Comments on these leak-down test results

[alexiskai]

This is a '29 coupe that runs a little poorly at low RPMs. I've been through all the carb and intake tests, those are fine. Timing is correct. So I started running tests on the cylinders.

A vacuum test indicated "worn/loose valve guides." A compression test showed 60-65 psi on all cylinders except for cylinder 2, which showed 40 psi on a dry test and 45 on a wet test.

To do the test, I cranked each piston to TDC in turn. I used an OTC 5609 cylinder leak tester. The compressor was set to 90 psi, but I never actually dialed the gauge up to full pressure because all four cylinders leaked 50% or more. All 4 cylinders showed hissing in the crankcase. Cylinder 2 also showed hissing in the exhaust. No hissing heard in the carb or bubbles in the radiator.

Assuming I did the test right, my guess is that all 4 cylinders are just worn after 90 years of use. I don't think the engine has ever been rebuilt, certainly not to the extent of installing oversized pistons. So presumably the rings are not sealing well? Is there any repair for this other than a rebuild?

The exhaust on cylinder 2 – I assume this is a burnt exhaust valve? Is that something I could repair without replacing the valve? Could I pull the head and clean up the valve seat or something?

The car does still run, so despite the clear problems with the pistons I may elect not to fix them if I can fix the symptom of poor running at low RPM.

Thanks for any suggestions.

[David R.]

Sounds like it might be time to pull engine and go through it. Might just need freshened up. Hone cylinders, rings, lap valves and check clearances?

[Patrick L.]

60# indicates a good cylinder. The difference with #2 is concerning. This engine would have non-adjustable lifters if it has not been rebuilt.

Have you checked the valve lash ? If so, what is it ? I think that needs to be done. The valve may not yet be burnt as it would then show zero compression. Work on just that valve may be all thats needed.

[rotorwrench]

Leakage past the exhaust valve could be a damaged valve or seat face but it could also just be some carbon keeping the valve from seating all the way. The rings & cylinder walls likely have a good bit of wear for the overall low compression readings.

I do regular leak down checks on the aircraft engines. We use 80 psi input with the pistons at TDC on compression stroke. The Lycoming service instruction lists 65 psi minimum reading with no more than 15 psi difference between cylinder readings. If I have a low one, I always run the engine and pull some power with it before rechecking the pressures. This will usually clear carbon from valve seat surfaces and allow compression rings that might have ring gap alignment to move out of that alignment. If results are the same, it's time to make repairs.

On an auto engine, it's not as critical that things be up to that level of safety. If it still runs on all four and doesn't use too much oil then it's still operable for a time. If it would double as a mosquito fogger then it might be time for an overhaul.

[chrs1961815]

Definetely time for new pistons and rings, if not more.

[alexiskai]

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Originally Posted by David R.

Sounds like it might be time to pull engine and go through it. Might just need freshened up. Hone cylinders, rings, lap valves and check clearances?

Problem is, that is well beyond my abilities or toolkit. Would have to have a pro do it, so then we're into the big bucks. I agree that the engine would see significant improvement from new pistons/rings/honing, etc., but it's hard to justify, especially in this economic climate.

Originally Posted by Patrick L.

Have you checked the valve lash ? If so, what is it ? I think that needs to be done. The valve may not yet be burnt as it would then show zero compression. Work on just that valve may be all thats needed.

Haven't pulled the valve cover or the head yet.

Originally Posted by rotorwrench

Leakage past the exhaust valve could be a damaged valve or seat face but it could also just be some carbon keeping the valve from seating all the way. The rings & cylinder walls likely have a good bit of wear for the overall low compression readings... If it still runs on all four and doesn't use too much oil then it's still operable for a time. If it would double as a mosquito fogger then it might be time for an overhaul.

Oil consumption is low, no smoke when it's running.
------------------------------------------------------

So it sounds like I should pull the head and check out the exhaust valve on #2, looking for damage or buildup. If it looks OK there, I should pull the valve cover and measure the lash. I'm extremely conscious of the potential to make problems worse by trying to fix them, so I want to proceed cautiously.

One thing this is making me question is my plan to eventually put in a HC head. With the cylinders in this poor condition, it really seems like I'd get much less benefit from a HC head than I was expecting unless I have the cylinders repaired at the same time.

[Kurt in NJ]

If you fix the valve sealing problem it should run smoother at low rpm

[alexiskai]

Originally Posted by Kurt in NJ

If you fix the valve sealing problem it should run smoother at low rpm

That's what I'm hoping. The leak-down test isolated it to the exhaust valve, so that was useful. Now I have to decide the least invasive way to diagnose and repair it.

[Jack Shaft]

In doing engine work its important to set a goal,what is the result you are looking for..most set a goal of a perfectly performing engine,with the comfort of reliability.Often when one valve is worn the other 7 are right behind it.Another issue when using 'least invasive' as a guideline is you fail to see the engine as a whole...worn valves? worn cylinders? then you should inspect your bearings. I understand feeling leery about performing tasks your uncomfortable with,we all are,but let your expectations be your guide and build to then.

[MikeK]

If you do get to the point of teardown, one thing that I found on a few lousy rework jobs relating to compression and blowby issues was way too much ring end gap, either on only one cylinder, or just some rings.

I guess I'm just crazy for measuring such things after the fact on teardown, but that's just me. If someone goes just 0.030 over on an A and uses a std. sized ring out of the box you end up with about 0.020 too much end gap and reduced wall tension.

[30 Closed Cab PU]

You might be able to see what's going on with an inexpensive borescope through the spark plug hole?

[Patrick L.]

One quick thing you could before taking anything apart is to warm the engine, set at a high idle speed and use a spray bottle of water set to a very fine mist and spray carefully into the carburetor. That will help clean any carbon in the cylinders.

[alexiskai]

Originally Posted by Patrick L.

One quick thing you could before taking anything apart is to warm the engine, set at a high idle speed and use a spray bottle of water set to a very fine mist and spray carefully into the carburetor. That will help clean any carbon in the cylinders.

Didn't I hear that MMO would also have that effect if delivered that way? Or does it clean some other thing?

[rotorwrench]

MMO softens hard carbon. Sticky valves can be from carbon build up and same with sticky rings. MMO in the fuel will improve an engine with a lot of carbon build up.

H2O may or may not affect valve guides or sticky rings. Folks used to use cleanser mixed with water to try and reseat worn rings. Results are generally mixed. Some folks will try anything to keep from investing more money. Most of this type of stuff is a temporary fix.

[Y-Blockhead]

Originally Posted by alexiskai

Didn't I hear that MMO would also have that effect if delivered that way? Or does it clean some other thing?

You could try Seafoam. I add 1 oz. per gal. at fill up.

I have never used the spray https://www.amazon.com/Sea-Foam-SS14...s%2C231&sr=8-5 but if your looking for a quick fix...

[Patrick L.]

Originally Posted by alexiskai

Didn't I hear that MMO would also have that effect if delivered that way? Or does it clean some other thing?

I wasn't talking about valves sticking, just dislodging and removing some carbon.

Also, Removing the side cover is not hard, manifolds don't have to be removed. The valve lash can be checked.

[alexiskai]

Originally Posted by Patrick L.

Removing the side cover is not hard, manifolds don't have to be removed. The valve lash can be checked.

Only caveat is that I have the cover-mounted oil filter, so it's more complicated than usual. Still haven't figured out the cleanest, simplest way to take it off.

[Patrick L.]

Originally Posted by alexiskai

Only caveat is that I have the cover-mounted oil filter, so it's more complicated than usual. Still haven't figured out the cleanest, simplest way to take it off.

OK, yup.

[Jack Shaft]

Remove oil filter,remove brass nut in center of fllter inlet,remove cover.Brass nut has siamese internal and external threads,go easy putting it together.The easiest was to fix sticky valves is to remove sparkplug and spray penetrant directly at the valve while turning the engine over with the starter...key off,plug out,spray,reach across.engine and push starter rod by hand.

[alexiskai]

Originally Posted by Jack Shaft

Remove oil filter, remove brass nut in center of filter inlet, remove cover. Brass nut has siamese internal and external threads, go easy putting it together. The easiest was to fix sticky valves is to remove spark plug and spray penetrant directly at the valve while turning the engine over with the starter... key off, plug out, spray, reach across.engine and push starter rod by hand.

PB Blaster OK for the penetrant?

[Jack Shaft]

Originally Posted by alexiskai

PB Blaster OK for the penetrant?

yep,as long as its flammable..you can actually see the valve work too.80/20 against it fixing your issue but its worth a try

[alexiskai]

This is worth exploring – when someone says "sticky valve" does that mean the valve is stuck closed, or open, or both? The problem here is the valve is remaining open during the compression stroke.

[Jack Shaft]

#2 cylinder? before you spray take flashlight,shine down on valves and turn engine over ,key off, with starter and you can watch the valve work.

[alexiskai]

Yeah, I think that's why 30CCPU was recommending a borescope, same idea. OK, I'll see what I can do to inspect the valve visually without removing the head. That seems like a good next step.

[katy]

when someone says "sticky valve" does that mean the valve is stuck closed, or open, or both?

Usually means that the valve is slow in seating because the stem and/or guide are carboned up or rusty, also may be caused by a weak or broken spring. The valve may close fully during cranking but when running it doesn't close fast enough.

[30 Closed Cab PU]

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Originally Posted by alexiskai

Yeah, I think that's why 30CCPU was recommending a borescope, same idea. OK, I'll see what I can do to inspect the valve visually without removing the head. That seems like a good next step.

As an FYI instead of shining a flashlight and looking through the spark plug hole I recommended using a borescope - personally my with my eyesight can not focus/and see fine detail well enough. The Borescope gets close and magnifies what you see. It also has a light for Illumination.


Looking through the hole is worth a try if you have good eyesight.


Either way with the ignition off you can use the starter/starter rod, or if you have a helper can hand crank. Easiest is to pull all the spark plugs making it easy with no compression to crank over the motor. And while you are at it inspect the other cylinders.


Just make sure that if you still have penetrant in the cylinders to cover the spark plugs holes with a towel or something. If not you may end up with penetrant blowing all over the place. Do not ask me how I know.

[alexiskai]

I've ordered one of those snazzy scopes that uses your phone as the display, so I'll post an update with pictures from the #2 cylinder and we can see whether a diagnosis is indicated.

[Jim Brierley]

Quit fooling around, the #2 exh. valve is burnt, and the seat probably burnt a little too. Keep running the car and it could do major damage to that seat and block. Changing the valve only is not a good idea, the seat will need to be ground too. You might get some shop to do a house call and grind the seats?

[Patrick L.]

Originally Posted by alexiskai

This is worth exploring – when someone says "sticky valve" does that mean the valve is stuck closed, or open, or both? The problem here is the valve is remaining open during the compression stroke.

Its hard for a valve to stick closed, things have to get 'bent' for that to happen.
They stick open. So, what is the lash, once again ? Is the valve working freely in the guide ? To me, these have to be answered before taking anything apart. But, the sooner this is fixed the better.

[katy]

the sooner this is fixed the better.

Ditto

[alexiskai]

Message received folks, fixing this valve is my top priority. I have no idea how I'm gonna do it but we'll take it one step at a time.

Edit: First step is to get in there with the borescope, see if the problem is visible. Second step is pull the valve cover and check the movement and lash measurements that way.

[40 Deluxe]

Originally Posted by alexiskai

PB Blaster OK for the penetrant?

Spray carburetor cleaner works much better. In my experience valve stems get a coating of a varnish-like substance that penetrating oil doesn't cut very well. A solvent like carb cleaner is better.

[40 Deluxe]

Originally Posted by Jim Brierley

Quit fooling around, the #2 exh. valve is burnt, and the seat probably burnt a little too. Keep running the car and it could do major damage to that seat and block. Changing the valve only is not a good idea, the seat will need to be ground too. You might get some shop to do a house call and grind the seats?

Jim, how about going back to the suction cup stick like you used when you were a pup? My uncle had a fancy one. It looked like one of those old hand cranked drills but had a suction cup on the end and the shaft oscillated back and forth. Was faster than rubbing your hands together!

[alexiskai]

Originally Posted by 40 Deluxe

Jim, how about going back to the suction cup stick like you used when you were a pup? My uncle had a fancy one. It looked like one of those old hand cranked drills but had a suction cup on the end and the shaft oscillated back and forth. Was faster than rubbing your hands together!

I think it'll all depend on how damaged the valve seat turns out to be. Those sticks were for polishing a seat that was basically intact.

[Jack Shaft]

Originally Posted by Jim Brierley

Quit fooling around, the #2 exh. valve is burnt, and the seat probably burnt a little too. Keep running the car and it could do major damage to that seat and block. Changing the valve only is not a good idea, the seat will need to be ground too. You might get some shop to do a house call and grind the seats?

the minute the wet/dry compression test showed 40/40..

[40 Deluxe]

Originally Posted by alexiskai

I think it'll all depend on how damaged the valve seat turns out to be. Those sticks were for polishing a seat that was basically intact.

I agree, but was just thinking how it was often done back in the Model A's heyday when few people had the money for a first rate valve job.