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Leak-Down Tests - What's Good ???

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dklawson

dklawson

Yoda
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As before, this topic is related to problems I'm having with the Mini but I know there's more A-series engine advice here so I'm hoping the Spridget owners can help.

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. There was leakage as expected, but each cylinder held the output pressure (downstream) at 85 psi. 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.
 
https://www.xs11.com/tips/misc/misc3.shtml
https://www.geocities.com/dsmgrrrl/FAQs/leakdown.htm
https://www.wcengineering.com/articles/leakdown.html


None of these mention leaking into the oil galleys. I would guess that it would be an unlikely event. If the head gasket as you mention was blown between a cylinder and the oil passage to the head i would suppose it would make a bubbling sound in the crank case , until it clearrd the galley way of oil. They all pretty much say that more than ten percent is generally indicative of a problem.

mark
 
Thanks. The links you posted echo what I found using Google. (The first link is the one I used to build my tester).

The percentage I'm seeing doesn't particularly surprise me even on this recently rebuilt engine. When I put the rings on I noticed their end gaps were a bit wide. I asked an engine builder/racer in the Mini community about it and he commented that the extra gap shouldn't cause a problem on a street engine (which mine is). Since all the leakage is heard through the pushrod tubes I'm assuming it's all past the rings. As mentioned, I was afraid I'd find damaged valves and I certainly don't think that's happened.
 
Think I just might consider a new set of valve guides along with a valve job.
 
You may try tapping your rockers with a hammer while the leakdown tester is hooked up and charged with air pressure. There could be just enough carbon build up to let a little air out through the valves.
 
I would squirt some thick oil into each cylinder, and re-test. If the problem is the rings, the oil ought to slow down the leak enough to show a change on the differential pressure. I assume that you are using a hose to listen through? It can be hard to pinpoint a leak source with out one. No leaks through the tail pipe(exhaust valve)?

If the noise is through the pushrods pull the valve cover and hook-up the air: check the valve guides for signs of air leaks. Listen @ the dipstick and all breathers too...
 
If you had a valve leak bad enough to hurt the running of the engine then you should hear it. I've done this on motorcycles plenty of times and you will hear it. I've changed valve seals with a deal that blows pressure in to keep the valves up when you remove the springs and you will have some leak down and yours does not sound too bad at all. Especially, with them all being the same. Have you verified that you have spark at each plug? When the engine spins over can you hear it going chug, chug, chug, at the exhaust pipe? You can take an extra spark plug and remove each plug wire one at a time and connect it to the extra plug and hold the outside to ground and verify a good spark.
 
I'm a leak down freak, been using one for years,, test my race engine after every event. 15% ain't bad for a street car, so I wouldn't wory about that too much. We shoot for much lower numbers on the race cars and buy high dollar rings to get better seal, but I wouldn't worry about 15% on a street engine.
 
Yup-yup. If the end-gaps were wider than spec's that could be the entire reason. You've hardly got a "leaker" so sounds as if your problem is NOT the mechanicals. At least not the "hard" bits. Does the engine even have enough time on it to be sure the rings/walls are well broken in?

With 100 PSI, if it were going into the oil galley I'd expect a rise from the gauge. Yours ~is~ mechanical, yes?

The "doesn't want to run anymore" comment is worrysome. But the problem would seem to be somewhere other than the valves/rings/cc's. Even with a funky valve you'd get NOISE but the other three lungs would likely light off. You're back to fuel or iggy issues.
 
No leaks through the valves, I was listening with a tube to the possible spots (carbs, tail pipe, dip stick tube, pushrod holes, adjacent plug holes) All the leaking appears to be past the rings. BTW, the valve cover was off... that's how I know the air was coming up through the pushrod tubes.

I agree that mechanically I seem to be OK. I'm going to install a different dizzy and try restarting with a new ignition system. If that doesn't fix it, I'll try a new coil. If still no joy I'll look at the carbs again. I hate tuning carbs. Fuel pump is pumping and the filters and float bowls are full.
 
I had a very old set of plug wires on my car several years ago. I changed out the cap,rotor,points condensor, etc and still couldn't get it to start. Finally changed the wires (really didn't know that they could get that bad) and have had a first turn starter ever since.
Bill
 
If there's fuel, it just HAS to be iggy. Good luck and let us know what you find!
 
Hap Waldrop said:
I'm a leak down freak, been using one for years,, test my race engine after every event. 15% ain't bad for a street car, so I wouldn't wory about that too much. We shoot for much lower numbers on the race cars and buy high dollar rings to get better seal, but I wouldn't worry about 15% on a street engine.
Click to expand...
I guess you guys use those gapless rings that overlap? I bet that gets the pressure up there. dklawson Hearing air at the pushrod holes, you are just hearing the air leaking by into the crank case around the rings. Also listen to these guys about plug wires, especially on a mini. All those wires and cap are close to the grill and take in all kinds of crap when it rains. Trust me on this, Britain has 300 rain days a year and I helped lots of people out with this very problem on lots of different austins, some of them almost new. The mini sets very low to the ground which means the snoot takes in more road grime. Could there be some corrosion in that nissan distributor you have? I had a subaru that just quit on me with no fire and I removed the cap and loosened and cleaned the pickup unit that gets triggered by the star shaped thing in there (was mounted on a gold plated plate) and put it back and it ran like a champ.
 
Does this engine have Points and a condenser? IF it does change them out with new, if that doest help change the cap rotor and plug wires. If that doesnt help change the coil.
From your descriptions of the "Leak down test" I doubt you have a valve problem, sticking or otherwise. Ignition can cause stumbling, stuttering, backfiring, and yes even make you think you have anything from a stuck valve to bad gas.
I vote an Ignition problem is your culprit.
Kerry
 
AweMan said:
Does this engine have Points and a condenser? IF it does change them out with new, if that doest help change the cap rotor and plug wires. If that doesnt help change the coil.
From your descriptions of the "Leak down test" I doubt you have a valve problem, sticking or otherwise. Ignition can cause stumbling, stuttering, backfiring, and yes even make you think you have anything from a stuck valve to bad gas.
I vote an Ignition problem is your culprit.
Kerry
Click to expand...
In his earlier post he said he had a japanese dist. The only reason I could see goin to it would be to get away from points and condenser. My subaru was an old 82 model and gave about the same problems as a bad condenser, started running rough, backfiring, etc. then finally just quite. The pickup unit under the cap was not grounded well. Most of those japanese units in the 80s were similar, thats why I sugested this.
 
Yep even electronic ignitions go bad. The "pickup" not grounded will cause a "no start" "no run" situation. a loose ground will cause a rough running engine, possibly stumbling, stuttering, and back fireing sceneraio. Grime on the gate {pickup} can also raise havoic. Try the ground first make sure it is secure. If that doesnt do the trick a new pickup may be in order.
I still vote for an electrical problem rather than a valve problem
Kerry
 
Thanks for all the suggestions. 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 rub 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.

Next time I'll take a look at both bowls before doing such extensive tests. I suppose the main reason I didn't start with fuel as the potential problem is the car didn't exhibit the lean, backfire through the carb, symptoms that I've seen on other cars with blocked fuel delivery.
 
I do think that was all it was. As soon as I freed the stuck needle valve the car started on the first turn of the key, required very little choke and immediately settled into a stable idle. I took the car on several short (but increasing length) test drives and it never so much as hiccupped. I'm glad it was something simple and embarrassed that I didn't check the floats sooner. However, as I mentioned, it didn't exhibit the symptoms I associate with lean running and blocked fuel supply.
 
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