Rough idle, low vacuum reading ?

[Tinker]

Originally Posted by tubman

It's not so much the lift of the cam that effects vacuum, but the duration and increased overlap.

Certainly so. Why having lower compression when running a supercharger is a thing. Also why you increase valve spring tension. I understand that duration could be problematic.


But why run that cam? and does it lower engine vacuum? There was a suggestion that it is normal for some cams to have lower vacuum readings and run well.

[jimvette59]

Hi Tinker I remember adjusting the carburetor just as you posted but also listening for the highest sound of it sucking air. That was in the 50s. Yes Longer Duration does lower vacuum.

[flatjack9]

With the larger overlap with most aftermarket cams, idle quality is not as good because of dilution of the air/fuel charge. That's why more advance is beneficial.

[40cpe]

Originally Posted by tubman

"40cpe" - Since you have a 276, can we assume that your cam is other than stock?

I don't see where Russ mentions what kind of cam he has. Let's make sure were not comparing apples and oranges.

Sorry I neglected to say, I do have an L100 cam. I only mentioned my readings because the original poster is running the L100 as he mentioned in post #11. I'll add that it runs a steady 17 vac at 50-60 mph.

[russcc]

Thank you all. Lot of good info there.
Seems like 550 RPM and 13-14 inches of vacuum is kinda low on both counts, is that a stock cam, and is 13-14 inches of vacuum normal ? Woops, Sorry I missed the 2nd post about the L100 cam in the 8BA
I was wondering if the crab distributor may have some limitations in my application.

[flatjack9]

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The crab distributor is not ideal for your application. I would set it up with about 8* initial advance, and limit the centrifugal advance to around 24* total.

[Tinker]

So I'm going to ask a stupid question. What is the advantage to overlapping duration? Certainly air/gasses must be released with more air introduced.