Idle vacuum reading. - Chevelle Tech
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post #1 of 38 (permalink) Old Jul 6th, 20, 10:28 PM Thread Starter
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Sam
 
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Idle vacuum reading.

I have another thread going about my idle issues but it's getting long. I've been told to get a new distributor as the one I have is an Ebay special. I've got about 1000 miles on the distributor before I rebuilt the engine and it seemed fine. I don't doubt a new distributor would help but honestly I went way over my budget rebuilding the engine. It would hurt to spend $200 on a new distributor and it not fix the problem. Here's a video of my idle vacuum in park without the vacuum advance connected. The timing is at 20 degrees and its idling at about 800 rpms. If you guys think the distributor can cause the erratic vacuum idle then I'll buy a new one.
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post #2 of 38 (permalink) Old Jul 6th, 20, 10:31 PM Thread Starter
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post #3 of 38 (permalink) Old Jul 6th, 20, 10:47 PM
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Re: Idle vacuum reading.

Doubt that's the distributor. Have you adjusted the carb?

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post #4 of 38 (permalink) Old Jul 6th, 20, 10:58 PM Thread Starter
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Quote:
Originally Posted by oldcutlass View Post
Doubt that's the distributor. Have you adjusted the carb?
I have several times. The engine runs strong once I get past idle.
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post #5 of 38 (permalink) Old Jul 6th, 20, 11:08 PM Thread Starter
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I did notice when adjusting the carb using the vacuum gauge the mixture screws don't seem to have much effect on the vacuum. They will kill the engine when turned in to far though.
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post #6 of 38 (permalink) Old Jul 7th, 20, 4:57 AM
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Re: Idle vacuum reading.

Did you try to turn the-distributor slightly in both directions and see if the vacuum increases. With any type of high lift cam this is where I start my timing procedure. Turn the distributor until I get the highest vacuum reading and lock it down. I’ve seen a lot of those eBay distributors fail.
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post #7 of 38 (permalink) Old Jul 7th, 20, 5:36 AM
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Re: Idle vacuum reading.

I'll take a stab.

You have a large(r) cam - larger than the one that is in my 502. Your vac gauge will not be stable with a larger cam which is typical. If you have a 22* mechanical special distributor, you could have as little as 8* of base - no good. Try 30* and see if you have an improvement. I would disconnect vac advance for this test to make sure its not coming in/out at idle, and make sure the mechanical advance is stable at idle. 800 idle speed is probably also too low imo.

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post #8 of 38 (permalink) Old Jul 7th, 20, 7:38 PM
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Re: Idle vacuum reading.

Yet another Top Fuel/Funny Car tune for the street.

And, 20 degrees initial, with vacuum advance disconnected, WAY OFF THE RIGHT WAY TO DO IT.

First off, I will assume the distributor is a large coil in cap HEI.

Did you install an aftermarket "Curve Kit" with different weights and center, and a set of softer springs? If so, what were the numbers on the weights and center chat came with the distributor that were removed?

And, way, way too much initial, 20 is too high, we make up the IDLE timing by correctly using the vacuum advance timing on full manifold vacuum. Stuff like 12/14 initial, with 12/10 vacuum degrees, for 24 degrees IDLE total.

If you have my vacuum advance modifications package, it is all in there.

And, to a certain point the more initial/idle vacuum, TO A CERTAIN POINT, increases idle and off idle vacuum, TO A POINT.


Idle rpm's also contribute to vacuum levels, too high, gives a too low vacuum levcl, correct makes good idle vacuum levels. Vacuum levels will also rise when the throttle is raised off idle slowly as well.
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post #9 of 38 (permalink) Old Jul 7th, 20, 7:43 PM Thread Starter
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I ran the idle timing to 30 degrees without vacuum advance it made an improvement. I added vacuum to the can bringing the idle timing to 36 degrees. Definitely an improvement brought the idle up to a 1000 rpms and gave me 11" of vacuum gauge still is fluctuating not as much though. The idle is smoother It will now idle in gear on it's own without dying. The timing does bounce with the vacuum advance connected. I need to get the vacuum advance working right. The tach is isn't steady not moving much maybe I have a slight miss? I also noticed the air fuel gauge is steadier.
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post #10 of 38 (permalink) Old Jul 7th, 20, 8:44 PM Thread Starter
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If you have my vacuum advance modifications package, it is all in there

I do not have your modifications package how would I be able to obtain it? I thought I was getting somewhere with more intital guess it's back to the drawing board. I haven't changed the any of the springs or weights in the distributor. I'm looking at getting another distributor and plan on getting a crane vacuum advance kit.
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post #11 of 38 (permalink) Old Jul 7th, 20, 8:49 PM Thread Starter
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Is the accel hei distributor any good?
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post #12 of 38 (permalink) Old Jul 7th, 20, 9:13 PM
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Re: Idle vacuum reading.

Mr. Ray obviously does not understand timing requirements for performance sbc/bbc's. These older engines with larger cams NEED more timing despite what he says and everyone here knows it except him. Sam just confirmed it with his quick test. I'm sure Mr. Ray will say its all in Sam's head and the improvements are not possible...yeah yeah yeah


Sam, what I would do is run say 18 or more initial (hopefully your starter can handle), and see where your total ends up. Will probably want 34-36*.

Then plug the additional amount. I would just make sure your timing is relatively steady at idle and not moving around so much indicating too weak of springs or a vac can that's dithering in/out.

Again, you have a large(r) cam and a smaller engine - you are going to have a rougher idle and the vac guage will move a bit. But 11" of vac tells me your tune is pretty dam good already. After you get the timing nailed down you should go over the mixture screws a couple of times to perfect them.

PS - all the center plates and weights talk is also overrated - run whatcha got.

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post #13 of 38 (permalink) Old Jul 7th, 20, 9:33 PM Thread Starter
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I just went out and brought the intial timing down to 14 degrees it's not happy AT all. I had to turn the idle screw way to far to get it to idle and in gear its wants to die again. The vacuum also dropped to 10". It sure seems like 36 degrees at idle is what it wants.
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post #14 of 38 (permalink) Old Jul 7th, 20, 9:40 PM
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Re: Idle vacuum reading.

Quote:
Yet another Top Fuel/Funny Car tune for the street.
Those dang young whipper-snappers with their fancy racy cars speeding around!

Quote:
Mr. Ray obviously does not understand timing requirements for performance sbc/bbc's.
From what I've read, I think most just ignore it. While I'm sure we're all (car enthusiast) grateful for his contributions to help develop the HEI, "Overall" performance parts have moved past the 60's and 70's. I work with Engineers all day listening to "It cant do that / it wasn't designed to that" and the reply is, well, it just did.

Completely agree with Vince. I and MANY others run locked out timing, vacuum can capped off with carb. My idle info is 800rpm, gets 14/15" vac at the manifold and sits at 180* degree engine temp in Florida summer every day around town. Driving temps at 165*. Not one hesitation or stumble in Idle or WOT. So what I don't get 20MPG.

Yes I have a high TQ starter.

Mike
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post #15 of 38 (permalink) Old Jul 8th, 20, 1:15 PM
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Re: Idle vacuum reading.

69, what a crock of idoicy in your diatribe. I completely understand timing for all typoes of conditions and engine parameters, obviously, you have very little, to NO clue.

Early 11.00:1 Corvette and Chevy 283 used a dual point distributor with no vacuum advance, solid lifer cam with very late inlet valve closing, which made low idle vacuum, too low for vacuum advance use for the advances for the times (NO adjustable vacuum advances at that time). So, with the inlet reversion and other low vacuum factors, timing was set to 18 in/hg. A better situation is now available, conservative initial, and adjustable vacuum advance that adds timing to a reasonable 22 to 24 deg/BTDC initial timing for the application and the low vacuum.

"I ran the idle timing to 30 degrees without vacuum advance it made an improvement. I added vacuum to the can bringing the idle timing to 36 degrees. Definitely an improvement brought the idle up to a 1000 rpms and gave me 11" of vacuum gauge still is fluctuating not as much though. The idle is smoother It will now idle in gear on it's own without dying. The timing does bounce with the vacuum advance connected. I need to get the vacuum advance working right. The tach is isn't steady not moving much maybe I have a slight miss? I also noticed the air fuel gauge is steadier."

It is refreshing that you completely avoided reading what I posted right above this post, it shows I outlined what would happen, and why it would occur, but you did it anyway, proving I am not wrong, and NOT STUPID.

Also, if you can't figure out where the email address for the vacuum and other timing package is, you would be blind, because it has been posted in numerous places here, and other websites. But, to help the chronically sight impaired, here it is...AGAIN.

[email protected]

Man, this is getting extremely old, if you don't want to do it right, just stay quiet and go make a mess out of it, or, listen to someone that shows no knowledge of it, 69, and pals.

BTW, 69, any idea why a Top Fuel/Funny Car nitro engine runs .018 to .020 plug gaps with a 60 ampere magneto pair, AND 60 to 65 degrees locked out timing? That much spark energy is lethal if a person is shocked by one. Fuel teams are required to have ONE team member that is CPR certified to revive a person shocked by one of those systems. The key here is, why the 60 plus degrees of timing. I didn't think you knew.
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