Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

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?

Re: Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

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

Re: Comments on these leak-down test results
 

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.

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

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

Re: Comments on these leak-down test results
 

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

Re: Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

Re: Comments on these leak-down test results
 

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

Re: Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

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

Re: Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

Re: Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

OK, yup.

Re: Comments on these leak-down test results
 

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.

Re: Comments on these leak-down test results
 

PB Blaster OK for the penetrant?