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Old 05-15-2006, 01:28 AM
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Timing/idle chevy-350???????

What should the timing be set at for a mild chevy 350 (mild cam, etc). Also, what should it idle at. Its a 3-speed manual tranny.

I'm assuming its a stock cam, don't know the compression yet. Thanks.
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Old 05-15-2006, 03:42 AM
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10-12 with the vacume advance disconnected
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Old 05-15-2006, 04:16 PM
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What year 350?

If it is an older style 350 with old style heads then set the total timing 38 to 40 degrees (without vacuum advance connected). Then let the initial timing fall where it may. Much more important to get the total timing right.

Ideal initial timing for a stock to mild cam is 10 to 12 (without vacuum advance connected). You may need to re-curve the advance mechanism to get both the 10 to 12 initial and 38 to 40 total.

You will need a timing light with the adjustment knob or a balancer with timing tape to see 40 degrees.

Vortec heads or other aftermarket Fast Burn heads require less total timing because of the fast burn chambers (makes sense). 34 to 35 degrees total is all the newer heads need.
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Old 05-15-2006, 04:29 PM
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Quote:
Originally Posted by 454C10
What year 350?

If it is an older style 350 with old style heads then set the total timing 38 to 40 degrees (without vacuum advance connected). Then let the initial timing fall where it may. Much more important to get the total timing right.

Ideal initial timing for a stock to mild cam is 10 to 12 (without vacuum advance connected). You may need to re-curve the advance mechanism to get both the 10 to 12 initial and 38 to 40 total.

You will need a timing light with the adjustment knob or a balancer with timing tape to see 40 degrees.

Vortec heads or other aftermarket Fast Burn heads require less total timing because of the fast burn chambers (makes sense). 34 to 35 degrees total is all the newer heads need.
If you are going to power time the vehicle to get total advance, shouldn't you leave the vacuum advance connected and rev the engine up to where it is fully advanced to time it? Just curious.
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Old 05-15-2006, 04:47 PM
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At WOT the vacuum drops is just about zero which will make the vacuum advance not work. But since the total timing is set in neutral and there isn't much load on the engine, then high rpms can be obtained with part throttle, so the vacuum advance will add to the total timing quite a bit.

The vacuum advance on the ported carb source works during part throttle only.

Manifold advance works at idle and reduces as the throttle is opened.

In either case, vacuum advance goes away during wide open throttle (wot).
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Old 05-15-2006, 04:59 PM
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Quote:
Originally Posted by 454C10
At WOT the vacuum drops is just about zero which will make the vacuum advance not work. But since the total timing is set in neutral and there isn't much load on the engine, then high rpms can be obtained with part throttle, so the vacuum advance will add to the total timing quite a bit.

The vacuum advance on the ported carb source works during part throttle only.

Manifold advance works at idle and reduces as the throttle is opened.

In either case, vacuum advance goes away during wide open throttle (wot).
OK...it's been awhile since I did any hands-on stuff like this but now that I think of it, none of the distributors that I used had a vacuum advance on them. Which would explain why I didn't remember unhooking it to power time the engine. *lol*
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Old 05-15-2006, 08:05 PM
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Thanks for all the replies. The engine was manufactered in 1970. The timing panel on the block is kind of confusing (i wish it was easier to read and had numbers):

A------0-------R
-|||||||||||||||


Its a points distrib (im planning on getting a HEI soon)

We timed it without the vaccum until it idled smooth, but it still seems like its idles a bit rough and the throttle response could be better. I'll try the preceeding suggestions and let everyone know. Thanks!
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Old 05-16-2006, 09:09 AM
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Well, "0" is top dead center and "A" is advance and "R" is retard. I am not sure how many degrees there are between each of your marks but an easy way to tell is to measure the circumference of the damper and divide by 360. If you have a 6.75" diameter damper each degree is going to measure about .060" so if your marks are spaced apart by .060" then each mark is one degree.
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Old 05-17-2006, 09:35 AM
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Ok, last night we replaced the old distributor with a mallory HEI and also replaced the plugs. With the vacuum advance plugged, we set the idle to exactly 800 and timed it at 12. It was idling perfectly. Then we reconnected the vacuum advance and it ran pretty poorly, the timing was fluxuating pretty bad. I'm not sure what we did wrong.

The new distrib was put in the same position as the old one, and I'm almost positive the firing order is correct (no backfiring whatsoever).

Any ideas? Thanks.
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Old 05-17-2006, 09:48 AM
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You need to plug the vacuum advance into the ported source on the carb. The ported source will have NO vacuum at idle.

The ported source gets the vacuum from above the throttle blades. Manifold vacuum is below the throttle blades.

Once the throttle is openned, the ported vacuum source will be getting vacuum from under the throttle blades (manifold vacuum).

Watch the timing mark with the timing light as you plug the vacuum advance into the ported source. It shouldn't move the timing mark. It it does then the throttle blades are open too much and your carb requires a few small holes drilled into the front throttle blades. But first make sure that you are on the right port (no vacuum at idle) before making any mods to the carb.

The carb will have both ported and manifold vacuum ports. You need to use the right one.
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