You can not crank these engines using the ignition key, with ungrounded plugs. To do so invites ignition damage. SO: Remove all four plugs and ground them then crank the engine over. Alternatively, leave the ignition key off and remove all four plugs. Ignition will not fire and will be safe. Then, "jump" start the engine at the solenoid with a remote starter switch or a short length of jumper wire. insert the compression gauge and crank at least four turns or until the gauge stops rising. Do this on all four cylinders with a fully charged battery. Slow cranking will affect readings.
Now, some people insist that the carbs must be fully opened. However, since a 2 cycle engine has ports in the side of the cylinder, the cylinder always goes to atmospheric pressure on each stroke. Thus carb butterfly position does not count. The slight vacuum generated in the crankcase/manifold will not affect compression readings at cranking speed.
