duncan1958 wrote:Is it possible that the pick up coil is not at the right height?
Incorrect pickup alignment is typical of the issue reported. A worn mag bearing will also give the same result. I suggest that this should be the first thing to check.
Compression pressure measurement on a Lammy is always going to be ambiguous. This is because the rather innaccesible spark plug hole doesn't allow you to screw a gauge directly into the plug hole. Consequently, you ineveitably have to use some form of flexible hose connection between the head and your measuring gauge. By adding in this flexible hose and a bourdon tube gauge, it in effect increases the compressed cylinder volume by a few CC's. So the the reading you get will always be less than true. The kit that you use will no doubt differ slightly from someone elses kit giving different readings on the same engine! So, your reading of 130psi, although it looks good can only be considered as arbitrary.
I use a piezo electric pressure sensor that screws directly into the plug hole. The 0-10V output signal from this sensor is relative to 0-150psi. A digital multimeter with peak maximum voltage detect is then used to determine the peak output voltage.
EG. A peak voltage reading of say 8.5v = 15 x 8.5 = 127.5PSI.
Granted.... This kind of kit is expensive and beyond the average persons toolbox but this is what it takes to be accurate.
This measurement is the corrected measurement. Theoretically, the pressure rises upon closure of the exhaust port. So, your exhaust port timing will have an affect on the measurement you get, as well as the compressed volume of your cylinder head.
In practice, compression begins prior to the closure of the exhaust port which expains why the mathematic calculations for corrected compression differ somewhat from the
practical measurements.
With those £15 compression testing kits with their various adaptors, extension hoses and connectors that you can get from fleabay, a reading of 75 - 90PSI is typical on most 2T Lammy engines. This reading isn't true but can be used to determine if you have a reduction in compression, providing you know what the reading was originally when the engine was new and good.