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Carb to distributor vacuum
bumpy - 12/4/15 at 09:28 AM

On my Pinto engine with Weber 32/36 carb I noticed, when doing the ignition timing, that there was no change to the timing position or engine revs whether the vac line on the distributor was connected or not.

I checked the vacuum pull at the carb by putting a wet finger over the opening of the pipe and it seemed OK (but perhaps not great). I also sucked by mouth on a tube connected to the distributor and could watch the vac advance working.

I can get a cheap vac gauge off ebay but have no idea what the vac reading should be at the carb - does anyone know?

Also any other suggestions as to what is going on?

Thanks

Is it normal that a good vacuum at 1000-1200 rpm could reduce to a poor vacuum at 800rpm. If not, where should I be looking?

[Edited on 12/4/15 by bumpy]

britishtrident - 12/4/15 at 12:40 PM

High (ie lowest pressure) vacuum occurs at low throttle openning with a light engine load. With the throttle fully open manifold vacuum is only slightly below ambient atmospheric pressure.
At idle with the throttle closed vacuum advance should be near maximum. With the vac advance connected snap the throttle open at idle and then ignition advance should initially fall back then advance as the centrifugal advance takes over.

Maximum vac occurs on the over run under engine braking.

bumpy - 12/4/15 at 01:42 PM

quote:
Originally posted by britishtrident
High (ie lowest pressure) vacuum occurs at low throttle openning with a light engine load. With the throttle fully open manifold vacuum is only slightly below ambient atmospheric pressure.
At idle with the throttle closed vacuum advance should be near maximum. With the vac advance connected snap the throttle open at idle and then ignition advance should initially fall back then advance as the centrifugal advance takes over.

Maximum vac occurs on the over run under engine braking.


Is it normal to have a good vacuum at say 1000 to 1200 rpm which can work the dist advance and then drops off to a poor vacuum at 800 rpm which cant work the dist advance?

If not, where should I bee looking

r1_pete - 13/4/15 at 08:03 AM

quote:
Originally posted by bumpy
quote:
Originally posted by britishtrident
High (ie lowest pressure) vacuum occurs at low throttle openning with a light engine load. With the throttle fully open manifold vacuum is only slightly below ambient atmospheric pressure.
At idle with the throttle closed vacuum advance should be near maximum. With the vac advance connected snap the throttle open at idle and then ignition advance should initially fall back then advance as the centrifugal advance takes over.

Maximum vac occurs on the over run under engine braking.


Is it normal to have a good vacuum at say 1000 to 1200 rpm which can work the dist advance and then drops off to a poor vacuum at 800 rpm which cant work the dist advance?

If not, where should I bee looking


It depends on the carb and the original design, some installations the vacuum port on the carb is blanked by the throttle butterfly at tickover, then as soon as you open it a little, you get max vacuum.

bumpy - 13/4/15 at 11:39 AM

quote:
Originally posted by r1_pete
quote:
Originally posted by bumpy
quote:
Originally posted by britishtrident
High (ie lowest pressure) vacuum occurs at low throttle openning with a light engine load. With the throttle fully open manifold vacuum is only slightly below ambient atmospheric pressure.
At idle with the throttle closed vacuum advance should be near maximum. With the vac advance connected snap the throttle open at idle and then ignition advance should initially fall back then advance as the centrifugal advance takes over.

Maximum vac occurs on the over run under engine braking.


Is it normal to have a good vacuum at say 1000 to 1200 rpm which can work the dist advance and then drops off to a poor vacuum at 800 rpm which cant work the dist advance?

If not, where should I bee looking


It depends on the carb and the original design, some installations the vacuum port on the carb is blanked by the throttle butterfly at tickover, then as soon as you open it a little, you get max vacuum.


Thanks Pete, that's exactly what we've discovered by having a look at the 32/36 off the car.

The vacuum take off for the distributor is close to, but very slightly above the throttle butterfly. At 800 rpm the vac hole is not exposed to the full vacuum that exists below the throttle plate.

As the butterfly opens a bit at 1000+rpm the hole is exposed and the effective vacuum increases