Help me understand these vacuum readings

[mosu]

With timing at 10 BTDC and a curb idle around 700 rpm, my engine vacuum is 15-16" Hg and the needle is vibrating rapidly +/-1" Hg.

If I keep the throttle open at ~1200 rpm, vacuum is ~19-21" Hg with the same rapid vibration. Cracking the throttle open makes it spike to 27-28" Hg then back to a steady value.

I would have expected a reading closer to 18-19" Hg at idle. Granted, I can't get it to idle at ~900 rpm without the having the carbs pull gas from the main jets so I'm trying to find the highest idle speed at which it's pulling only off the idle jets (nothing coming out of the mains).

Can someone help me interpret these readings and the rapid needle vibration?

[Frank Reiner]

Care to let us know about the engine in question? Perhaps begin with make & model.

[mosu]

Sorry, didn't realize I missed that.

1968 280S, M130 engine with 88k miles on the clock. Valves are adjusted to cold spec. Chain has 4-5 degrees of stretch.

[Frank Reiner]

The gauge readings as you describe them are not what is almost universally called "vacuum", but are, in fact, manifold pressure (absolute). Note that the lowest pressure, 15-16" Hg, corresponds to closed throttle, and the highest, 27-28", which is very close to atmospheric pressure (29.92"), corresponds to a large throttle opening.

The needle vibration may be laid to two causes: the pulses in the intake manifold resulting from the opening and closing of the intake valves, and the lack of dampening of the gauge mechanism.

[mosu]

Thanks Frank!

On my old '73 M110, the idle reading was at 19" Hg. This engine is lower for similar compression. Is that concerning? I thought the idle reading should be around 18-20" Hg.

[Frank Reiner]

With varying valve open duration and valve overlap, so too will the manifold pressure vary.

[mosu]

Gotcha. What number should a healthy M130 show?

Also, how strong a vacuum signal is needed to pull gas effective from the idle jets?

[Frank Reiner]

Any engine that exhibits an idle manifold pressure of 15"Hg (absolute) would be considered to have mild valve timing, and to be in good condition.

As an after thought, can you post a picture of your gauge, not connected to engine?

If you wish to avoid flow from the main fuel circuit at idle, try lowering the float level about 1/16" for starters.

[mosu]

I'll grab the photo this weekend when I go home this weekend.

I didn't realize that the float level is that sensitive. I'll check and see what washer I have under the floats now. Thicker washer = lower float level?