5 replies to this topic

[dklawson]

Last week after my car acted up I started a thread on this and a couple of other message boards asking what the symptoms of stuck valves are. Most the replies indicated that I lacked some key symptoms of a stuck valve. However, several people encouraged me to perform a compression or leak-down test. My car has decided it doesn't want to run anymore so I was moved to perform the leak-down test today. I'm looking for input from any of you who have done these tests to tell me if my numbers sound good or not.

I'm using a home-made leak-down tester based on the more-or-less common design of a pressure regulator with one output gauge, a pipe nipple with a 0.040" (1mm) hole/orifice in its bore, followed by a second pressure gauge that connects through a flexible hose to a spark plug adapter.

Each cylinder was tested the same way and each gave the same result. With the engine cold, the piston was brought to TDC so both valves were closed on that cylinder. The car was placed in gear so the engine couldn't turn. Air was supplied to the regulator and the regulator's output adjusted to 100 psi (6.8 Bar). There was leakage as expected, but each cylinder held the output pressure (downstream) at 85 psi (5.8 Bar). For each cylinder the leakage could not be heard in the carbs, nor in the exhaust, they were not leaking into the adjacent cylinder, nor were they producing bubbles in the coolant. In each case the leakage could only be heard coming up the pushrod tubes. This indicates to me that all my leakage was through the piston rings. I don't think it's leaking into the oilways but if it were... how would I know?

I was looking for leakage through valves that wouldn't seat but didn't find any. Now I'm curious though. What I've got appears to be 15% leakage around the rings. The engine has very low mileage on its rebuild and I'm assuming everything is OK based on the leak-down tests but I'd like to know what are "good" numbers and what are considered "bad". If I have a problem with the head gasket such that the pressure is going into the oilways... how would I know? Thanks for the input.

[Dan]

I believe that 7-15% is normal on a healthy A series, 7% to 8% when newly built. Last time I had a test done I had one cylinder at 51% leakage (no that isn't a typo) and the engine was still running fine and pulling hard. That was a head gasket on the verge of failure.

[dklawson]

I was relieved that I didn't hear anything coming out of the carbs or tail pipe.

The percentage my car is leaking didn't surprise me very much. When I fit the rings to the pistons I noted they had wider than expected end gaps. I wrote K. Calver about it and he commented that the gaps I measured shouldn't be a problem on a street engine (which mine is). With the engine cold like this I figured the percentages would be a bit higher.

Thanks for letting me know that up to my 15% isn't out of the ordinary. As I said, I was relieved I didn't hear hissing through the carbs or tail pipe or find bubbles streaming up the radiator. I guess my next step is to look at the ignition, then on to the carbs if necessary.

[Dan]

It'd be interesting to compare a hot test to this cold one. I'm not sure on how much difference there would be as I've only ever done hot tests.

[dklawson]

The engine won't run now so that hot test will have to come later. The leak down test was part of my attempt to find WHY the darn thing decided not to run.

An engine builder in Georgia told me that 15% leakage by the rings on a street engine isn't bad. I'm going to move on to the ignition (and if necessary) fuel systems.

[dklawson]

I may have the answer to my problem.

I checked the leak-down values and was going through the ignition checks without finding any problems.

When my problem first started I checked to make sure I was getting fuel BUT I only looked at the level of the fuel in the "back" carb's bowl. I don't know what possessed me to check today but I took the cover off the front bowl and discovered it was empty. A little bit of work to free the needle valve and the car fired up normally and immediately settled in to its normal idle.

So... I had a valve problem... just not in the cylinder head. The pi$$er about this is that I had recently replaced those fancy Grose Jets with regular rubber tipped needle valves because my Grose Jets appeared to be leaking. Darned if you do, darned if you don't. Too much fuel, then not enough.