WILL RUN WITHOUT #4

[excess.11]

If the "crank oil splitter"??......is the pcv valve..and that is where the air loss is observed....it points to cylinder wall or piston ring issues.
I m with jonesy and would do a hot test as well. Would nt hurt to chech the operation of the pcv valve as well ...that it is not stuck open.
Did you do a leak down comparison on all 4 cylinders or just number 4 ?.

[Jonesy :-)]

10% is a good result, leakage out the crankcase breather suggests valve sealing is good and not totally unexpected with 100psi up its arrŕ.
Was that a cold test? If so a hot test would be good for comparison. Higher %age leak down when hot might support the cracked head opening up with thermal growth theory.........maybe?

[excess.11]

In the words of the "Queen of Hearts " from "Alice in Wonderland"...
"Off with his Head " I say.....

[steptoe]

have just done leak down test, {after new toy/tool has arrived}
shows about 10% leak, but we,ll play with it a bit more next days off

did jam 100 psi in it and air poured out crank oil splitter

done all that Ras

[rasputin]

Ok I have not been watching this for a while but if the above stated stuff is not working so far, well how about a leak-down test done both cold and warm? That should eliminate a lot of possibilities. The other thing that comes to mind after watching your video is, Does the lobes on #4 both intake and exhaust match with the other lobes on the camshaft? I could be that the cam has worn enough to make a difference. I know I would be a long time analyzing before I would measure to check. Just some thoughts from over here.

[steptoe]

done all that Mitchell, all seems good,

interesting thoughts Jonesy,, will have to check them out

[willrcr15]

If you think it's a cracked head, could it just be a head gasket on that end, maybe a head bolt / stud has let go.Would you check all the tensions to see if ones loose ?
A compression test would show something wouldn't it if a crack is affecting the cylinder temp that much ?
It might be something cheap & simple, fingers crossed 

[Jonesy :-)]

Question
If its cracked around #4 spark plug, would that cause the spark to track off and give dodgy firing? but have no affect on 3

do you know any one that does NDT and have access to dye penetrant for crack detection? A good engineering shop should be able to do it if not
ABEN Technical services (in Bundy) do this sort of work, or a cheaper option might be to go online and buy a 4 part kit such as Flawcheck
all this is of course if the crack is not visible to the naked eye, it may not be apparent until it heats up and thermal growth takes place

[excess.11]

There would be air movement in the crankcase from piston travel....but would be largely negated by the opposing pistons ...and generally air expansion due to heat.
The crankcase breather should take care of that operational  pressure.

The crankcase pressure your mentioning seems too much if your feeling it blow from 200mm.

I m thinking if it s a cracked head that no 3 cylinder would be affected as well...but it seems via the vid it s not.. seeing as you mention no external oil leaks ....it s an internal pressure leak which may be valve/seat issues rather than a cracked head.
Looking forward to the head removal and inspection.

[Dan Tyson]

thanks to mr google, and some research,, it,s all pointing to a cracked head, to many symptons not to be

I have a spare if you need it. No buckets though and it's off a '78 but it's yours if you don't have one.

[Jonesy :-)]

I've got a spare if you need it

[steptoe]

thanks to mr google, and some research,, it,s all pointing to a cracked head, to many symptons not to be

[steptoe]

Jeff, it does has positive crank pressure, how much is to much, {you can fell it blowing on your hand at 200mm}
oil is still nice and golden, no sign of milkyness

Mitchell, carbs have fresh kits, I have a set up for float hieghts, will recheck everything next days off before tearing head off

[willrcr15]

If you survive your testing & don't get a result I'd go back to the carbies & check the fuel levels in the bowls with a clear tube connected to the outlet on the bowl & bend it up the side of the carby with the end to the same height on each, put a texta mark on the tube at each fuel height, it'll give you a look at what's going on inside, maybe that cyl is too high or just different to the rest. Our little FZR 250's give us the same hell, we find the std rubber carby kits go hard & we get leaks but there's a mob called Lite Tek that are doing Viton kits that don't go hard & precision made gear but no kits for XS's. Just another angle to consider.
But keep up the good work, it's an interesting adventure you on !

[excess.11]

A test i used to do on early VW air cooled engines to check for a cracked head was to remove the oil filler cap and place the palm of your hand over the filler tube ...and run the engine to feel for positive crankcase pressure. Also check the oil was nt a milky colour and frothing via the oil on the  dipstick.  An earlier warning sign on these flat four engines and due to the pushrod tube seals being on the lowest part and underside of the cylinder head was they would leak due to the pressure.
With the xs1100 it may  show up via weeping or leaking seals as an indicator.