Thunderbird 900 Spitting Out Its Coolant

It's got all the symptoms of a failed head gasket.
Any decent dealer will be able to do a pressure leak down test to prove one way or the other
The one discoured plug suggests to me that is the cylinder coolant leaking into and compression is leaking out of blowing the coolant out.

 

Really sorry to hear you haven't sorted this Stu.

If you are having trouble sourcing a new stat could you not just remove yours completely and run the bike without one. Purely to eliminate the possibility of the stat being the cause.

I vaguely remember doing this in the early 70's on old minis I used to run.

Although speeder may be onto something with the head gasket idea.

 

Might be worth getting one of these

 

That's where I was going @speeder but didn't want to frighten him off straightaway.

 

There are a few things that can and do cause overheating, you have eliminated a few, therefore for me I would firstly check the radiator, it may appear to be ok, however sediment can and does build up in the core on older machines, especially if they have been laid up, this sediment turns hard and restricts flow, leading to overheating, I would drain the whole system and flush it through, some people insist on using distilled or de ionised water, I have only ever used mains water via a suitable attached hose, although I don't live in a hard water area. That said do workshop staff use distilled water when changing coolant, you would need a huge amount in stock and great cost. You say the thermostat was sticking but did open at the correct temperature, at least that is a clue, if it is sticking but eventually opens it could lead to overheating, personally when I test thermostats, any sign of a fault I replace, It is rare I would put back any removed thermostat that I had taken out for testing, just for peace of mind, you say they are hard to come by, those thermostats are not made by Triumph, they are fitted to a number of different models and manufacturers, so get the number off the stat and look at an alternative. Given it blows out smoke on start up, if grey in colour it could be the head gasket, although it would be there all the time, when a machine spits out coolant is is usually down to excessive pressure, caused by a fault with either the radiator, thermostat sticking, or not opening correctly, restriction in the engine water jacket or failure of the head gasket. Always go for the simplest to try first before moving to the most serious, in your case unfortunately would be the head gasket.

 

Boy oh boy you're having a right old time of trying to sort this out.
I've a 900 Thunderbird, that had a similar problem to you. A new radiator cap fixed it right away.
But that was because I contacted Clive Woods https://www.mk1speedtriple.co.uk/clive-wood-independant-triumph-specialist/
He is brilliant, he is so helpful, I was on the phone to him for well over 40 mins last year, he was running through everything that I'd done, and giving me advise along the way too. One thing he told me was don't fill the coolant overflow bottle above the minimum line. Since following his advise, I've had no problems at all.
Give him a ring, he really is a great guy, and I'm sure he, like others on this forum, will help you to get to the bottom of this.
I really hope that you'll get it sorted.

 

Yes I have used this system numerous times, mostly on cars, but have done so a a couple of bikes, and it does work.

 

I get why you want to try the sealant, firstly it acts a little like the liquid used to inflate tyre punctures, it is a short term measure, and will leave a mess inside the water jacket, if a head gasket has failed then the only real option is remove the head and replace the gasket with either an OEM item or top quality replacement, it is not worth cutting corners, the head needs to be checked for any sign of warping, or cracked valve seats, also I would replace the timing chain and tensioner. If you are unsure about taking the work on yourself then look for a reputable independent workshop who can do the work for you. To do the job it does unfortunately need reasonable skills and more importantly the correct tools, you only need a sized head bolt to snap and without the right tools you could be in difficulties. I use an induction heater kit for such jobs, you may be able to hire one if the need arises. I really feel for you, the work is all straightforward, however think it through before starting work. Even my 4 year old STRS threw up a few really stubborn and sized fasteners, and 1 item I couldn't get the CH in to heat it up, and that took 4 hours alone. Keep the forum posted on your progress and hope it all goes ok.

 

You can still get good cylinder pressure even with a failed head gasket, it depends on the area of the gasket leak, is it across say two cylinders, or a corner fail, they can go at the water jacket joint, if it has failed between 2 cylinders yen this would be indicated via a low compression test reading, this is why I prefer doing a pressurised test via a compressor, using this method you get accurate indications, as an example if the engine had a leaking or burnt out valve, you would hear the escaping noise of the air coming through either the exhaust or air box depending on an inlet or exhaust valve, failed gaskets are also detectable, as are piston or ring wear etc etc. £120 an hour labour is reasonable, some dealers are now charging £170 and more per hour, dealerships have large overheads to cover.

 

I know some are a little confused re my post regarding the pressurised air test, so to clarify during this test, compressed air is introduced into the cylinder, and the escape path of the air is monitored. Air escaping through the intake or exhaust indicates leaking valves. A blown gasket often results in adjacent cylinders having low pressure. As a time served mechanic in my early days I often used this equipment for both 2 and 4 wheel machines, the system I had back in the day was a sun diagnostic and test unit, as equipment advanced it became viable to purchase basic kit to do the work. I have attached a link that details how it works, there is no rocket science in using this method and you can't damage the engine internals. Hope this explains things more clearly. The attached picture shows the very same unit I used.

How To: Cylinder Leak Down Test - Owners World