On piston aircraft engines the technique for leak down test includes a
device called a differential cylinder pressure tester. (Aircraft Spruce and
Specialty has them for $60.00, 1-800-824-1930, www.aircraft-spruce.com) .
This has two guages on it, one showing how much air is going in and how much
air is left. Aircraft use typically 80#. Limit for rejection of cyliner is
60#, but these are air cooled engines and I don't really know what the limit
would be for water cooled. I recently checked my '74 midget and found all
cylinders to be mid sixties. No matter what cooling, the engine must be
warm and the piston must be at TDC on it's upward stroke. If it is after
TDC the rings will have moved up in the piston grooves and will give
erroneous readings. All this said, it looks to me also that the head is
coming off. Aircraft Spruce has a great catalogue that is worth the price
GJB '74 Midget
From: William M Gilroy <firstname.lastname@example.org>
To: email@example.com <firstname.lastname@example.org>; email@example.com
Date: Monday, October 12, 1998 10:55 PM
Subject: Leak down test questions
>I have have received several suggestions about what could be causing
>the low compression in cyl. #2 and #3 in my 77 Midget. These
>causes could be:
> 1. Worn rings.
> 2. Blown head gasket.
> 3. Burned/leaking valves.
> 4. All of the above.
>The only way to really tell is to run a leak-down test. I gave
>it a quick try tonight and I have some questions. The basic
>idea is to charge the cylinder with air and see how fast it leaks
>out. I am also supposed to listen at the tail pipe and carb. for
>leaking valves, listen at the dipstick for worn rings, listen
>in adjacent cylinders or look for bubble coming from the
>radiator for a blown head gasket. I think that covers most things.
>I tried the above but my connection at the spark plug hole was leaking
>and it made too much noise to hear other leaks (I think). So now
>I have the following questions.
> 1. How many PSI should I charge the cylinder to? (I used ~40 PSI)
> 2. How fast should the air leak out?
> 3. Will the volume of the cylinder affect this? If show how do
> I figure in the location of the piston in the cylinder?
>I am going to try a leak-down test to see if I can identify the
>problems. I will then pull the head and look at the head gasket. If
>it is blown I will just replace the head gasket and then run another
>compression check. If the head gasket is blown how will I tell? Will
>be obvious even to me :-)
>That's about it for tonight. Thanks for the help.