Compression and timing

Darcy · Feb 7, 2008

Just got back from a little engine work. V8 Chevy OMC. 1986.

Getting ~120 psi on all cylinders except #3 at 50 psi and #4 at 90 psi. Squirted some oil into both, with no change in compression.
#3 spark plug looked different than all the others: damp (I assume oil, didn't evap like gas) and the conductor looked like it hadn't seen much spark.
Does this sound like a valve problem? Where do I go from here? No milk in the oil pan, and no oil consumption.

Also, checked the timing, and it was roughly at 30 BTDC (off the scale). I put it down to 13 BTDC per the OMC book. Is that way off, or is it normal to have it drift that far? The lock bolt on the distributer seemed tight enough, so I was surprised to see it so far off. Would that have caused significant power loss, and is there any damage I should look for?

Also, when I pulled the drain plugs for the exhaust manifolds, I got a whole lot of rusty looking debris coming out. I'm talking fingernail sized flakes of hard oxidized material. What's going on here?

Thanks in advance!!!

Chris1956 · Feb 7, 2008

Re: Compression and timing

Darcy, Are #3 and 4 next to each other in the same cylinder bank? If so, you might suspect a bad cylinder head gasket.

JustJason · Feb 7, 2008

Re: Compression and timing

if that spark plug your talking about looks clean... as in steam cleaned clean. and the rest are averagly sooty. then you have a head gasket leak.

You need to do a leakdown to figure out where your compression is going.

TilliamWe · Feb 7, 2008

Re: Compression and timing

Chris1956 said:
Darcy, Are #3 and 4 next to each other in the same cylinder bank? If so, you might suspect a bad cylinder head gasket.

No they are the second cylinder back on opposite sides, which is interesting.

If you can't or don't know how to do a leakdown test (I'd have to read on here how to do it, too), take off your valve covers and try adjusting the valves on the low cylinders. And I mean loosen them up to the point where they tick just a little. If the compression comes up, you have valve issues. If it doesn't, it's probably not valves. Valve problems are easy to cause on the Vortec engines, but I didn't think the older ones were as prone to it.

whywhyzed · Feb 7, 2008

Re: Compression and timing

....

Don S · Feb 7, 2008

Re: Compression and timing

My guess is you have tuleped valves, probably due to the timing issue. Loosening any rocker arms will not make them seat when they are warped like that.
I would also venture to guess the weights and springs in your distributor are rusted and frozen up. At 3000 rpm, the timing should be around 30?, but drop back down to ~ 10? BTDC at idle. The springs pull the weights back in at lower rpm.

Don S · Feb 7, 2008

Re: Compression and timing

Here is the cylinder numbering on a SB chevy. Are we still talking about cylinders #3 and 4?

Windykid · Feb 7, 2008

Re: Compression and timing

I would adjust the valves, do another compression test and go from there! I agree with Don, but the other cylinders have a compression in the 120. If compression comes up in 3 and 4 recheck the timing and inspect the distributor.

dcg9381 · Feb 7, 2008

Re: Compression and timing

Windykid said:
I would adjust the valves, do another compression test and go from there! I agree with Don, but the other cylinders have a compression in the 120. If compression comes up in 3 and 4 recheck the timing and inspect the distributor.

How do you adjust the valves on a V8?

You can run a leak down test - similiar to compression to figure out if you're leaking from the block, head, or HG.

Haut Medoc · Feb 7, 2008

Re: Compression and timing

dcg9381 said:
How do you adjust the valves on a V8?

You can run a leak down test - similiar to compression to figure out if you're leaking from the block, head, or HG.

Tightening/losening the nuts on the rocker arms......
On a GM engine anyway.....

Darcy · Feb 7, 2008

Re: Compression and timing

Don S said:
Here is the cylinder numbering on a SB chevy. Are we still talking about cylinders #3 and 4?

Yes Don, still #3 and #4.

#3 Spark plug (50 PSI) wasn't 'Steam cleaned', but the ceramic around the conductor didn't have that sooty look. In fact, it looked more dirty than the others, but unburned. Obviously not seeing the same combustion as others. These are all new plugs with maybe 30 min- 1 hour of idle time.

When I was doing the timing, it would walk +/- 1`. Is that normal? Sounds like I definitely need to check out the advance mechanism. Damper seems to be on tight

If the valves are tuliped, what kind of work are we talking? Will I be able to just replace that valve, or will I need to machine the heads as well?

Are the OMC and Clymer books good enough that I can pull the heads on my own? As long as the info is there, I am confident I can follow directions...

Thanks again!

Haut Medoc · Feb 7, 2008

Re: Compression and timing

I would imagine the OMC book would be OK it it wer eoem.....
Clymer's or Seloc often leave out/misstate important stuff.....
Merc manual #9 would actually be helpful for the engine, it is after all a GM underneath all of that OMC stuff.....
R&Ring the heads should not be too difficult at all.....

I don't think 1* of 'wobble' is significant & I would more likely attribute it to worn bushings in the distributer than the advance mechanism/advance weights....
Since it is OMC, I am assuming a points type ignition, correct?.....
You could just replace the valve, but on a 20 year old engine, why not spend the $ & get the heads redone?....
The flakes out of the manifolds is normal, however how old are they?....
Those flakes are the manifolds/riser disintegrating from within....
They do not last forever......

Darcy · Feb 7, 2008

Re: Compression and timing

Manifolds are probably original. There is a hairline crack by the riser that gets a little damp. How long can I let it go on like that?

I'm willing to let some stuff like the manifolds be not perfect, as long as they aren't going to fail and cost me way more $$ in the long run. It isn't an ocean boat, so a failure on the water worst case is a tow back to the marina.

What cost is involved in machining the heads? Should I be concerned about putting money into this 20 year old engine? Will the investment last provided its done right?

I'm trying to avoid sinking a whole lot of $, as I only paid $4000 for the whole shebang. But on the other hand, I do want to maintain it and keep what I already have in good shape.

TilliamWe · Feb 7, 2008

Re: Compression and timing

Letting the manifold fail is just about the worst thing you can do to a marine engine. The problem is that when they fail internally, you don't know it, until you go to start it the next time and it hydrolocks due to one or more cylinders being full of water. And if you let a cylinder sit full of water long enough, well, you know that'll ruin the motor. If you have a cracked manifold, you seriously need to consider replacing them.
Don says that adjusting a tulipped valve won't make it seat. I will respectfully disagree with him, cause I have done it, 4 times. It's not a solution, but a simple way to identify a specific problem. As far as having the heads done, yes, have them reconditioned if you have valve issues. But also check your local shop, replacement heads might be the way to go. For example, I paid $159 to have 1 Vortec head reconditioned, and when the other was shown to be cracked, I paid just $179.99 plus tax for a replacement reconditioned head. And it was available right away, I didn't have to wait two days for them to fix mine.
Best of luck to you.

Don S · Feb 7, 2008

Re: Compression and timing

I'm willing to let some stuff like the manifolds be not perfect, as long as they aren't going to fail and cost me way more $$ in the long run.

Defective manifolds and risers cause many people to buy new ENGINES every year, because they let them go too long.
They may already be your biggest problem, especially if it's ever been used in salt or brackish water.

Flukinicehole · Feb 7, 2008

Re: Compression and timing

With your timing that far out either Dons right about your weights or the dampener spun the outer ring. The rubber in dampeners breaks down and the outer ring can spin.

dcg9381 · Feb 8, 2008

Re: Compression and timing

This is the "easy" answer - you sure you were getting your inductive pickup off of the #1 cylinder?

I've seen rubber spin - usually a few degrees, never seen one spin that far off..

Darcy · Feb 8, 2008

Re: Compression and timing

dcg9381 said:
This is the "easy" answer - you sure you were getting your inductive pickup off of the #1 cylinder?

I've seen rubber spin - usually a few degrees, never seen one spin that far off..

Positive I was on #1. It may not have been as far out as 30, but I was guesstimating because there are no marks past 16.

The engine idled much better after the timing fix. I'm sure I've made an improvement.

So my to-do list so far is:
Check out advance mechanism.
Check valves on low cylinders.
R&R/ replace heads
pull exhaust manis and check/replace.

Did I miss anything?

Haut Medoc · Feb 8, 2008

Re: Compression and timing

That's a good start.....

Compression and timing

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