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Vacuum is produced when the pistons move down and attempt to draw air into the cylinders. The amount of vacuum created varies based on the position of the throttle butterflies in the throttle body (or carb.) When the throttle is in WOT the butterflies are standing straight up and there is almost no resistance for air to enter the intake, hence low vacuum. When the butterflies are closed, such as at idle or when decelerating, there is a lot of resistance for air to enter the intake, hence high vacuum. So as the butterflies move the vacuum rises or lowers along with them which is why there is no "normal" vacuum reading when driving.Originally Posted by Dojo
Thank you! Maybe I should hunt for a vacuum leak after this trip - when on WOT or close to it the vacuum needle tends to jump a lot. Is that normal?
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@Eunospeed 13.5 mpg on highway is also the best we ever got with this van. We already run synthetics in the engine (Mobil 1 5/10W30, differential, transmission (ATF +4) and bearings.
Also the ingnition coil, spark plugs, distributor cap, rotor and wires are new but with little to no impact on mileage as far as I've noticed.
Maybe we can try "side-gapping" the spark plugs https://www.instructables.com/id/Save-Gas-And-Incerase-Horsepower-By-Side-Gapping-S/
Had anyone tried this on a B-Van Magnum engine, is there any real benefit to it except shorter-life plugs?
Side gapping your plugs isn't going to help you. It might, initially, but, the spark will be concentrated on one side of the electrode, leading to premature wear, and expanding gap. Fine for racing, where they change the plugs after every race anyway, not so much for a vehicle you leave them in there for 30,000 miles or so.Originally Posted by Dojo
Thank you! Maybe I should hunt for a vacuum leak after this trip - when on WOT or close to it the vacuum needle tends to jump a lot. Is that normal?@Eunospeed 13.5 mpg on highway is also the best we ever got with this van. We already run synthetics in the engine (Mobil 1 5/10W30, differential, transmission (ATF +4) and bearings.
Also the ingnition coil, spark plugs, distributor cap, rotor and wires are new but with little to no impact on mileage as far as I've noticed.
Maybe we can try "side-gapping" the spark plugs https://www.instructables.com/id/Save-Gas-And-Incerase-Horsepower-By-Side-Gapping-S/
Had anyone tried this on a B-Van Magnum engine, is there any real benefit to it except shorter-life plugs?
@alloro The vacuum needle at open throttle doesn't follow the movement of the pedal/intake butterflies but jumps/oscilates very fast between 3 and 7 on the vacuum gauge. I wonder if that's normal since the engine throttle certainly doesn't move that fast?
@HeyYou That's exactly my thinking regarding side-gapping but at ~1$/plug I would try just to see if there's any real benefit from it. Besides, I understood that's not such a good idea to leave cooper plugs for that long in these engines... Problem is with "DYI" side-gapping. I would have to resort to a professional for such an operation since there are quite some things that can go wrong besides hurting my fingers
@HeyYou That's exactly my thinking regarding side-gapping but at ~1$/plug I would try just to see if there's any real benefit from it. Besides, I understood that's not such a good idea to leave cooper plugs for that long in these engines... Problem is with "DYI" side-gapping. I would have to resort to a professional for such an operation since there are quite some things that can go wrong besides hurting my fingers
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That's due to a burnt or sticking valve or a weak valve spring. It is telling you that for at least one cylinder the valve isn't closing fast enough to seal or just isn't sealing altogether.Originally Posted by Dojo
The vacuum needle at open throttle doesn't follow the movement of the pedal/intake butterflies but jumps/oscilates very fast between 3 and 7 on the vacuum gauge.
This is how the vacuum gauge looks at cruising speed, the only time it stops wiggling is when we're going downhill or I take the foot off throttle:
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You have a valve that's not sealing or closing, or although less likely...a cracked ring or scored cylinder. Your next step is to do a leak-down test on all of the cylinders.Originally Posted by Dojo
This is how the vacuum gauge looks at cruising speed, the only time it stops wiggling is when we're going downhill or I take the foot off throttle: