L28 Compression

[boomer3]

I did a cold/dry compression test on the L28 in my '76 Z last fall. I got around 120-125 across the board. I think that the bottom end is running somewhere in the range of 150k miles, and the head was rebuilt about 8000 miles ago.

 

My throttle response seems a little lacking, and it's burning a bit of oil on acceleration from what I can tell. I re-shimmed the valves as I could hear a little noise, but nothing was out of tolerance too much. I think she ran a little better after shimming, but this didn't solve the compression/oil burning issue. As I understand it, oil under accel. is typically rings, on decel. it's typically head. Everything is pointing toward the bottom end potentially needing a little work. I haven't checked the advance on the timing, it could be that it hasn't been bumped to spot 2 or 3 on the sprocket. If it weren't burning oil, I would just assume that it's a timing issue, but I'm suprised that it's burning some oil at the mileage it has! These engines usually go forever without needing rings from what I can tell.

 

Please chime in with your advice/what you think about this. I think the next step should probably be to try to advance timing and see if that bumps up the dry compression test. Beyond that I should probably do a wet test and see if the oil rings are shot.

[NewZed]

You're describing three things that might not be related.  Oil burning, cylinder pressure and throttle response.

 

Quarts per mile is a common indicator of wear.  How much are you burning?  In general, these engines seem to run well while burning lots of oil though, so your lack of throttle response is probably due to something besides worn rings.  Oil-burning would come from bad bottom (oil scraper) rings, poor compression and blowby would come from bad top rings.  The wet test will affect the top rings, not the bottom.

 

Cylinder pressure numbers vary a lot due to all of the different gauges out there.  One person's 125 is another's 160.

[30 ounce]

What head do you have that uses shims??

[boomer3]

Thanks for the info, I am a bit illiterate when it comes to diagnosing engine wear. You have helped clarify multiple things here.

 

I am mixing a few things together here for sure. I would say i'm burning about a quart of oil every 2-3k miles, so not very much. I can see quite a decent puff of blue when I rev her up. I have always presumed that burning oil is synonymous with blowby, (again my lack of understanding). If it is able to make it past your oil rings, won't you burn oil regardless of whether your top rings are good or not?

 

It's a litte un-nerving that there isn't a decent benchmark for compression testing and engine. I understand that as you change the variables, (ie engine temp, wet/dry, etc etc), the numbers will vary, but if it's a difference of 35 psi, then one of the methods should be ruled out as ineffective I'd think. I'm happy to be told i'm wrong though.

 

Regardless of all of this, the engine still seems to haul quite well given how tired out I have been presuming it is.

[boomer3]

What head do you have that uses shims??

 

Sorry I should have said checking the clearance, (feeler gauges). Not shimming.

[ram75280z]

Based on the information given, amount of consumption & when you see smoke, I would say you likely have leaky valve stem seals. If it only smokes when you 'rev it up' from idle speeds, what's happening is the oil is pooling on the back side of the intake valve when idling then being sucked into the cylinder when you rev it. Rings will generally smoke consistently under all conditions with more smoke as you drive it harder.