Diagnostic testing on '82 1100

Guys....

just a quick question - how is a compression LEAK DOWN test different from the standard compression check ?

Peter

PRM068 wrote: Guys....

just a quick question - how is a compression LEAK DOWN test different from the standard compression check ?

Peter

A leak down test pressurizes the cylinder with a steady supply of air with the piston at TDC compression stroke. You have 2 gauges. One reads line pressure, the other trapped cylinder pressure.

It is completely inconsiderate of cam lobe effect or piston dome effect.

A standard compression test is affected by both.

The advantage of a leak down test is that you can listen to where the air leaks and determine the correction process with a more enlightened approach. It is diagnostic in function and form.

Your pistons are rated at a static compression ratio. However, the live engine sees a dynamic compression ratio, significantly lower than that advertised static ratio. That is due to valve timing and lobe shapes.

A piston with a given static compression ratio will yield different compression measurements when the cam duration is either shortened [higher reading] or lenghtened [lower reading], all else remaining the same.

Closing the intake valve earlier will yield a higher compression reading as well.

While both tests can be used as health indicators, the leak down test allows you to know the condition of the sealing elements of the cylinder without any knowledge of the static measurements of the sealing components.


Hope that helps you...

An interesting test that may enlighten your understanding is if you have a single cylinder motor that has easy to adjust valves. Set the lash to factory settings and check compression. Then fiddle with reducing lash and increasing lash, taking a compression measurement after each change.

Larry C

A leakdown test injects air pressure through the spark plug hole to pressurize the combustion chamber.

Can perform a poor man's leak down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed. Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun (rubber air nozzle) held into the spark plug hole. And of course keep holding it in position to keep air from coming back out the spark plug hole while listening for escaping air at other places. The air compressor should not be running while listening because the noise will likely drown out any sound of escaping air.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (perhaps worn piston rings or cylinders).

Air heard escaping from head gasket area indicates indicates loss of compression past head gasket (perhaps due to a blown head gasket).

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

Good Fortune!

A good OEM starter with a good battery will crank an 1170 motor just fine. The only people who need Honda starters in a street compression Kawasaki at the ones that don't know what they're doing... It's just that simple.