Amal. You change the main jet. The bike runs a little different. You change it again. The bike runs a little different again. Three hours and four jets later you are no closer to a good-running bike than when you started, and you have spent fifty dollars on brass.
The reason is that you started in the wrong place. Each carb setting affects the range above it. Tuning the main when the pilot is wrong is like setting the toe on a car with a bent control arm. You can chase the symptom forever.
Here is the order that works. We do it in the shop. It is not original to us — every Amal manual and every shop that has lived with these carbs prints something close to it — but it gets ignored often enough that it is worth repeating.
How an Amal divides up the throttle
Before you touch anything, know which circuit controls which slice of the throttle. The Amal has four overlapping fuel circuits and each one owns a range. Tune them out of order and you chase symptoms across ranges that have nothing to do with the screw you are turning.
Pilot circuit runs 0 to about 1/8 throttle — idle and the first crack of the grip. Slide cutaway runs 1/8 to roughly 1/4 — the bottom end of acceleration. The needle and needle jet run 1/4 to about 3/4 — the meat of normal cruising and rolling on. The main jet runs 3/4 to wide open. Anything you feel in one of those ranges, with everything else dialed, points at the circuit that owns that range.
Pilot circuit first
The pilot controls idle and the first eighth of throttle. With the bike warm, set the pilot air screw at 1.5 turns out as a starting point. Set the idle (throttle stop) screw for a stable idle around 900 rpm. Then come back to the air screw and adjust by quarter turns for the highest stable idle, retouching the throttle stop to bring rpm back down each time.
If you cannot get a stable idle no matter what the air screw does, you have a pilot circuit blockage, not a tuning problem. Pull the pilot jet (Monobloc and pre-1968 Concentric MK1) or inspect the pressed pilot bush (post-1968 Concentric MK2 has a fixed bush rather than a removable jet) and verify you can see daylight through it. If you cannot, replacement is six dollars; cleaning is an hour with an ultrasonic.
Slide cutaway, 1/8 to 1/4
The cutaway is the chamfered cut on the air-filter side of the throttle slide. As the slide lifts off the bench, the cutaway shapes the airflow before the needle has lifted far enough to do real work. Most Triumph twins came stock with a 3 or 3.5 cutaway depending on year and model.
You cannot turn a screw to change the cutaway — it is part of the slide casting. If the bike stumbles or hesitates from 1/8 to 1/4 throttle with the pilot dialed and the needle in a sane position, a different cutaway slide is the fix. A higher cutaway number (4 instead of 3) leans the bottom of acceleration; a lower number richens it. Order the slide from Amal or Burlen and swap it in.
Needle and clip position, 1/4 to 3/4
The needle is the tapered brass rod hanging off the bottom of the slide. As the slide lifts, the needle pulls out of the needle jet and lets more fuel through. The clip position on the needle determines how rich or lean that whole range is.
The rule worth memorising: lower to lean, raise to richen. Moving the clip to a higher groove drops the needle deeper into the jet, leans the mixture. Moving the clip to a lower groove raises the needle, richens the mixture. Most stock Triumphs run the middle clip. If the bike flat-spots at quarter throttle, go one groove leaner. If it stumbles off idle and the pilot is set right, go one groove richer. One notch at a time. Ride after each change.
Main jet last, 3/4 to wide open
On a stock airbox and stock pipe Triumph at sea level, the factory main is almost always correct. The reasons to change it are the obvious ones: a less restrictive air filter, an open pipe, or you ride somewhere thinner than the bike was jetted for.
Read the plug after a sustained wide-open run, not after a parking-lot blip. Tan or light brown means the main is right. Black and sooty means rich, go down two sizes. White or grey means lean, go up two sizes. Stop when you get to tan.
Altitude jetting, roughly
Below 3,000 feet, leave the main alone. Between 3,000 and 6,000 feet (most California desert riding, much of the Southwest interior) drop the main about 5%, rounded to the nearest available jet size. Above 6,000 feet, drop another 4% for each additional 3,000 feet of elevation.
Above 4,000 feet you may also need to drop a needle jet size, but try the needle-clip position first — one groove leaner usually gets you most of the way there without buying parts. Big elevation changes (Death Valley to Mammoth in a weekend) are where altitude jetting actually matters; if your usual rides are within 1,500 feet of where the bike was tuned, the standard jet will be close enough.
Balance the two carbs after each side is set
On a twin-carb Triumph (T120 onward), tune each carb to itself first, then balance the pair. The right tool is a manometer — a U-tube of mercury or a pair of vacuum gauges, one on each intake. The cheap tool is an ear and a piece of clear vacuum hose held between the two ports. Either way, the cables come up off the slides at the same point and the engine pulls evenly through the rev range. A bike that runs fine on one carb and rough on two is almost always a balance problem, not a jetting problem.
You cannot tune around a worn part. A slide that rocks in the carb body is a rebuild, not a jet change.
When to stop tuning and rebuild
Pull the slide out of the carb body. Look at the front face of the slide — the side that faces the air filter. If it is worn enough to see a flat spot or a polished groove where the body meets the slide at top dead center, the carb is worn. If you can rock the slide side-to-side in the bore with the slide in place, the carb is worn.
A worn Amal will not hold a setting. You can get it to run okay for fifteen minutes and then it goes off again as the slide chatters in the bore at different positions. Surrey Cycle Works in the UK re-sleeves the body with stainless inserts and matches a new slide to the sleeve. The work is about £200 per carb shipped, and it lasts another forty years. New carb bodies from Amal direct are an option but cost most of the same money.
The five-minute test
A tuned Amal pulls cleanly from idle to redline with no flat spots, no hunting, and no stumbling. The plug reads tan after a hard ride. The bike starts cold on the second or third kick and warm on the first.
If yours does not, the problem is one of the four steps above, and you can tell which by where the bike fails. Idle and the first crack of throttle is pilot. The middle of the range is needle. The top is main. Anywhere it feels like the bike is fighting itself, suspect the slide first.