I have read on here a few times that it is difficult to get a usable compression reading on engines with decompressors. On the FJ180 the manual in the link provided by Grumpy above gives the instructions on performing a compression test. How does this motor compare with a Briggs on giving a reading whoch is useful?

Here is the test
Quote
Cylinder Head
Compression Measurement �Before measuring compression, do the following.
○Thoroughly warm up the engine so that engine oil between
the piston and cylinder wall will help seal compression
as it does during normal running.
○Stop the engine. �Disconnect the spark plug cap and remove the spark plug. �Attach the compression gauge assembly firmly into spark
plug hole.
Special Tools - Compression Gauge [A]: 57001-221
Compression Gauge Adapter [B]: 57001
-1159
�Ground the spark plug to the engine.
WARNING
To avoid fire, do not ground the spark plug in proximity
to the plug hole. Keep the plug as far away as
possible from the plug hole.
�Opening the throttle fully, run the engine by pulling the recoil
starter grip several times until the compression gauge
stops rising. Read the highest compression value.
Cylinder Compression
Service Limit: 196 kPa (2.0 kgf/cm�, 28.4 psi) (MIN)
If the compression is higher than the specified value, the
piston rings, cylinder and valves are probably in good condition.
If the compression is too high, check the following.
1. Carbon build-up on the piston crown and cylinder head
- clean off any carbon on the piston crown and cylinder
head.
2. Cylinder head gasket - use only the proper gasket. The
use of a gasket of incorrect thickness will change the
compression.
3. Valve guides and piston rings - rapid carbon accumulation
in the combustion chamber may be caused by worn
valve guides or worn piston oil ring. This may be indicated
by white exhaust smoke.
4. Malfunction of ACR (Automatic Compression Release)
with the camshaft.
If the cylinder compression is lower than the (MIN), check
the following.
1. Gas leakage around the cylinder head - replace the
damaged gasket with a new one and check and check
the cylinder head warp.
2. Condition of the valve seating.
3. Valve clearance.
4. Piston/cylinder wear, piston seizure.
5. Piston ring, piston ring groove.