Q&A re Vacuum Distributors

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Question -

Looks like I'll be installing a new distributor soon. I suppose the timing and all will be different with a vacuum advance distributor--

Rob's response -

Easy - if you are using a single vacuum distributor - static timing is 7.5BTDC. If you are using the double vacuum distributor, 5ATDC (that AFTER), set with a timing light and the engine idling. You can't set this distributor engine-off, as you need the idle (front) vacuum line working to pull the distributor plate back to the idle setting.

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Question -

Something I've never gotten straight in my head is the requirement to plug the vacuum line(s) during timing. I take it that one of the two (if you're using the double vacuum distributor) must remain attached. Is the other one plugged? And if so, which way (i.e., is the distributor port plugged or the carburetor port plugged?) It seems to me that the 34 PICT/3 carby was designed for use with a double vacuum distributor, since it has two vacuum ports (I'm still baffled by the port down on the intake manifold).

Rob's Response -

Yes that port on the manifold has me foxed too.

(We learned later that the vacuum port on the intake manifold is for a line going to the air cleaner.)

As I understand it, with the double vacuum carby hooked up, you have both vacuums still on and set the idle at 5 ATDC with a timing light and the engine running. With the single vacuum distributor (or with the double vacuum which has the idle retard circuit permanently disabled/plugged), you set the timing with the engine off static to 7.5BTDC. I suppose you COULD plug the retard line and set the double vacuum distributor to about 7.5BTDC (static) then reconnect the retard vacuum line, but that's not how I've seen it written. Does Muir or something else talk about temporarily disconnecting a vacuum line just for the timing?

(Muir is unclear. The point is to plug the carburetor vacuum port to prevent air being sucked into the carburetor, creating a too-lean fuel/air mixture and affecting the idle speed.)

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Question -

I'm confused. The specs say that the timing for the double advance distributor should be set at 5 degrees ATDC at idle (850 900rpm) with both vacuum hoses attached. Our engine absolutely refuses to run at this setting and demands 7.5 degrees BTDC, which is 12.5 degrees advanced beyond the specification. I don't understand.

Rob's Response -

You are dealing now with both vacuum and centrifugal effects on the distributor. At 1200rpm the idle circuit of the carby would be shutting down and the low speed ciruit starting to operate, so the IDLE vacuum (the retard vacuum line) is probably inoperative at 1200rpm, and setting the 5ADTC would not be possible, as this retard vacuum circuit is designed to work with the throttle shut idling at 900rpm.

And at 1200rpm, the centrifugal advance would be just starting to work, so you can't set it with the engine running at 1200 for 7.5 either it probably needs to be higher than that. You could try either...

Setting is at 7.5BTDC with the engine off (when both retard vacuum and centrifugal advance are inoperative). It's not supposed to be set this way, but you could then check the max advance, and if it is around 28 30, it HAS to be somewhere near right for the idle.

Setting the high revs at 28 30, and hoping that the idle will come in about right. Until you get the car to idle at about 900 (and neither of us is sure how to accomplish THAT yet), you are going to find it difficult to set the timing the 'normal' way for that distibutor/carby set up.

Question continued -

I can tell you this: It doesn't come anywhere near the 30 degree mark on the pulley when I rev it up -- I would say on the order of 25 26BTDC. But if I advance it such that it approaches 30 degrees BTDC at 3000 rpm that's going to advance the timing at idle even more. I'm missing something here!

Rob responded -

As I said above, you're 'missing' the fact that the centrifugal advance is starting to advance the timing at 1200rpm, so the 'idle' timing at 1200rpm SHOULD be higher than 7.5 . Hope that makes sense. The 009 is set STATIC, and you're now setting the vacuum distributor DYNAMIC, with two variables vacuum and centrifugal advance.

Question -

I guess I'm going to need some tutoring here, too. How can you tell that the retard circuit is not functioning properly?

Rob's Response -

You probably won't be able to test this until you are able to get the idle at about 900rpm. Then with the timing light connected you should see it jump from 5ATDC to 7 8BTDC as you crack the throttle open and the retard vacuum line 'disengages'.

(We later learned that this system was suffering from air inleakage around the throttle shaft in the carburetor, making it necessary to set the idle high just to keep the car running, and rendering it impossible to correctly set the timing.)

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Last revised 6 May 2004.