timing jumpy with manifold vacuum hooked up.

[steveo3318]

I have read TONS of info on timing and finally have my 383 about dialed in where I want it. But I have a very simple question. Can the size of a cam make the vacuum jumpy and make timing jump a little at idle?

My timing is smooth as a button with manifold vacuum unhooked at idle. But hook it up and it goes up about 8 degrees "as it should" but it jumps a little almost with the lope of the cam when I hook up the vacuum. Is this jump ok? It might be a deg. or two max.

383 11.1 compression, 67 cc world heads, 308/296 510/510 roller cam. If any more info is needed please let me know. I just don't like seeing it jump a little on the light!!!

[geezer#99]

What's your initial timing?
Have you turned the idle down and adjusted the mix screws when you plug it into manifold port?
What rpm does it idle at with the vac plugged and plugged in?

[steveo3318]

initial timing is 12, 22 mechanical advance a little to close to 4000 rpms, "Waiting on some crane springs" going to get it all in by 2800 or 3000, and getting a total of 8 extra degrees out of the vacuum. all in I am running 42. when I had the vacuum advance stop plate at 14 degrees it drank gas and ran like a turd. I swear I am still not running enough timing though but I swear I can hear in spark knocking at cruising. I think I am going to start giving it more and more timing by 2 deg intervals until I really hear the detonation.

when I plugged in the vacuum I had to bring the idle down and reset idle mixture screws yes. It idles around 700 rpm. If I unplug it she drops to around 500 rpms.

[70SBC10BOB]

First. Your base timing is set with vacuum unplugged from the distributor.

Second. Vacuum to your disturber should be ported vacuum, not manifold.

Third. If the timing jumps around when you connect vacuum, your vacuum advance diaphragm has a hole in it.

Bob

[Lattimer]

Originally Posted by 70SBC10BOB

Second. Vacuum to your disturber should be ported vacuum, not manifold.

Disagree with this. Before emissions controls came to be, vacuum advance was on the manifold. I run mine on the manifold and the truck runs so much better its amazing.

[70SBC10BOB]

You are correct provided the cam etc. are set for that emission set up. Manifold vacuum will be advanced to the max at idle. From there you have to count on the centrifugal advance to take over.

The jumping timing is probably a hole in the diaphragm.

Bob

[70SBC10BOB]

Manifold vacuum may be low and causing some jumping because of valve overlap of the cam.

Check you cam specs or call the builder and ask what advance he recommends and distributor curve.

He may want ported vacuum, he may want manifold vacuum or he may suggest centrifugal advance only.

Bob

[steveo3318]

Base was set with vacuum unhooked. Distributor is brand new street fire and constant vacuum with my hand pump and gauge. No holes no leak down. My builder did say not to run vacuum advance just to run mechanical all in by 3k with base starting at 10 to 12 and see what she likes.

As far as ported or manifold vacuum goes I have always ran manifold and had great result with it vs ported.

[steveo3318]

I personally think it's the cam. I just wanted to make sure they can do this.

[geezer#99]

Or your timing chain has some slack in it.

[steveo3318]

No chain is brand new double roller. Motor has less than 3000 miles on it. Steady timing mark is steady if vacuum is not hooked up