Is this your first winter with this bike perhaps?
Some bikes are cold blooded. My Ninja750 had to warm up FOREVER before it ran properly. I'd have to adjust the choke several times to keep the idle from climbing to neighbor-alarm levels.
But let's discuss what's going on:
1)Engine fires up ok but idling speed suddenly shoots up to 3k revs. Idle speed knob doesn't help but choke helps brings the revs down.
An engine is THROTTLED by limiting the amount of air it can suck in. The idle screw may hold the main throttle plate open a tab, OR control some sort of limited air bleed AROUND the throttle.
FWIW, modern, fuel-injected cars ALL have a stepper-motor based, computer-controlled idle bypass valve because maintaining idle is A) so difficult under varying conditions of air temperature, altitude, fuel grade, etc., B) So important towards meeting emissions goals.
So your bike, when the idle skyrockets, is getting too much air. How does that happen. Well, your throttle cable could be sticking a little. Or your throttle shafts are a little worn. Or your intake boots are not sealing, they're cold and hard (they can be softened in an overnight bath of zylene and wintergreen oil). Once they warm up, the leak goes away, and everything operates normally. Or your intake manifold gasket has a leak. I'm sure I haven't listed EVERY possibility, but these are common.
You can often FIND an idle air leak by taking a can of aerosol brake cleaner, which is flammable, and spraying AROUND your boots, etc. If you get a sudden surge in rpm because of the additional "faux fuel" you've introduced you have found your leak.
Or you can take cigarette or vape smoke and see if if it gets sucked INTO your machine at one of these locations.
Lastly, in the auto world, we use a smoke machine to PRESSURIZE the intake manifold 1-2 psi with smoke and see where smoke leaks OUT.
Things like pleated (accordion-like) snorkels are notorious, in the auto world, for developing cracks over time. Molded vacuum lines, which get hard,and crack due to engine heat. etc.
>Try blipping the throttle & the engine croaks (croaking sound). Persist with it & the engine dies.
This indicates the engine isn't getting enough fuel to run. YOu provide it a big slug of cold air when you blip the throttle. Unless the choke is full on, it can't enrich the charge sufficiently. Note that on a cold (30-40F) morning your COLD engine may require FIVE TIMES as much fuel to run as it does when warm. Yeah, that's a LOT of enrichment!
2)Engine fires up ok. Try blipping the throttle & the engine croaks. Give it a bit of choke & the engine will rev quite nicely as it should be.
What I just said!
Release the choke & try blipping & the engine croaks. Try maintaining that throttle level & the engine croaks out.
3)Today it took 6 kicks to get it to start
Six kicks? Are you sure your choke is working properly? And what about your fuel bowl. Is it dripping, (bottom is wet, dirt on bottom indicating it has BEEN wet?) so you're trying to start a motor with no gas in the carburetor bowl, and having to "pump up" the fuel?
>& idling was super low
So today your intermittent vacuum leak wasn't there, and since you'd adjusted the idle for when it WAS there.... you got a super low idle.
& the engine died. Pull the choke & kick again, the engine starts with 3k idling revs & we're back to 1) above.
Are you starting to see why SOME of us will ONLY purchase fuel-injected bikes anymore? @#$#'ing around with chokes gets old. I'd simply give up and spritz a little starter fluid sometimes rather than break my leg kicking a bike over and over.
>Even when the idling speed seems to hv stabilised, when i try to move off, the throttle input causes the engine to croak & i will immediately stall.
Indicative of insufficient choke, OR...an engine that just hasn't warmed up enough to ride yet. Air cooled motors...yeah, motorcycles and the engines on them were designed to ride in RIDING weather from 65-95F degrees. Optimized for that. Cold blooded.
But once the engine is at a certain temperature, much warmer then when i usually start moving off anyway, all these symptoms disappear & the bike behaves normally.
Yep!
Ok, so now you know what's going on, the causes and cures, and can take appropriate action. Adjust & lube your throttle & choke cables & mechanisms, check for sticking throttle shafts, look for a drippy carb, cheat with starter fluid, make sure the choke DOES fully engage
Now a final word. Choke is a double-edged sword. It will permit an engine to start and run when cold.
But all that excess, LIQUID fuel, that doesn't vaporize and burn will scrub the oil off the cylinder and cause excessive wear of the rings and cylinder.
Fuel both gets ATOMIZED by a carburetor, that is, made into tiny drops through the emulsion tube/venturi action, etc, and VAPORIZED by the heat of the intake manifold. Cold fuel, cold engine, you end up mostly with LIQUID fuel which does NOT combust going into the engine. Remember, gasoline/air VAPORS burn, not LIQUID gasoline.
So you want to use choke as sparingly as possible, and turn it off as SOON as you can get the engine to hold a reasonable RPM. Idling for 20 minutes in the driveway with full choke is NOT a way to make a bike last 20 years.
The replacement of choke by software-controlled "cold start enrichment" in fuel-injected engines, which lasts 2 minutes or less, with major reductions after 30 seconds, is why most modern engines (car) will last more than 100,000 miles. Back in the "carb" days, an engine was typically worn out, due for rebuild and TRADED IN by 100K. Sheesh, lot of cars will go 250K today and it's NEVER the engine, rings, main bearings...it's always little @#$ that wears out.
Ok, ready now for the quiz?
1) The best way to deal with cold weather starting is
A) Move to Arizona for the Winter
B) Park the bike in a heated garage
C) Seal up any boot air leaks and use choke as needed
D) Buy a fuel-injected bike and spend all the time you save on cold mornings choking your chicken instead of your carburetor.
Addendum:
>tune down the carb for better fuel economy.
You can't get better fuel economy (miles per gallon, mpg) by leaning an engine out, UNLESS you're the designer of that engine and can adjust ignition advance and all sorts of other factors to prevent damage. I don't know what your mechanic did but running a 2-stroke lean is a sure-fire RECIPE for engine damage.
And 2-stroke fuel (with added oil) is EVEN harder to vaporize on a cold morning than normal "gas." One trick we did with chain saws was to take the plug out, warm it with a cigarette lighter, put it back in and THEN start the saw. The hot plug VAPORIZED just enough liquid fuel to get the saw to start.
