1973 Campmobile - Vacuum Bouncing between 0" and 10&quo

[mylopolis]

My '73 Campmobile vacuum sucks (pun intended). It bounces between 0" and 10", as measured from different places. I even tried using different gauges. I've replaced every piece of hose I can find in the engine compartment, including the little L boots. Any help would be appreciated. Thanks in advance!





1 (Intake manifold side of central idle circuit): Bounces between 15 and 0
2 (Central idle circuit side of left carb): Bounces between 10 and 0
3 (Brake Booster line): Bounces between 10 and 0
4 (Central idle circuit side of right carb): Bounces between 10 and 0
5 (Intake manifold side of central idle circuit): Bounces between 15 and 0
6 (Vacuum Retard line from left carb): stable 10"
7 (Airbox T into central idle): Bounces between 10 and 0

I can only assume I'm losing 5" from the brake booster, but why does it bounce 0 to when testing directly connected to the manifold, on either side? Shouldn't that be stable?

I've got the EGR valves (D) disconnected and the right carb EGR hole capped. I've checked (visually) the EGR valves, and they move upon vacuum and seem to be closing. I know the EGR line is leaking, I'm missing a bolt stud to the exhaust, but with the EGR valves disconnected that shouldn't be a problem, right?

Any thoughts?

[RSorak 71Westy]

I would not suspect a vacuum leak at all but a poorly adjusted/non-sealing valve. Does this engine run well on all 4 cylinders?

[mylopolis]

I would not suspect a vacuum leak at all but a poorly adjusted/non-sealing valve. Does this engine run well on all 4 cylinders?

It didn't, previously. It wasn't getting fuel to the right carb due to a bad cutoff jet. That's fixed now, and it's running _okay_ on all 4.

I've adjusted the valves, but don't know how to test if they're non-sealing. What I find strange is that it's consistent across both sides.

[Amskeptic]

it's running _okay_ on all 4.
What I find strange is that it's consistent across both sides.

Central idling circuit dual carb engines have this HUGE vacuum leak called the . . . Central Idling Circuit.

Vacuum is created by the engine attempting to draw in more air than the throttle plates will allow. The central idling circuit has no such thing as a throttle plate, just calibrated tubes. They also utilize retarded timing which helps to lower the vacuum. 10" vacuum is about right for these engines.

The vibrating needle on the gauge is just reacting to individual intake pulses. The best "damped" reading is obtained from the thermostatic preheater valve (the hose I call the "reference hose").
ColinThisShoreIsFun

[mylopolis]

The vibrating needle on the gauge is just reacting to individual intake pulses. The best "damped" reading is obtained from the thermostatic preheater valve (the hose I call the "reference hose").
ColinThisShoreIsFun

Thanks Colin,

I measured that, what I called #7, and it also bounces rapidly between 0" and 10". Should I not be worried then? This is acceptable for my engine/carb setup?

[Amskeptic]

The vibrating needle on the gauge is just reacting to individual intake pulses. The best "damped" reading is obtained from the thermostatic preheater valve (the hose I call the "reference hose").
ColinThisShoreIsFun

Thanks Colin,

I measured that, what I called #7, and it also bounces rapidly between 0" and 10". Should I not be worried then? This is acceptable for my engine/carb setup?

We have some cause for concern then. Try removing each spark plug wire one at a time to see if any cylinder is giving you this back pressure. Suspects would include worn intake valve guides, poorly seating valves, some inexplicably-missed vacuum leak near an individual intake valve, a failing exhaust valve. . . the important thing here is to not go chasing willy-nilly after all items on the list, but to visualize what is happening here and then intuitively seeking the source of this air pressure pulse. A vauum gauge is reading the integrity of a low air pressre environment that relies on every element of the entire system. One stupid intake valve leak can introduce a hint of combustion pressure into the intake environment and that ticks off the vacuum gauge in a hurry.
Colin

[mylopolis]

We have some cause for concern then. Try removing each spark plug wire one at a time to see if any cylinder is giving you this back pressure. Suspects would include worn intake valve guides, poorly seating valves, some inexplicably-missed vacuum leak near an individual intake valve, a failing exhaust valve. . . the important thing here is to not go chasing willy-nilly after all items on the list, but to visualize what is happening here and then intuitively seeking the source of this air pressure pulse. A vauum gauge is reading the integrity of a low air pressre environment that relies on every element of the entire system. One stupid intake valve leak can introduce a hint of combustion pressure into the intake environment and that ticks off the vacuum gauge in a hurry.
Colin

Thanks for the suggestion. I'll try and give the plug wire removal process a go tonight and measure vacuum from the "reference hose".

[dingo]

http://www.secondchancegarage.com/public/186.cfm


scroll down to check out the different scenarios to refine your bouncing needle.

(actual numerical values wont relate to your pdsit contraption)

[mylopolis]

Well, whatever my issue was, it's gone away with my teardown! Got her running again (finally) last weekend and seeing a constant 15" vacuum :) Now off to follow the carb syncing process.