So Close!...
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The head to frame mount is good.
The radiator cap is good. Temp is now excellent until the radiator stars emptying out of the expansion tank
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Squish varies from head to head (I’m on my fourth different head&barrel) but none are altered from factory.
On all the gaskets that have failed it’s been the black coating. It goes soft at certain spots. Every time but once it has always failed between the head and the gasket. The seal between the gasket and barrel has been tight and dry, which I think is weird.
It’s always between a coolant channel and the bore, never coolant to outside edge.
I’m going to get a new piece of plate glass and some fresh wet&dry from 180/240 up to 600 or so.There has to be a physical cause for this, but I can’t figure it out yet. I just can’t see in my mind how the bottom end can be a factor in this.
I’d love to have a standard exhaust to try using. Also a different stator assembly, just in case a previous owner has bodged an advance on the ignition somehow?
There is a very pronounced rough spot at just cracked open throttle. The engine knocks and sputters when trying to maintain a consistent speed at this amount of throttle. I don’t know if this is relevant, but it’s been around as long as my head problems 🤪Thanks for everyone’s patience with this nightmare!
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Reason I ask about the head mount, is my Mick abbey modified head. Needed the bracket holes enlarging, because of how much had been skimmed off the head.
If you've been skimming the heads and barrels, the squish gap will have closed up. It may be a insignificant amount. But with how many times you've been doing it, it would be worth checking.
With how old these bikes are now, you can't guarantee things are to standard specs. Skimming your head and barrel will raise the compression ratio.
Also after reboring a cylinder and fitting oversize pistons, you increase the cc slightly also affecting the compression ratio.
All this will have a slight effect on jetting, add in newer ethanol fuels burn slightly leaner as well.
Not convinced these are the reasons for your issue's but worth ruling out.
Could the piston actually be making contact with the head?
I would pressure test your radiator with compressed air (don't need alot of psi) while submerging it in water, to check for leaks. -
Cheers guys, I appreciate all the help, food for thought!
God knows I need it!! 
The radiator (& cap) was behaving like it should. Holding fluid right up to the neck every time I checked it, and believe me I checked it a lot! Expanding and contracting like it should. It wouldn’t vent through the expansion tank if the cap wasn’t venting pressure. It also wouldn’t have had a full rad if it wasn’t contracting and returning the coolant when it cooled. Also the outer seal on the caps is good otherwise it’d leak from the cap and wouldn’t make the Schweppes noise when I open it hot.
I really appreciate any ideas, I just don’t think the caps to blame. It won’t stop me putting on a new one if I can find a genuine new one though
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@SpookDog but the cap isn't supposed to vent pressure. It's supposed to hold in the pressure. The expansion tank is only used in cases of over heating. If as you say the cap is venting pressure then something is not right there. The coolant is pressurised as it boils at a much lower temperature when it's under pressure. I even more so think it's the cap now. Try a new cap and make sure it's the correct pressure rating.
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I don’t think that’s right bud. It expands and contracts to aid cooling. No manufacturers would waste money on an overheating tank. An overflow pipe maybe...
There are two rubber seals. One to make the radiator airtight, the other inner one allows expanding fluid to pass back and forth...
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@markus-w said in So Close!...:
@SpookDog but the cap isn't supposed to vent pressure. It's supposed to hold in the pressure. The expansion tank is only used in cases of over heating. If as you say the cap is venting pressure then something is not right there. The coolant is pressurised as it boils at a much lower temperature when it's under pressure. I even more so think it's the cap now. Try a new cap and make sure it's the correct pressure rating.
PV=nRTCoolant boils at a higher temperature under pressure.As atmospheric pressure decreases, water boils at lower temperatures. At sea level, water boils at 212 °F. With each 500-feet increase in elevation, the boiling point of water is lowered by just under 1 °F. At 7,500 feet, for example, water boils at about 198 °F.Therefore as the pressure inside the coolant system increases, so does the boiling temperature.
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@SpookDog I know that bud but they are set so they open at a certain pressure and different caps are set to different pressures due to different strength springs (dt125r cap should open at 12.8psi with a tolerance of +/-1.5psi) and if it's old and worn and opening too soon/late then it won't regulate the temperature correctly. Springs tend to weaken as they age. It's surely a coolant issue you have. I may well be wrong but given the trouble you've had a £15 cap is surely worth a try?
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@markus-w
Hey! Don’t get me wrong, I appreciate all and any ideas. I’ve got 3 old, original caps. I’m definitely gonna find an New original one just to be sure that I cross i’s and dot the t. But I’m not sold on it being the cause.
I’m doubting my own workmanship more at the moment... -
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One thing that I’ve never done is re~torque the head after riding the bike up to temp. I’ve just put a ring spanner on the head nuts and given them a tweak. It was surprisingly easy to tighten them (Very surprisingly easy!!) Looking back it was never hard to undo them when tearing down the top end. I haven’t seen anything about re~torquing in the Haynes. Only ever seen it recommended with car head gaskets that are those silver multi layered ‘compressible’ stuff...
Does everyone else retorque after running up to temp?...
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@SpookDog haha my suggestion to you, hopefully it works , I know it is really a car thing but in theory heated metal expands especially alloys, be nice to know what the torque wrench says with it heated vs what It said unheated although hard to get one in a built bike
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@MH-Bikes-N-Bits using a 1/4" drive torque wrench (£20-£30 from a cycle shop) and 1/4" drive universal joint and 12mm socket you can get to all the head nuts with the engine in situ, even the one at the front which is close to the frame.
Alternatively you can remove all the engine mount bolts apart from the swingarm pivot and rotate the whole engine around it a few degrees to improve access (I did this once to replace the O-ring in the steel water pipe without disturbing the waterpump cover as I had reason to believe one of the cover bolts was seized)
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