A quick test for crank seals.... a few words

Another in my controversial series of DIY hints and tips... crankshaft seals...a popular subject.
When I suspect dodgy crank seals in any running multi cylinder 2 stroke engine I'm repairing, or buying, I have come up with a "ready reckoner" way of assessing their condition, by doing the following.
Get yourself an automotive type vacuum gauge, does not need to be accurate, even one of the green, orange, and red "fuel economy" type is fine as we only need comparative measurements.. This is connected to the engine side of the inlet manifold of one cylinder, many manifolds are fitted with a blanked connector for this purpose, via a length of small hose. This hose is fitted with an oil line ball valve (non return) from a T500 or similar. Remove any air filtering equipment so you have good access to the carburetor inlets. Start the bike, and at around 1500 RPMs, choke the carburetor, sealing it with your palm.  Note down the reading. Repeat this with the other two cylinders, swapping the gauge to the correct manifold , also taking note of each reading.

Now, to study the results.... similar here to a compression test.

If the left cylinder shows a significantly lower reading than the center and right, we can suspect the left outer seal.

If the right cylinder is significantly lower than the center or left, we can suspect the right outer seal.

If the left and center are significantly lower, we can suspect the center left seal.

If the right and center readings are significantly lower, we can suspect the center right seal.

If all readings are very near the same, we can assume the seals are probably ok, unless all readings are ridiculously low.

A "calibration" test with a known good similar engine and your vacuum gauge would be the obvious next step. The VERY rich mixture drawn into the cylinder under test, floods the piston/cylinder, and helps seal the piston against the cylinder reducing errors with leakage from this area. I have also noticed that worn engines do have significant differences between crankcases pressures, suggesting seals do wear randomly, and not necessarily all at the same rate. This procedure checks the seals under a vacuum only, not compression, but either way, if leaking badly, they will need replacing . Any large variation between cylinders then suggests remedial work.

This tip is offered as an indication only of bottom end condition..... I have found it useful.