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Vacuum system and what does it do?

AUSMHLY

Obi Wan
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64 BJ8 vacuum system.
What is it's purpose and how do I test it to make sure it is working?

1. The copper vacuum pipe that connects the vacuum unit on the distributor and the vacuum advance adaptor on the rear carburetor is clean. I shot some carburetor spray through it to make sure it's not clogged.

2. The vacuum advance adaptor on the carburetor is clean also. (The Carbs were rebuilt by Joe Curto a couple years ago.)

3. The rubber hose that connects the adaptor to the vacuum pipe is clean and makes an air tight connection.

4. I don't know if the vacuum unit is working or not. How do I test that?

With the car warmed up, it idles around 850. However during driving, when sitting at an intersection, it may drop down to 550-600. Then later be at 800-850. It's not consistent.

I've checked for vacuum leaks by spraying carb cleaner on all the hoses and their connections, the inlet manifold gasket area, the check valve fitting on the manifold and the brake booster. I've sprayed all areas of the throttle shaft too. None of those area seem to have a problem, for the idle does not change, does not drop.

Why does this happen...

1. I disconnected the vacuum pipe from the vacuum advance adaptor and it made no difference in the idle. I put my finger over the pipe on the vacuum advance adaptor and it made no difference in the idle.

2. I disconnected the vacuum pipe from the vacuum unit on the distributor and it made no difference. I put my finger over the opening on the vacuum unit and it made no difference.

I'm under the impression that if a car has an uneven idle, hunts or an idle changes as the engine's temperature climbs or drops it maybe because of a vacuum leak.

Cheers,
Roger
 
5

57_BN4

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Hi Roger,

Vac is only present on the distributor hose at light throttle therefore the system has no effect at idle. The port where it goes into the carb is upstream of the closed throttle but when the throttle is opened a little then the port becomes exposed to vacuum.

Take off the dissy cap and suck on the vac line, you should see the advance plate move and stay moved until you release the vacuum. If it doesn't move or moves then returns to the rest position your vac canister on the distributor is leaking. That won't affect your idle though unless the mechanism is sticking somehow.

I got a load of water in the fuel last week and it played silly buggers with the idle. I mopped out the float bowls and pumped the tank out a couple of times into a jerry can to clear out the water.

Andy.
 

Patrick67BJ8

Darth Vader
Silver
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64 BJ8 vacuum system.
What is it's purpose and how do I test it to make sure it is working?

1. The copper vacuum pipe that connects the vacuum unit on the distributor and the vacuum advance adaptor on the rear carburetor is clean. I shot some carburetor spray through it to make sure it's not clogged.

2. The vacuum advance adaptor on the carburetor is clean also. (The Carbs were rebuilt by Joe Curto a couple years ago.)

3. The rubber hose that connects the adaptor to the vacuum pipe is clean and makes an air tight connection.

4. I don't know if the vacuum unit is working or not. How do I test that?

With the car warmed up, it idles around 850. However during driving, when sitting at an intersection, it may drop down to 550-600. Then later be at 800-850. It's not consistent.

I've checked for vacuum leaks by spraying carb cleaner on all the hoses and their connections, the inlet manifold gasket area, the check valve fitting on the manifold and the brake booster. I've sprayed all areas of the throttle shaft too. None of those area seem to have a problem, for the idle does not change, does not drop.

Why does this happen...

1. I disconnected the vacuum pipe from the vacuum advance adaptor and it made no difference in the idle. I put my finger over the pipe on the vacuum advance adaptor and it made no difference in the idle.

2. I disconnected the vacuum pipe from the vacuum unit on the distributor and it made no difference. I put my finger over the opening on the vacuum unit and it made no difference.

I'm under the impression that if a car has an uneven idle, hunts or an idle changes as the engine's temperature climbs or drops it maybe because of a vacuum leak.

Cheers,
Roger
I check mine by setting a stationary Fast Idle by placing a screwdriver at the carb linkage area and setting the engine to run about 2500 rpm(the screwdriver should hold it in place), then with a Strobe Timing Light I look at the timing marks while disconnecting the small rubber hose at the rear carb. It should drop from its approximate 35 degrees which can be seen on the harmonic balancer and then with engine still running reconnect the hose and you should see the timing jump backup to approx 35 degrees.
 

Bob Hughes

Luke Skywalker
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I got a load of water in the fuel last week and it played silly buggers with the idle. I mopped out the float bowls and pumped the tank out a couple of times into a jerry can to clear out the water.

I have heard of trying to eek out the fuel a bit but these down under people take the biscuit. :highly_amused:

:cheers:

Bob
 
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