Sorry all.
I've seen
sooo much incorrect advice, magic potions, voodoo and philosophical / metaphysical renuminations on how to get a modern carbed Triumph Bonneville/Thruxton to start, so I cannot resist any longer...
1.) Cold starting definition - anything below say 40 degrees Farenhiet (4.5 degrees C).
2.) Cold starts - the engine case is cold, carb is cold, manifold is cold, so much of the air/fuel mix doesn't atomise correctly, so you will need a very "Rich" mixture, somewhere around 6:1 or so.Until she runs awhile and warms up.
3.) The Bonneville has Carburtetor "De-Icers", which are not to be confused with heaters, and have absolutely, positively zero (nothing) to do with cold starts. So fuggetabowdit.
4.) Keihin carburetors utilise a Cold start "Enricher", which is an additional "spurt" of fuel. The Keihins do not have a conventional "Choke", as in a butterfly closing off the venturi to only allow the accelerator pump - which the Keihin does not have - to spray raw fuel in the manifold. Therefore say "Enricher", not "Choke" from now on.
OK, those facts beknown, lets move forward...
A.) Make sure your idle speed is set to the lowest allowable. The idle speed set screw works by opening up the throttle butterfly slightly, allowing more "air", and a higher idle speed. "Air", we do not want in this case of a cold start. We want lots of raw fuel, little air. Also, never touch nor twist the throttle grip, which only opens and closes the butterfly, having zero to do with fuel addition, but instead allowing more "Air" (bad for cold starts) in.
B.) Pull the "Enricher" Knob fully out. Hold the bike up balanced horizontally, not tilted on the kick stand. This allows both float bowls to be exactly even, and helps.
C.) In very northern climes (say Illinois / Missouri / Indiana) on up, you may want to consider #42 pilots (stock is #40). These give a little bit better rich mix at start. You'll need to readjust the mixture screws, or MPG will suffer slightly.
D.) I give the bike a little side-to-side shake just to ensure the floats are unstuck, and the needles are wet.
E.) Press starter... Varoom, she should start. If not, something listed above is wrong.
F.) If no-start, and you tried everything above, your floats are probably set too low. Re-adjust per Manual.
My bike wouldn't start above around 45 degrees F (7.2 Degrees C) when brand new. An "Expert" dealer mechanic worked on it three times (warranty!!!) to zero improvement. In the end, he said it was because of "Cheap British gas". The bike was built in Thailand (2007), and the Petroleum Industry in Great Britain would also be disappointed to hear such comments. He also opined that "They don't have cold winters in England, and these bikes weren't built for it", an odd, enlightening and very unique perspective. (I've since changed Dealers and now feel significantly more confident in their abilities.)
I rejetted to the #42's, re-adjusted the mix screws, and followed the common sense approach as noted above. Since then, she has never, ever failed to start first time, and in northern Illinois cold such as zero degrees F, sometimes -5 or so.
Always remember, the bike wants lots of raw fuel to compensate for poor atomisation and condensation of the fuel on the cold walls of the carb, manifold, and cylinder head runners, and little air velocity, until the manifold and cylinder head walls heat up, then things go back to normal. Anything that allows more air (throttle/butterfly opening) or less fuel (small jets, low set floats) when attempting to cold start will make it difficult, if not impossible.
Lastly, don't forget to push the "Enricher " Knob back in after 2 or 3 minutes of running - it's not automatic, and you'll continue to run rich if you leave it out.
Ride safe (after she starts!)
Bob
Added on December 20th, 2010: One more item to consider. Hinckley Bonneville's - assuming you haven't raised the compression ratio significantly, as in up to 11.5 to 12.0 : 1 -run on 87 to 89 Octane fuel. I see where many feel that adding 93 octane (premium) is a "good" thing, and makes their bikes faster, increased fuel economy, better this n' that, etc. All of which is scientifically, thermally and chemically impossible, but there you go. You may want to reconsider at least for the winter months - ala <40 degrees F, or <5 degrees C. Read in this Forum the Innovation Center/Down at the ol' Fillin' Station thread about the effcts of the slow burning premium fuel being used in a fast burning designed low compression (9.2:1) motorcycle engine. The pre-programmed ignition system is looking for a faster burning fuel to ignite at a precise timing interval, and "premium" devotees have provided a slow burning, higher octane fuel. A recipe for a poor start. That said, try using the OEM recommended fuel, and see if it helps. Can't hurt!
Cold Starting a Hinckley Bonneville
. One just had to flood the Amal's until they started copiously regurgitating gas, let one's right leg hit the kick-start lever 2-3 times as if one did NOT really expect the bike to start (just to get that excess of gas sucked in by the engine upon intuitively reaching the max compression point ) and then hit it again with all one's "now or never" might... as if one's life (aka the respect of fellow riders) depended on it. 
. Best. 
