Vacuum source for dizzy

[WALTER]

Hi,
If I use the intake manifold with the PCV valve on it, where do I tie in for the vacuum advance. Or should I vent the timing cover through the 'Y' tube attached to the carbs and use the later manifold with all the holes in it. I'm so confused.

Thanks,
Walter

 

[58Custom]

Does your forward carb have a small hose nipple on it anywhere?

 

[WALTER]

The only hose nipples on my carbs are the two facing the center that the hoses with the 'Y' fitting attach too. I found one on some nasty old carbs I have, so I know what you are talking about.

W

 

[58Custom]

Don't call them nasty, old carbs have feeling too, y'know.

Well, my forward HS2 has a vacuum port which is placed so that it is souced at the engine side of the venturi, so that it would be the equivelent of "timed" vaccum on a fixed venturi carb. This port would not have vacuum at idle but would at cruise.

Manifold vacuum is OK if you have a well tuned engine. Ported vacuum is good and will not affect idle speed if the engine decides it does not want to idle well that day.

If all you have is manifold vacuum, use it. See how it works. On my Fords, the mild ones, I like to use manifold vacuum but I limit it to a total of 10 degrees above initial advance. I can do that because the vacuum servos have an adjustable stop. As for the vacuum servo in the MG (Lucas?) dizz, I dunno if it has that adjustment.

At cruise, ported and manifold vacuum works pretty much the same. High vacuum when at steady state, less when accellerating, the idea being that lean cruise mixtures need more ignition lead. Obvously, I do not know how the engines earlier than the MkIII midget were plumbed, so I am kinda talking outta my shorts here. But at least I am honest about that.

 

[WALTER]

Thank you Tom. And I did apologize to my nasty old carbs. We're good again.

Walter