68 Z/28 Engine Smoking - Any Ideas?

[1969z280]

I'm wondering about the machinist who would have cut an original bore 320 for sleeves in order to save $300 worth or pistons as well. It seems the sleeves would have cost twice that much and eliminated the originality. JMO Ed

[68 RS Z/28]

I kind of thought that boring it out and going with oversize pistons would hurt the value - wouldn't really be a 302 anymore, that kind of thing. Anyway, I'll try replacing the valve stem seals and see if that helps. Thanks for the help!

[Salvatore]

Why would you think that it was not a real 302 being 30 over??? Who would really care about that? I get nervous when people talk sleeves for no reason IMO but that is just me. If you look at it by not being original because it would be bored.....its not original anyhow because of the sleeved motor. Hopefully a sleeve is not moving around in the bore. Try the umbrella seals. They don't need to have the head cut. But they do move up and down with the valve because they are not fixed to the head. Should be better than nothing though. Good luck!

[SuperNovaSS]

Why not try a leak down test before chasing a ghost? It will give you a lot better idea of what is/is not going on. Didn't you say only one guy referrenced blue smoke? Is the engine using any oil?


Jason


Jason

[GreenLS6]

I'd do a compression test first-check balance between cylinders.Look for something obvious. Then a leakdown.
O-ring seals SUCK. Machine for teflon seals or at least get the good rubber sleeve seals(they stay put and last longer) Depends on the valve springs you have though(Internal diameter) If it's only smoking when you stop then check the needle and seat or spray carb cleaner around the intake gaskets. You have higher vacuum at idle a may suck oil from a warped intake . Although a MO doesn't have much vacuum anyways. But it sounds good doin it!!
Good luck

[njsteve]

On the warped intake idea, you can also have a leak on the underside of the intake that sucks oil from the lifter valley into the intake runners without ever giving any indication of a vacuum leak on the outside of the intake.

My vote is for the dried out rubber o-ring valve seals. After sitting a long time, those things can just fall apart on their own. They would also give an inconsistant oil down that would occur randomly and not all the time, depending on oil flow/engine rpm.

[Lynn]

If you have already torn into the engine then disregard this. However, I would approach it this way.

First, I doubt it is valve seals based on this statement: “What it is doing is smoking occasionally, sometimes really badly. This seems to happen only at idle speeds.” Normally, an engine with bad valve seals will smoke on initial startup when cold.

Because it only seems to smoke at idle, I would suspect the carb is “loading up” a bit. Depending on how high the idle speed is set, it might not actually stumble before smoking. Could be a power valve, needle and seat, float. Eric can help you diagnose the carb. There is no one better with a Holley.


If you want to eliminate rings, valve guides and valve seals without tearing into it, take it for a spin, get it up to operation temp, lightly accelerate in second gear up to 5000 RPM, and let off the gas, but don’t hit the brake at all. Let the engine brake. If it has poor ring seal, bad valve seals etc. (or even a leak at the intake underneath so that it can draw in oil into an intake port, as someone has suggested) then it will billow gray smoke, guaranteed. If it doesn’t, you know it is not internal to the engine, and save a lot of tear down time chasing rabbits that aren’t there.

Hope this helps.

[SuperNovaSS]

Good advise.


Jason

[Hotrodpaul]

If the smoke is black, I would look at the carb, while a bluish smoke would be oil related. As suggested earlier, pull all the plugs and check the color. A dry black sooty residue indicates a rich condition, possibly a carb problem. An oily black residue indicates oil passing through the Intake Valve guides or Rings. Oil passing through the Exhaust Valve Guides will cause blue smoke but may not affect the plugs. If a particular plug shows these symptoms, look to that cylinder for the problem, such as rings, valve seals, or leaking Intake Manifold Gasket. Reading the plugs is the first thing I would do.

Paul

[king_midas]

Brake fluid?