Ported vs Manifold Vacuum Advance

[SkinnedKnuckles]

First off, I would like to acknowledge the wisdom of Randall & Poolboy and the "encouragement" that got me off my duff to find the ported vacuum tap on one of my carbs. I remember now it was there, but when I went to the triple set-up the vacuum retard was still in play, and besides I hadn't heard of ported vacuum anyway.

So this morning I rediscovered the tap and hooked it up. My impressions after a warm-up and idle adjustment:

• There is more power available when starting off in 1st gear which will help prevent stumbling. Makes sense because now the timing is increased on take-off rather than decreased. That's also why according to a single reference ported vacuum was adopted in the 1930's.

• Warm starts seem easier. Don't know about cold starts - only had 1.

• RPM's drop like a rock when shifting if you totally let up on the accelerator. Maybe I shift too slow when cruising. Not a biggie, just takes some getting used to.

• Idle stability - no apparent change.

• Cruising performance - no change.

• HC emissions - decreased :wink:

So there you have it. I was pleased with running vacuum advance off the manifold (retard) port for years, but am more pleased now. Thanks, guys.

 

[poolboy]

Forums are great. Tell you what, when I removed the red fan in favor of an electric fan, my rpms between gear changes stayed up noticeably higher. I think the air resistance on that red fan tends to put a load on the engine that's especially noticeabe..when it's gone..

 

[TR3TR6]

Brent:

You mention triple carbs. Are those webers or something else?

 

[SkinnedKnuckles]

No - triple Strombergs. The Good Parts kit.

 

[SkinnedKnuckles]

Yes - a bump. Ran an experiment to determine the vacuum in the Z-S carb mix chamber. That's the space between the piston and the throttle butterfly valve that the carbs try to maintain at a constant value. These are CD carbs & I've been curious what the actual constant depression was. Asked a couple of times, but got no response. It's 3" of water column vacuum. Not much. That's measured from the ported vacuum tap in off position. The vacuum could be slightly higher at the jet, maybe 4", but probably not given the way the piston holes are drilled & milled. Just some geek info.

 

[poolboy]

As one geek to another, I've measured the vacuum at the "valve cover evacuation ports" on the right side of the carbs. It's roughly 1/10 of the manifold vacuum, but enough to relieve the build up of crankcase pressure.