This has just started happening. Had a really good run but it was wet so bike got pretty dirty (a month ago).
Gave her a dam good wash and used a new angled brush that really gets right in there

Is there anything likely to have water in it/on it's contacts that is worth checking?
Is this telling me it's time to do something - spark plugs, valve clearances etc?

When it’s started for the first time in a while, my 990 track bike fires up on one cylinder initially before the second chimes in, and a fair bit of steam comes from the exhausts until it’s fully warmed up - this can happen in summer so it isn’t condensation burning from the inside of the exhaust. I should investigate further and identify which cylinder is the reluctant one and check the plug for evidence of water in the cylinder ... hopefully I don’t have ahead gasket issue, but I can’t rule it out until I’ve investigated.
I’m not suggesting that Rich’s bike is suffering the same as I’ve never given my 990 a deep clean like he describes, but if the issue persists it’s less likely to be directly related to the cleaning activities...

Could a blowing front pipe cause this???

I just went to try and work out which cylinder is missing, and I think it's the rear.
Whilst holding the headers I felt exhaust blowing on my arm from the front header.
Hmmm....... KTM's ey...

This could have been like this since I've owned the bike BTW, there is a scuff/scratch on the mid pipe where it runs down towards the rear so maybe the pipe got hit by something before I owned her.

Is the coolant contaminated with oil or coolant level dropped?

Coolant not contaminated.
Coolant level has dropped in the radiator but strangely not in the reservoir.

Had a few coolant leaks recently so earlier this year I replaced hoses and clamps (that was at 16.974 miles.), they leaked after a couple of runs but since re-tweaking them they've have been fine.
So after that's I topped up the coolant in the reservoir and put the res at Max fill level.
The radiator cap was also replaced as I think that was leaking too.

So I just went out to check it now, there was a big suction sound when I cracked the rad cap, and the level is down to just below fan switch with the bike level.
So I think it has used or leaked that fluid somewhere as I don't think any has gone back to reservoir.
Current mileage is 17,776 miles, so done 800 mile since changing the hoses.

What I don't understand is why it hasn't drawn fluid back in from the reservoir, unless it was actually under pressure and not vacuum?

What should I do now?

Run the engine with the cap off and look for bubbles?

Sounds like more than one issue, ie cooling system, broken front header and running on one cylinder - latter two could be related though, eg blown head gasket......

Check that cooling system is fully bled (did coolant level in expansion tank not change at all? Is hose between expansion tank and rad kinked/crimped/blocked?) - if all OK, could be dodgy new radiator cap, unlikely but did happen to me once.

Weld up/replace front header.

Establish which cylinder has problem, ie grab hold of headers - check that there's no water in the spark plug well and that drain hole is clear of any blockages - I had a couple of similar symptoms in past which turned out to be, firstly, a small oil leak past spark plug shaft o-rings partially filling spark plug well and, secondly, a failing pulse generator - swop over front and rear coil packs (are they fully bolted up?) and, if no difference, spark plugs also. No fault codes? Water in BGC or coil pack connectors?

I'm down in Somerset tomorrow if that might help.

I can't tell what's going on with the coolant TBH.
It could be that I still have leak, I had several leaks last time and the level had dropped in radiator but not in the reservoir.
My rational for this was that maybe it was venting through the leaks when hot but also as it cooled it could pull air back in via the hose leaks easier than it could through the radiator cap.
This should have stopped now though as there isn't any evidence of leaks around the hoses, but it is also possible it's still burping.

I looked at the final vent line from the reservoir and the bottom is wet, so it's been venting it seems.
With the cap off I blew on the end of the vent pipe and coolant flowed freely back up into the radiator, so I continued to do this until the radiator was full, which took almost all the fluid in the reservoir (was at the Max level) - so at least now I'll see if that level rises.

The intermittent thing started after it's last mega clean so I'm hopeful that I'll find water somewhere.

I'll get the pipe off and welded and then monitor the coolant usage, and cold starting.

So Establish which cylinder has problem, ie grab hold of headers - Will do, I think it's the rear but will double check.
Check that there's no water in the spark plug well and that drain hole is clear of any blockages - can this be done without pulling the tank up?

a failing pulse generator - swop over front and rear coil packs (are they fully bolted up?) and, if no difference, spark plugs also. dunno, I will look into it.

No fault codes? None on the display, haven't got TuneECU, Can codes be stored in the ECU but not shown via the clocks?

Water in BGC or coil pack connectors? I'm not sure what a BGC is?

I'm down in Somerset tomorrow if that might help. Thanks for the offer but it's a little too soon for me I'm afraid. I'm gonna be focused on getting that dam pipe off first but that might have to wait until Wednesday now as I need to get the garage sorted to free some space to work.
Once the pipe is off I'll start making my way around the bike looking for water where it shouldn't be.

This is what it started like this morning anyway. Which was when I realised the front pipe was blowing so asked for it to be turned off.

https://youtu.be/hlk0asIrP3I

It seems to be getting better each time, as in the 2nd cylinder catches on sooner.

I won't now be trying it again until after the exhaust is repaired.

I've just got back in and for some reason thought I'd just remove the rad cap and check the level again.
The bike is on a bit of wood so it sits more level.
Coolant level has dropped....

It's been about 6 hours since topping the rad off and now it's about 1 cm down.
I'll check again in the morning but as there are no leaks visible on the outside then surely it must be going into a cylinder.

I'l check again in the morning to see where the level is.

Just had another idea...

I have one of those ebay USB endoscopes that plugs into your phone.
I can stick that down the spark plug hole to look for water in each cylinder, that might be more conclusive.

Search Blown Head Gasket here - coolant loss issue is more often than not just burping properly.

You could be right - phew...

I topped the rad up last night, it took surprisingly little coolant to top that 1cm back to the lip. I then rocked the bike around and a few more bubbles appeared, so I then leaned it over as far as I dare (until the bar end rested on a cabinet) and she burped a little and level dropped again, topped that up and it hasn't dropped since.

I guess the engine has been burping since I changed all the coolant hoses at the start of this season (about 4 months ago), I still don't know why it's not pulling back from the reservoir after cooling... I could try it with the old rad cap back on as that certainly allowed it to.

So this investigation will pause now as I need the get the exhaust sorted, will start that job tomorrow.

Expansion tank flow and return is disrupted by air in system.

I think this is sorted now.
Tried 2 cold starts since and it goes straight up to normal revs running on both pots.