Vacuum at idle for healthy M130 engine?

[Riquiscott]

I asked this question before and haven't gotten any responses, but I may have been asking too many questions in one post.

What should the manifold vacuum reading be for a healthy M130 (2.8L SOHC) engine? My engine is running pretty well, but I'm only getting 15" of vacuum at idle, which seems low to me.

The vacuum reading is rock-steady, so I don't think I have any burnt or sticking valves. I'm basically looking for a little better fuel economy, and I think the low vacuum might be caused by not having enough initial ignition timing. Right now, it's set at 7 degrees BTDC, with the vacuum advance hose connected. I need to check tomorrow to see if the vacuum advance is connected to ported vacuum or manifold vacuum. I guess it's probably too much to hope that the mechanic accidentally plumbed the vacuum advance to a manifold vacuum port by mistake :)

If 15" at idle is normal, then I'll look at making other changes, like opening the plug gap a little bit. The engine has dual Weber 32/36 carbs, and a Mallory electronic ignition, so I don't know how useful the stock ignition timing numbers and spark plug gap settings are going to be for my engine.

Any tips would be appreciated,
Scott Gardner

[mercmad]

15"HG sounds OK,but try retarding the spark with the engine running to see if there is a change in the level,at a lower idle it should go to 28"HG.
Opening the gap won't change the vacuum value,but will make it ping .
If the engine is in good conditon i would go for the stock plug range.
Have you checked the vacuum advance/retard is working by aiming a timing light at the vibration damper?.
If it not going to full advance by the specified engine speed,IS the actual vacuum canister working?
Are the mechanical advance weights free and not stuck?
Vacuum levels while the engine is running at speeds above idle have no bearing (,or relation too,) on vacuum at idle.
But something has just occured to me:
Timing Chain
Have you done a basic timng tiimng chain wear check?
set the vibration damper to zero on the timing cover pointer.
The cam gear timing notch should be lined up with number 1 cam bearing tower.
There must be NO slack in the timing chain.
Replacing the chain requires removing the tensioner so replace it if you have to change the chain.

[Nutz 4 Benz]

20-22 in./Hg. is ideal and your 15 in./Hg. reading may due to leaking carburetor gasket,low compression,late ignition timing or leaking manifold gasket.

[Riquiscott]

I'll check the carb gaskets and manifold gaskets by spraying some starting fluid around them. I'm pretty sure it's not a compression problem since the reading is so steady, but I'll pick up a compression gauge and check that as well.

I'm leaning towards late initial ignition timing as being the problem, but at 7 degrees BTDC, I'm already 11 degrees more advanced than the factory setting (4 deg ATDC). I'm not sure if the initial timing numbers should be different since I have a Mallory electronic distributor instead of the stock points. I'm pretty sure that the electronic distributor will let me open up the plug gap for a little bit fatter spark, though.

The mechanical and vacuum advance mechanisms both work, and Mallory recurved the distributor to a factory advance before it was installed.

I don't think there's really anything "wrong" with the engine, and it runs smooth and strong. I'm just investigating because it takes a little longer to start than I'm used to, and the fuel economy is only about 15-20 MPG.

The timing chain and tensioner were replaced last month, and when I check the timing with a light, the timing mark is rock-steady, so I don't think there's any slop in the chain.

Thanks for the tips - I'll let you know what I find.

Scott