My money is on the centre main bearing. Either the cap has a problem (unlikely) or the thrust washers are toasted and/or it's spun the bearing.

I had a similar problem when I built my 1360 auto block the first time 2 years ago. Nothing to do with the block being an auto, prob was I set the endfloat at .003" as the books say. .003" is NOT enough if you are going to rev it. <edit> because 1275 cranks flex the middle crankwebs a lot.
When revved to 7000 at Wakefield Pk 1000 miles later the thrust washers got bronze peeled off by the crank, the centre main overheated, and it almost spun the bearing in the block. I brought it home on a trailer, thanks Mini Mad. The S crank was bent .007" and needed the mains ground, again.
It won't happen now, it was built with .007" endfloat this time. GR uses .008" for race motors.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

thanks guys,

i still struggle to understand how it does it even when the ignition is off.. can anyone say compression can affect this?? if what is described is the issue??

letting the car roll(ignition off') in forth at 2400rpm flated the accelerator loose 20psi, take my foot off the accelerator back to 60psi.. that absolutly baffles me dont understand it maself ahh well

ya help was greatly appreciated, supose ill take the engine out cheerz

When you have your foot off the accelerator the motor is pulling vacuum, when you open the throttle the vacuum drops.

I wonder if it could be related to breathing somehow?

Morry mate,,, it can`t be related to engine breathing,,, think about it????!!!!!!!!

it`s got to be a crank float problem,,, foot into it ,,,& crank goes one end,,,

but take it off ,,,& it goes the other

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

I think that Kevin is on the mark.
If the thrust brgs were both worn IE say .050" each and the crank is centralised the clearance total is 2 X .050" = .100"
If the crank moves to one side hard up to one thrust then the clearance is still.100"
I know that this end movement can cause some oil pressure fluctuations but not 20lbs and then remaining constant.
I think that it is a oil passage that is being open and closed off due to load rotation. It could be a shell that is loose and as load comes on it partially closes off the oil gallery and the oil pressure increases.
It could also be end float in cam shaft that has worn the pump faces allowing the oil to bypass and come out around the shaft.
I would say probably a loose main brg shell

Just another idea could the oil pump be loose where it's bolted in, allowing oil to bypass the gasket seal

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Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

Which is why I asked about the auto to manual block conversion , how was it done and did the person doing it know what they were doing ?

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This would give lower pressure all the time, not just when you put your foot down or push the clutch in.
But yes it is important for the pump spacer to be a good neat fit in the block, and you want a gasket both sides of it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

about the auto conversion

i have no idea who did the conversion but when i purchased the engine i replaced the oil pump putting in 2 new gaskets around the pump.

apart from that the engine is 100% original. i shall pull her apart this week