Deciphering engine compression numbers

[Spider951]

I removed the engine on my FI '81 (actually the engine is a replacement from an '82) to deal with a broken clutch disc (discussed in the maintenance section of this site). I took some engine compression measurements before removal and recently added some leak down measurements (all pressured to 100 psi) with the engine out, to wit: Cylinder 1: compression test: 170 psi, leak down: 93 psi; Cylinder 2: compr. 170#, leak down: 93#; Cylinder 3: compr. 168#, leak down: 92#; Cylinder 4: compr. 171#, leak down: 94#. Other observations: all cylinders leaking air on intake side and through oil dipstick and filler neck - not a lot but some. Also the engine Oxygen sensor badly covered with carbon as are spark plugs and end of tail pipe. Car emits light blue smoke at startup and at idle regardless of how long driven (though I don't drive it far enough on most outings); also audible "ticking - clickety" sound coming from engine (pretty much confined to cam towers best I can tell from my "Dr. Fiat" (read cheap) stethoscope).
My questions are: what do these compression numbers mean? I was delighted that the numbers were kind of on the high side , but does that mean a heavy carbon coating is "inflating" them? Can I assume that the rings are OK (with better than 90% on all cylinders in leak down tests), even with some air leaking out the oil holes (crankcase)? I suspect worn out valve guides and seals. I don't have much history on the engine. [It has Reed camshafts (to my utter surprise when I bought this engine) so it's obviously been tinkered with. I just adjusted the valves, new spark plugs, etc and have run the car since 2018 (search Reed cams on this site if you want to hear about that conundrum - I perhaps have the only Spider with Reed cams??). The OE '81 engine was kind of anemic (especially compared to the '82 engine) and on leak down test performed rather poorly (except cyl 1) (leaking exhaust valves and crankcase; but I did NOT do compression testing on it while it was still in the car... duhh)].
Will I know a lot more by removing the cylinder head to look-see at the cylinders (a major cleaning if not more is in store anyway)?

[18Fiatsandcounting]

Your compression and leak down numbers look fine to me. As for the other symptoms:

Carbon buildup: It won't change compression pressures as much as you think. Some yes, but only a few psi.

Carbon in general: It sounds like your engine is running rich or perhaps only driven on short drives.

Blue smoke at startup: Most likely worn valve stem seals.

Blue smoke at idle: Your compression/leak down numbers are good, so my guess is an overly rich mixture that is washing down some of the oil off the cylinder walls. Does it matter whether the engine is warm or cold in terms of the blue smoke?

Clicking sound: Sorta normal for a Fiat spider, but you could check the valve clearances. Typical specs are (when cold), 0.019" on the exhaust and 0.017" on the intake. Performance cams sometimes specify less, but in any case you shouldn't be greater than 0.019". Too much clearance can lead to noisy tappets which could be what you're hearing.

Summary: Sounds like your engine is in reasonable shape, likely needs some new valve stem seals, might also have an issue with running rich. Might benefit from an "Italian tuneup" every now and then (driving hard at 4000 or 5000 rpm for a while).

-Bryan

[Nut124]

The numbers alone seem OK.

What do you mean w cylinders leaking air at the intake? Can you see of hear this thru the intake port? What about air leaking thru the dipstick??

The intake valves should not leak air at 100psi unless valve clearance is zero.

[Spider951]

By leaking through the intake I mean I can feel and hear air coming out of the big intake port on top of the intake manifold when each cylinder was pressured up to 100 psi. I also can feel and hear a bit of air coming out the dipstick hole and around the oil filler cap (loosened for the tests). Getting each cylinder to TDC was a bit imprecise (maybe not doing it quite right??). Thanks.

[18Fiatsandcounting]

Getting each cylinder to TDC was a bit imprecise (maybe not doing it quite right??). Thanks.

If the piston for the cylinder you were testing was at TDC but at the end of the exhaust stroke (rather than the end of the compression stroke), then yes, you would bleed out pressure since the intake and exhaust valves are both slightly open at that point.

You don't have to be exactly at TDC for each cylinder under test, but you do need to be near the TDC when both valves in that cylinder are totally closed.

-Bryan

[Spider951]

I may try the leak down tests again. I did each cylinder several times, but I admit that finding the precise point at which both valves were closed was a bit elusive. However, all it would mean for the '82 engine is that the 90+ % numbers would just go up. But I need to learn to do it right. The method I finally settled on was to use the rubber hose from the compression tester screwed in to the spark plug hole, and feeling with my finger over the end when I reached the point at which air seemed to stop being expelled from the cylinder on approach to TDC, then turned the crank a "bit" farther, meanwhile watching the ignition rotor with respect to position for firing for that cylinder, assuming that point would be the most compressed part of the cycle. I also tried ye olde wooden dowel in the spark plug hole method to judge where TDC was. I guess I'm a bit ignorant of the precise point at which both valves should be closed with respect to TDC.

[Nut124]

If a valve was actually open, even a little bit, your leak down test result would be zero or near zero, not 90+.

Remember that there are two different TDCs for each cylinder; one where both valves are open and another where both are closed. These are 360 crank degrees apart.

[18Fiatsandcounting]

...but I admit that finding the precise point at which both valves were closed was a bit elusive.

I think you kinda did this, but the best way is to turn the engine until the ignition rotor is pointing towards the spark plug wire of the cylinder that you want to test. Distributor cap removed of course, so you can see the rotor. That will be the TDC point where both valves are closed.

If a valve was actually open, even a little bit, your leak down test result would be zero or near zero, not 90+.

Yes. But if Spider951's valve clearances are way too large (that would explain the clicking sound), then it might be possible to have the valves still closed at both TDCs within a given cycle. One TDC would be end of the compression stroke (valves tightly closed) and one TDC at the end of the exhaust stroke (valves just very lightly closed).

-Bryan

[Spider951]

I measured and adjusted valve clearances on '82 engine on 8-3-2019. The numbers (mm) were: Cylinder (measured - adjusted to):

Intake valves: Cyl 1 (0.46 mm - 0.46 no adjustment); Cyl 2 (0.44 - 0.44 no adjust.); Cyl 3 (0.46 - 0.46 no adjust.); Cyl 4 (0.381 - 0.43 adjust. 0.049 mm)

Exhaust valves: Cyl 1 (0.46 - 0.51 adjust. 0.05); Cyl 2 (0.46 - 0.51 adjust 0.05); Cyl 3 (0.46 - 0.5 adjust 0.04); Cyl 4 (0.51 - 0.51 no adjustment)
My manual says intakes should be 0.41-0.48 mm and exhaust should be 0.46-0.53 mm.

To Nut124's basic point, I think I did everything pretty close to correct. Because when I was off by even just a bit on the compression stroke TDC and then injected air from the compressor the psi readings were way down (like 30% or less) and I would move the crank a bit and try again. Thus the 90+ measurements can't be off by much (?).

My main concern has been all along whether or not the piston rings are OK. I kinda inferred that from the original 170ish psi numbers from the compression test, but I had read that a leak down test is more definitive. I didn't feel or hear a lot of leakage out the dipstick hole or oil cap, but there was some. So maybe rings are worn some but not a lot (?). I am hoping I can just rebuild (or purchase) the cylinder head and not do much to the bottom end. I expect to see a ton of carbon when I take this thing apart. Thanks.

[Nut124]

I'd say your head and rings are good. No need for a rebuild with those numbers. I would just try to get it running properly. Plugs center electrode insulator should be grey or light brown with Fiat FI.

If you are saying that a tiny move in the crank rotation changed leak test from 30% to 90+%, then you were, like Bryan said, at the wrong TDC. That would also explain the leaking intake valves.

Remove the cam box covers and redo the leak test. Make sure cam lobes are pointing away from the bucket when you test. At the wrong TDC, both lobes are laying flat on the bucket and both valves are marginally open.

Or remove the dizzy cover like Bryan suggested.