[alfazagato]

I have a 2000 Spider with an engine rebuilt some four years and 15000 miles ago. Recently, at the Weber Concessionaires I had the car on the rolling road to get the engine performnce back to what it was when I rebuilt it. It was discovered that there was a 70% cylinder leakage from no.1 cylinder. You could hear the air escaping through the carburettor. I removed the head and had it checked and there was a burnt mark on the seating face of no.1 inlet valve, this was refaced (with all the valves and seats) guides checked, buckets checked clearances checked and reassembled. I repeated the test yesterday and still 70% leakage but no noise of air escaping through the carb. This time I could hear escaping air in the chain cover i.e. from the sump. I sqirted oil into the combustion chamber to seal round the rings and retested. The same 70%!
Not in the head now, not rings, new head gasket....what's left? Tick over is still not as smooth as it should be but when running at over 2000 revs its fine. Any ideas before I strip the engine again?

 

[Tifosi]

Have you done a leakdown test?

If not, it may be worth trying. (if you search keyword 'leakdown' posts by username 'ghnl', he's posted a couple times with a homebrewed way to do it without all the super fancy equiptment)

 

[alfazagato]

Tifosi, Thanks for your contact. Essentially, ghnl is repeating hat I have found. My one concern is that after stopping the leak from the inlet valve I appear to have the same amount from round the rings. Could they be stuck? I can be from round the gasket as there would be bubbles in the radiator. There isn't! I forgot to mention all the other cylinders have less than 10% leakage which is what you would expect from a relatively newly rebuilt engine. More thoughts?

 

[Tubolare Zagato]

I would suggest making a leakage test (as you have done) but in combination with a compression test and perhaps a power balance test. Given the bad leakage results from cylinder No1, if you find that you have good compression in the same cylinder then most probably the problem is stuck piston rings that are not expanding properly. Maybe -but less probable- is that the rings are not installed correctly at the first time. Don't forget that the gaps should be placed into a Y fashion...
Also make a vacuum test and see if you have a fluctuating needle of the vacuum gauge. A sharp vacuum drop at regular intervals may suggest a burned intake valve.

 

[Tifosi]

I concur.

Certainly the rings can freeze (especially mulit-piece oil control rings that don't fit just right) or be out of alignment, or they may not have bedded in properly for whatever reason.

Have you got a small inspection light on a stalk or flexible whip that you can run down into your cylinder through the plug hole and actually take a look?

Try at TDC where you can see most of the crown, then at BDC and look for scoring along the cylinder walls and edges of the piston. (pay particular attention to the edge where the crown meets the skirt as that's where vac/intake leak damage usually shows itself first on the side with the leak)

Roll it over a few times too so you can see the backs of the intake and exhaust valve heads.

Of course the downside is that if you're getting air blowing back through the intake track, you've likely got or have done some damage by now. (it can take as little as a few minutes depending on scenario)

Things on the list of potentials that may or may not be relevant but of use anyway:

*Vac leak at manifold or any place post-throttle plate/body burning edge of piston
*Valve seat burned or pitted
*Valve proper burned or pitted
*Bent/hanging valve
*Stuck/wedged tappet bucket
*Broken or weak valve spring not closing valve completely
*Screwed up valve seat
*Broken ring(s)
*Fat gapped rings
*Ring gaps not spaced correctly
*Rings installed upside down
*Wrist pin retainer spring slipped and scoring (or forgotten during install letting the wristpin proper do the work for it)
*Extremely protruded plug bashing the piston crown or dinking the edge of a valve (though I'm sure you'd notice that right off)

Granted several of the above are worst case scenarios, but all fall into the 'can cause what you're describing' category.

 

[Tubolare Zagato]

Also keep in mind that if you had your car (and engine) stationary for quite a long time, this may explain the problem. It's amazing how easy it is for the piston rings to stuck into the rails if there is not any movement and oil circulation out there!

 

[crashmctavish]

The only piston with 70% leakage has a hole in it!
Go back to the valves & valve gear.

 

[Tubolare Zagato]

If a piston with a 70% leakage is a piston with a hole, then a valve with a 70% leakage is an absent valve!
Don't forget that with normal working piston rings it is absolutely normal to see a 20% leakage past them. This is because when the engine is running, the rings are sealing much better, so the actual leakage with the engine running is less than 20%.
To see if there is a problem in the valve train (like a partially stuck open inlet valve for example) you should do a compression test as well.

 

[crashmctavish]

nah that would be 100% on an absent valve!
Sounds to me like the guy who built the engine has got a valve guide not inserted correctly or its too tight & seizes the valve. I would take it back to the guy who built it for you.