Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
One big problem is the legth of flexible hose on the compression tester, i know you tried 2 testers but if they had about the same hose legth it could be your problem. The space left in head when piston at tdc is not much and could easily be doubled.Thats one reason i say try it and see
Where is the release valve on the test gauge? As Brian says the tube will be part of the volume ( and give problems of reading) if the control valve is away at the gauge end of the tube as the tube volume is then evacuated at each exhaust stroke.If valve is located right where the spark plug location is, then the tube remains charged and should then report peak psi, which should be close to specified data for the engine.
I have seen someone in the USA test for equal compression across all four cylinders by dropping the exhaust headers off and placing a ping ping ball (table tennis ball) in each exhaust port. Crank the engine over on the kick start and if all the balls hit the garage wall at the same time, compression was close enough to equal to not worry about it 😁😁😁😁😁