Just to recap... last year I had several coolant hose leaks but the reservoir wasn't dropping so I thought they couldn't be too serious.
Then I noticed my radiator was down a couple of inches and as it was at the end of the season I just moth balled the bike until this year.

In Spring I changed the hoses, cap & screw type hose clamps.
That was fine for a couple of runs but then it started leaking again, which must have been the clamps settling on the hoses as nearly all of them took about 1/2 a turn to become tight again.
No visible leaks now.

A couple of weeks ago whilst looking into some other issues I noticed that my reservoir was at MAX (which is where I'd filled it to) but my radiator was down to below fan switch.
So with the cap off I pressurised the reservoir which forced all that coolant in to fill the radiator again, I left the reservoir empty.
I also tilted the bike to the left and it gurgled a bit and the level dropped a little more, so I topped that up.
Until tilting the radiator was dropping while just sitting but after tilting and filling it stopped dropping when left.

A few days ago I had the first ride out since and it pushed a small amount of coolant out into the reservoir, enough to go just up to the MIN marker, but when it cooled it didn't pull it back into the radiator.
That must mean there's still air in the system right?

What should I do now??

Ride again to see if it pumps more out.
Undo the rad cap and top up the level again.
Try to elevate the front end with the engine running and up to temperature (thermostat open)?

A proper burping requires the front end in the air. A good long wheelie works for me.

As per my post on your earlier thread, get that front wheel up..........

When I filled it I did it in the Abba stand with the rear wheel out.
I suppose with the leaking hose clamp issue air could have gotten drawn back in and thus trapped in there.

So I guess it's time for another ride on the Abba stand.

Did this yesterday, bike on stand, rear wheel out, front as high as possible.

Got a little more coolant in and ran the engine until the thermostat opened, topped up again then on with the cap.

Result!
As the engine cooled it started to suck back from the reservoir.
Gonna do it again today whilst it's still on the stand just so I can see how much it pushes out when up to temp.

Plus I've got to change the bearings in the rear sprocket carrier so not in a rush to take back off the stand.

Don't forget to open the burp screws, upper left on the radiator and upper screw on the water pump. Raise the front wheel, remove radiator cap and wait for the air bubbles to stop. Refill

OK, maybe she isn't sorted just yet....

Went out for ride yesterday and it was hot.
1st stage (up to the first stop off) the coolant went up to just over the MIN marker on the res, which is as it did last time when testing in the garage.
2nd stage involved more traffic initially (multiple fan events) and then a spirited ride on back roads home.

Got home and tank is now up to MAX, checked this morning and it's still at MAX.
Took the rad cap off and air is drawn in and the radiator is down a couple of inchs again so I guess there wasn't enough vacuum to drawn the coolant back in - rats!

So I'm thinking it can't have been fully burped.
I guess I can't get it high enough on just the ABBA stand. I'm gonna have to rig something up using a ramp and my van to ensure the front wheel really is over 50 cm (ABBA stand got it to only 45 cm difference I think), and then go through the whole bleeding process again.

Well I did it again, this time using a half plank and some blocks. Got the front wheel a 50cm off the ground, did all the burp screws, set the res at MIN.
Started and idled to fan switching on and left to cool - level didn't change.
Took her for a run today which was a good test, lots of traffic, lots of fan events. Temperature gauge was above half & sometime 3/4 way up the bars.
The res is now at MAX it isn't dropping as the engine cools.

Engine luke warm so just took the cap off and there might have even still been pressure in there!

There is trace of something on the surface when I shine a torch in the radiator filler, could be oil but could also just be crap, almost like dust.

Anyway, I'm now thinking it has to be a head gasket... what else could cause this?

Soo pissed off with this bike at moment..

What do you guys think, are there anymore tests I can do or is now likely I've got a gasket gone (or a cracked head)?

Seems it only blows into the coolant when the engine is hot, and the hotter it gets more it does it.
Also the engine need to be under load, idling doesn't seem to do it, or maybe idling does do it but it's happening so slowly that it not noticeable until after several hours of riding.

Again, don't think you symptoms necessarily indicates blown head gasket, ie no bubbles in coolant with radiator cap off and engine running and no blow out when engine revved (?) - if the system is fully burped, and based on what are saying now, my guess is that it's dodgy radiator cap that's not allowing connection between radiator and expansion tank as radiator pressure/temp. drops.

Ok - see vid and tell me what you think pls.

Vid starts when the engine is up to temperature and thermostat opened, before that it was pretty calm in there apart from a few little bubbles.

https://youtu.be/tan5EuPyqkc

Is this normal for an SD that has air trapped or is it as I'm fearing?

No intermix? What does the exhaust smell like? Compression test? Leak down?

Possibly. What does the oil look like?

This guy is a bit of an idiot but look at the kit he is using to test for exhaust in coolant.

Oil looks normal in the sight tube, I haven't looked in the tank itself.

BTW - If that vid isn't conclusive I could go through giving it a full bleed again (even run the bike with the wheel in the air), and then make a new vid?

I’m having a hard time making judgment on the video because I’m not there. Is it exhaust bubbling out of the radiator or just vibration agitating the coolant. When ever I have blown a head gasket it has been an obvious event, bathed in coolant an no compression. The exhaust in coolant test kit would let you know if you needed to pull the heads.
If there is coolant in the oil than you most definitely have a problem. Look in the tank. Dip and see what the oil looks like.
When I burp I get the wheel easily over a meter in the air,( wheelie), then top up and all is good. Set level in reservoir to slightly higher than minimum on cold engine. Done.

Don't know if this is of any help but could be a relatively cheap way of checking for exhaust gases in your coolant. We used to use a fancy version of one of these years ago.
https://www.ebay.co.uk/itm/BT-3-BLOCK-C ... 0005.m1851

Did some more tests and bad news - head gasket gone or head warped.

This is what got me to this conclusion:
I re-bled the system and did good job this time. With the engine running and thermostat open (bike still on ramp) the coolant was calm in the radiator opening, it just got hot and steamed a little - all good.
I put the cap on and let it cool and the reservoir level dropped - I was optimistic.
Today I let the bike idle until the fan kicked in, took note of the reservoir level then left the bike to cool, the level dropped again - really optimistic now.
Took her for a spirited ride and pulled over to inspect, reservoir level higher than before now but as I didn't have the rad shrouds on I could see the whole side of it, and there were bubbles in there - shit..

Left her to cool (desk fan to speed this up), reservoir level didn't drop. Took the rad cap off to see if it had dropped and there was pressure in there, so much that it spat some coolant out.
So I have a small leak, one that only really shows up when the engine is under load, or only leaks enough to make a difference when the engine is hot & loaded.
Either-way I think that is proof positive now.

Bike to be mothballed now, will look at it in October as I should have time for this then.

Made a start on it now, got a little surprise when I pulled the airbox.



That gunk from the engine breather wasn't there when I last cleaned the filter back in April this year!

To be fair the engine has recently done a fair amount of short test runs, had extended idling to the fan and then switched off, and also been run with the front in air a few times. So this could just be pretty extreme but normal condensation, or it could mean coolant is entering the cylinder. The oil in tank looks fine btw.
Will look further into this tomorrow.

Plan is pull the spark plugs and see what they tell me - probably nothing.
USB endoscope - gonna have a look in both cylinders for signs of coolant ingestion, or even just wet in there.
Leak down test - that'll be the final test to see if I can narrow down which cylinder is the problem.

Wish me luck!

Just completed the leak down test and confirmed that I do indeed have a bad head gasket, the rear one.

At just 60 psi applied I could see the coolant level in the radiator filler rising.
I went up to 80 psi on the front and saw no level change so I'll just change the rear head gasket.

Kind of bitter sweet really, at least I know now what is wrong but it also means I now know what I have to do.

Front spark plug looked drier than the rear, rear had oily threads.
USB endoscope - not much help but the rear piston looked wetter than the front but both looked to be wet around the edges, probably just reflective carbon build up.

So I now have a few question but I'll start some fresh threads now.

Mine does the same thing. Coolant leaves the overflow tank and just goes away. No idea. Nothing leaking, no coolant in oil, and it's not burning it as the exhaust is clean. So idk.

Check your radiator when cold.
Mine would be holding pressure in there when cold, so would hiss when the cap was removed, it could even burp out coolant if it hadn't been run for long since filling.

Basically once filled & bleed it would push out coolant whilst running until the system stabilised. This happened until the coolant level in the radiator was down about 3 inches.
At that point the bubbles of leaked exhaust gas could collect at the top of radiator and leak out every time the pressure gets high enough.

I did one test run without the side fairings on and it was then that I could see air bubbling up through the coolant overflow tank, that's when I knew it had to be a head gasket.

All sorted now.

I replaced the rear head gasket and now the coolant tank hardly moves. It drops a little on cold mornings and raises a little when the engine is very hot but no huge swings in the fluid level anymore and the reservoir breather pipe is now dry after a run.

A lot of head-scratching over a fair amount of time, and a fair bit of work, but big ups for figuring it out and fixing it yourself to save a heap of money and learn loads in the process.
Well done Rich

Thanks Scotty

My local (and cheapest) KTM dealer quoted me 3 days labour plus parts to investigate and repair, which I estimate would have resulted in a bill of at least £1800, so I'm pretty happy I managed it myself.
I also got other jobs done whilst in there (the valve clearances and also put new sparkplugs in), plus some cleaning of parts you can't normally get at as I plan to keep this bike for while yet.