Total mechanical advance question

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Jul 27, 2019 | 12:26 PM

#21

BBCorv70

Thread Starter

Melting Slicks

Joined: Apr 2007

Posts: 3,189

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From: Tolland CT

I installed new springs as recommended, all in now by 2500 RPM.
The mechanical advance appears to have a range of 24 degrees.
Once fully warmed up,engine detonates with full advance at 38 degrees.
Had to back down to 36 degrees.
When I set my max advance to 36 degrees, I end up with 12 degrees at idle.

Ideally I'd like to idle at 18 degrees but would need to have the advance slot modified to achieve this.

Other than smoothing the idle a bit more, what would I gain between 1500 and 2500 RPM if I were to have the distributor modified?

Any noticeable increase in torque?

Car has 3.70 gears.

I can live with the slightly lumpy idle, wondering if it's worth modifying the distributor to limit the advance.
This is a weekend driver.

Jul 27, 2019 | 12:45 PM

#22

derekderek

Race Director

Joined: Jul 2015

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From: SW Florida.

if you are all in by 2500, check timing at 1500 with and without vacuum. you may be just fine.

Sep 7, 2019 | 09:31 PM

#23

leigh1322

Old Pro Solo Guy

Joined: Dec 2017

Posts: 7,966

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From: Marlton NJ

You could limit the vac advance to 10 degrees, via a stop, and then make sure it uses manifold vac, so it will kick in at idle. Then you may be fine with 10 or 12 initial, it'll be 20 with the vac.

Feb 26, 2020 | 05:58 PM

#24

jvmagic

Instructor

Joined: Jan 2015

Posts: 101

Likes: 3

From: CA; Bay Area

Quote:

Originally Posted by MelWff

You need to buy the spring kit that Lars recommends, Mr Gasker 928G. It comes with springs and a brass stop bushing. You will have to remove and disassemble the distributor to install the stop bushing. The original bushing was nylon and is either gone or on the verge of falling off. Using the original weights install the silver and black springs and then you can set your total timing to 38 degrees using a timing tape or dial back light.

If I follow this correctly, I need to install the stated springs/bushing and set my timing to 38 at say 3k RPM then at idle, set the timing to a certain reading or leave alone when I get 36 - 38 at 3k RPM?
Thanks in advance.

Feb 26, 2020 | 08:26 PM

#25

MelWff

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Joined: May 2006

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Quote:

Originally Posted by jvmagic

once you set the timing at 3k, you check what the idle timing is but do not reset it.

Feb 27, 2020 | 10:25 AM

#26

lars

Tech Contributor

Joined: Aug 1999

Posts: 14,356

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From: At my Bar drinking and wrenching in Lafayette Colorado

...unless the resulting timing at idle is way out of the desired/acceptable range, in which case the distributor itself needs to be reworked to produce an advance curve of the desired length.

Lars

Apr 21, 2021 | 04:59 PM

#27

Cliff R

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Joined: Jun 2010

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"You built the QJet for my '72 LeMans and I got the parts from you when I rebuilt the QJet on my '71 Corvette. Thanks for making my cars run so well."

You are most welcome. It's nice to now be officially retired, but I'm not out of the game, just slowing down some.

I've built the parts business to a point where I don't need to work on any carburetors so keeping that for a while. Still doing a few carburetors and custom tuning couple of Saturdays a month for the beer fund!.........

Apr 21, 2021 | 05:40 PM

#28

MelWff

Race Director

Joined: May 2006

Posts: 18,620

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Quote:

Originally Posted by Cliff R

is that Cliff R as in Ruggles? Have your Quadrajet book, excellent. Not sure though who you quoted at the beginning of your post because I don't see any LeMans mentioned in this thread.