Hi steve-d, A few thoughts. First are you still running chokes? Not sure. is it the '77 or your '73. The earlier had choke lever on left intake manifold attached to underside with 1/4" bolt. The hole goes through to inside manifold. So if bolt is missing it makes a big air leak. I believe the '77 has bar mounted choke lever. Later manifolds weren't drilled, but check to be sure.
Cold motor will often not idle reliably at all. Once warm same exact adjustments will bring idle to about 950-1100. It takes about 4-5 miles depending on how cold it is to bring motor to reliable idle. High speed riding like 10 miles of freeway 60-65mph will bring idle up 50-150 rpm. Again the exact same bike. Now you slow through easy riding roads. Idle goes back to normal in a mile or so. Suppose you are on same freeway ride. Bottom of off ramp your at 1200. Now you go down long hill on compression braking, say 3/4-1 mile. More is not too cold. Again motor cold idle is lower now about 925. Sometimes you think it may try to stall. Getting onto level or a few red lights, motor warms back up, idle goes back to normal. This is normal for a correct running bike.
In any case cold motor should not idle high. In fact cold you'll have to hold the rpm up as there is no provision for cold fast idle. Also if bike runs really well cold with no choke, it often is too rich when warm. However if you idle is stuck at near 1500 cold, & you have no air leaks the slides are too far open.
There has been mention of throttle cable play at idle. That is most important. Do you have one or 2 cables to twist grip? Normally would have 1 & a splitter to rear cables. Some get fed up with splitter & go to 2 cables & twin cable twist grip. No matter both work fine.
So.... right now with motor off reach down & lift the cable at top of carb. Gently at first. It should lift about 1/16" ish. But should have some play.
Then pull harder on cable, so hard you can feel sides lift. Now lower cable slowly, can you feel the slide bind. Moving cable by hand you may or may not feel slide hit stop screw.
Now open throttle a lot & close very quickly. You should hear decided click as slides hit the stop screw. You can feel it with finger on side of carb once you know what to feel for. If you don't hear the click something is binding or you don't have enough play in cable at carb top.
With air filter cover off you can stick finger in slide & feel for binding & feel & hear the click when you let it drop.
My hunch is you don't have enough play at cables. You have 3 cables. If front cable is too short, it will hold splitter up in junction box. Then it's like the twist grip is being held open some.
At this time, I'd like you to feel for play. Give it more play like 3/16" at least. Now hold one cable gently, back off rpm screw 2 turns. Does cable go down following the slide? Do this both sides. Now start bike & see where rpm is.
When syncing cables you may or may not know where to start with RPM screw. So you make a best guess so to speak. If rpm is too high you back out rpm screws in attempt to lower. This removes play from cable. So you always have to keep the cable play in mind when syncing carbs. You may/will have to go back & adjust cable play as you find the rpm & mixture on new build or new carbs etc.
I personally like to check slide opening at idle with drill bits to make them even. Sort of using them like feeler gauges. I first get lots & lots of cable play. I start from zero then tighten screw upwards until slide lifts a little. Measure with bit. Then set other side the same. Do click test with finger or throttle to verify slides drop to adjuster screw. Then set cable play to 1/8" or so both sides. Now open throttle to about 3/8". Stick drill bit in both sides. Just get them close for now.
Mixture screw base setting is 1.5 turns out. At 1-1/8 you're biasing idle mixture fairly rich. No matter you'll trim mixture to best hot idle later.
Now I start bike & ride it. Usually runs pretty well, maybe not perfect. Remember won't idle right for first few miles. After 5 miles I pull over & evaluate rpm & mixture. If rpm is close to what I like I do mixture screws first one side, then the other. Out & in feeling for best/smoothest/fastest idle. You do one side, then the other. Trim rpm, always thinking about running out of cable play... If rpm is too high or too low change rpm first, then do mixture screws.
Remember you set slides even in garage. So move the mixture screws same amount each side. Tip, if going up (inwards) with rpm screw hold throttle open slightly when you turn screw so slide is not touching screw as you turn it. Let throttle go. Blip throttle a few times & let rpm settle down. Turn rpm screws as needed evenly.
Then go back to mixture screws. So you end up in a loop. Rpm, mixture. Round & round until motor runs best at desired rpm. Ride bike another 5 miles. Pull over & recheck idle rpm & mixture. Trim as needed. These two 5 mile road tests will get you close, not perfect, but close. It takes a good 20+ miles to really warm the motor. It can change then, but not too much. Final adjustments will be done after 20 miles. You'll find the happy medium idle rpm by how bike runs. I tend to like about 1050 as it idles very reliably in all conditions. Some like it a bit slower. Personal choice. It's 100f+ often in my summers so faster idle is better in hot stop & go traffic. Yet again keep eye on cable play. For now just make play even by eye.
Now back in garage you'll adjust the true lift of slides. Again I like drill bit about 3/8". Very steadily hold throttle open. You can tighten the friction lock screw or have steady assistant hold throttle. Feel the slide opening with drill bits. Adjust both sides even at rear cable adjusters, or carb top adjuster as case may be. Always thinking about the cable play at idle. We don't want to end up with no play... If you don't have enough play loosen both adjusters more. Then use the 3/8 bit again. But don't touch the rpm screws.
Finally you have play at idle, the slides lift evenly. Now go back & check gap at idle. Using drill bits (set is by 1/64s is what you need to 3/8. To 1/2 is better). How different are slides? The 3/8 bit showed slides lifted evenly. That is very important. Idle the slides should be close, but sometimes motor runs better with one slide a little lower or higher at idle, just like one side may want a little richer or leaner mixture. It is how the bike runs best.
Seems this takes a lot of warm up road test time & fiddling. Yes it does. When you get right the motor will run so good you'll be delighted. It will stay adjusted a long time. With practice & experience this is not hard at all & not too time consuming. There is no short cutting the warming of motor. If you short cut warming & adjustments you cannot achieve best running.
Back to binding slides. After you get good cable play & sides are hitting stop screws as they should cold. Heat soak the motor. Again 20+ miles. Even at 100f it takes a good 15+ miles. 20 really. On 50f day it may take 50 miles or maybe really never heat soak fully.
So you have idle rpm good. Memorize or measure cable play both sides. Remember at idle play may not be the same as one side may have wanted a little more or less rpm screw. So look at each side in it's own right.
Now is starts hanging up. Idle rpm obviously too high. To check for binding slide if it hangs up blip throttle wide open for an instant. Let it go. The slide drops fast & hard from the spring pressure. Now rpm is normal. Or not... If rpm won't drop check the cable play. If slide(s) is sticking the cable will have additional play by amount slide is sticking.
I've only experienced sticking slides a few times. Each time after installing new slides in old carbs. The slide felt perfectly free on install & cold. But heat soaked on rare occasions the slide would stick at about 3/16-1/4 throttle. (grip was marked). Throttle grip I could feel lots of free play/lost motion so I know slide was partly up. Scare the pants off you in town or slowing for sharp curve. Blipping throttle would drop slide to normal. Twice I hit kill switch just in case as I blipped throttle. On a friends bike put old slides back in, was again ok. Got new carbs. On my bike I pulled new slide. Looked like it was catching on front of bore of throttle due to wear. I put a tiny chamfer on bottom of new slide. Kept riding it. It wanted to catch slightly after but very rarely & even a tiny blip would drop it. Finally wore into old carb & never stuck again. About 5k miles later Premier casting were back in stock. Got new carb. New premier carb has covered 10k with basically zero slide or bore wear. Worth the $$! Not surprisingly the new slide I put in old carb quickly wore to same as old slide.
Sticking slide is unnerving at best, kills you at worst. Only sticks at the worst time. Kill switch is a valid safety feature.
I don't know why you are sticking. Now you can feel cable play. If it's sticking & you don't have cable play the cables and/or junction block is at fault.
If your slides/carb bore are worn out, even if not causing sticking, I mean just worn out, there is a test that has shown reliable. Heat soak motor well, I mean fully heat soaked. Now pull over. Let motor idle 15-20 seconds. Normal idle 950-1050. If idle is too fast test is not valid. Now open throttle slowly as possible. I mean slowly as you can. If motor falters bad or actually dies before rpm picks up, that's worn out carb. However a quicker turn of throttle will raise rpm as expected, no dying. Do this test several times. Get the feel for it. This was told to me by member here. I've used this several times. Hasn't failed yet. I do same test over a few road tests on different days. If carb is worn the tests will be repeatable. Once you get the hang of it you can feel it right off the bat. New slide may improve short term. Don't even think about sleeving. Get new premiers . They were on back order recently. Waiting for castings.
In the mean time waiting for new carb, raise idle rpm to a bit faster than we'd like. Play with mixture screw. At best idle it will die opening slowly. You can change/cheat mixture & experiment with slow opening. Sounds counter intuitive, but in each case I found cheating mixture about 1/4 lean(out) , then raising rpm helped. Took me several thousand miles to get a full diagnosis & learn to believe the slow opening test was valid. Then a good 6 months for carbs to come off back order. I got really good at working around the dying, but certainly no fun to ride.
A good, not worn carb will raise rpm with no trace of faltering or dying. No matter how slow you turn grip motor will follow the grip & run well the entire time.
A thought about points... I run points, I like the reliability of points & you can start bike with dead battery
with points. I don't mind servicing points at all. The bad part of points is advance curve is not desirable with modern fuel. Especially California fuel.
Sometimes (often!) the advance springs are not quite tight or strong enough. This allows the advance unit to advance too rapidly. Also the slot in weights wear flat spot & even a dip where weight can sort of catch & stick slightly, then advance suddenly. This can start right about 1100 ish rpm. You feel rpm is a bit slow, so you raise it. Then it does some odd things like my creep up for no reason, but it's the weights advancing timing. Timing change has huge effect on idle rpm.
Triumph sent out a Tech bulletin about stronger/different springs. No longer made it seems. Lots of sellers sell the correct part #, but new springs prove worse than old... A quick & dirty test is put points cam where both rubbing blocks are putting pressure on cam best you can. Move cam to advance & let go. Springs should reliably pull cam back to zero every time. Again new springs can be worse than old. I shortened the spring loops both ends, both springs, by about half. They barely fit on pegs of AAU & weights. Looking in old posts by Pete here & RAT group he shortened them to change curves.
If loose/long/weak springs is part of problem, shortening will help. If not, it's never bad to shorten springs, but won't cure current problem.
Take you time, go one step at a time. This may prove simple or not. It certainly is not normal & can be fixed.