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Idle concerns with center mounted 2 bbl
Posted by: Colorsarge ()
Date: July 31, 2022 06:30AM

Working on a 140 with Holley 2bbl carb.it has the usual flat spot, hesitation issues, but the big problem is the idle. You can adjust it to stay running and it idles too fast. Try to lower it and it goes too low and stalls.

Innards of this engine are unknown, but, has a hot cam. These are the things I've checked:
Compression
Float level
Vacuum leaks
Has correct distributor, initial timing 18 degrees
Vacuum advance hooked up and working

Is this idle issue another inherent problem or is there something I'm missing. By the way, when I got the car from the previous owner, the vacuum advance was disconnected.
Thanks for your kind attention.

Frankford, De., eight miles north of Ocean City, Md.
1966 Corsa convt.

Re: Idle concerns with center mounted 2 bbl
Posted by: cnicol ()
Date: July 31, 2022 07:22AM

Low runner velocity and cool runners cause fuel to drop out, causing lean idle mixture I'd suggest bumping the timing up a couple of degrees. It's a common problem with center 2&4 bulb conversions. Adding intake runner heat is another cure.

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
65 Canadian Monza 4dr 110/PG 40k
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts

Re: Idle concerns with center mounted 2 bbl
Posted by: Wagon Master ()
Date: July 31, 2022 07:25AM

What is your manifold vacuum reading at idle?
What is the number stamped into the power valve?

Re: Idle concerns with center mounted 2 bbl
Posted by: Colorsarge ()
Date: July 31, 2022 07:42AM

Wagon master. Won't be near car again until Tuesday. I'll try to collect that info.

Frankford, De., eight miles north of Ocean City, Md.
1966 Corsa convt.

Re: Idle concerns with center mounted 2 bbl
Posted by: chuckvh ()
Date: July 31, 2022 09:52AM

Where is the vacuum advance connected? Needs to be connected to a ported source, on the carburetor as opposed to connected directly to the manifold. If it’s connected directly to the manifold, you’ll have too much advance and won’t be able to get the mixture and idle speed screws properly adjusted.

Chuck vonHofen
Brentwood, California

66 Corsa Turbo Convertible
64 Spyder Coupe
Too many boats
Amazingly Patient Wife

Re: Idle concerns with center mounted 2 bbl
Posted by: Colorsarge ()
Date: July 31, 2022 12:11PM

Advance to Venturi vacuum.

Frankford, De., eight miles north of Ocean City, Md.
1966 Corsa convt.

Re: Idle concerns with center mounted 2 bbl
Posted by: chuckvh ()
Date: July 31, 2022 02:23PM

Which Holley are you using? There are a number of different vacuum port arrangements, some with full manifold vacuum and some with timed vacuum based on throttle position. Just to clarify, with the engine running at idle, you should have no vacuum signal to the vacuum advance. You should be able to disconnect the vacuum line and see no movement of the arm and no change in spark timing. You should only see movement of the arm as you start to increase the throttle setting.

Chuck vonHofen
Brentwood, California

66 Corsa Turbo Convertible
64 Spyder Coupe
Too many boats
Amazingly Patient Wife

Re: Idle concerns with center mounted 2 bbl
Posted by: potvinguy ()
Date: August 23, 2022 03:16PM

I think the vacuum advance should be fully advanced at idle; that means the vacuum line should go to full manifold vacuum, not the timed vacuum above the throttle blades.

Re: Idle concerns with center mounted 2 bbl
Posted by: American Mel ()
Date: August 23, 2022 03:21PM

That does NOT sound right to me.
I believe that Chuck is correct, above.

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WA. state, 1 mile south of the Canadian border, I am not at the end of the world, but you can see it from here.

Re: Idle concerns with center mounted 2 bbl
Posted by: cnicol ()
Date: August 23, 2022 03:41PM

potvinguy Wrote:
-------------------------------------------------------
> I think the vacuum advance should be fully advanced at idle; that means the vacuum line should go to full manifold vacuum, not the timed vacuum above the throttle blades.

This advice is incorrect. To ensure "best case" idle stability, VA should be connected to ported vacuum and all base timing "baked into" the initial timing advance. If the engine requires more base timing to idle properly, a similar amount (degrees) should be subtracted from the mechanical timing maximum.

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
65 Canadian Monza 4dr 110/PG 40k
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts

Re: Idle concerns with center mounted 2 bbl
Posted by: jamesolefjensen ()
Date: August 23, 2022 04:24PM

I have connected my vacuum advance to both ported and full vacuum on my Holley 390 (8007). I didn't notice any difference on the road.

If there are idle stability issues, ported vacuum makes it easier to set the idle speed. I found the stock vacuum advance distributor I was using was very sensitive in the vacuum range my 140 would pull at idle (never measured but it's not much). At idle the vacuum advance would "hunt" which would cause the idle speed to change after it was set. (One could see the vacuum canister link move in and out at idle)

I switched to full vacuum when I installed an electronic distributor (thank you very much Seth). The vacuum canister on this unit stays fully actuated even under the relatively low vacuum my 140 pulls at idle. There's no hunting and the idle stays put when I adjust it.

Fun fact: With the vacuum advance on full vacuum, the throttle position can be closed slightly compared to ported vacuum.

There is a great debate raging on the Internet on Ported vs. Full Vacuum Advance. I am in now way advocating one approach over the other.

James
65 Corsa
SF Bay Area

Re: Idle concerns with center mounted 2 bbl
Posted by: jamesolefjensen ()
Date: August 23, 2022 04:35PM

I misspoke, I have measured the vacuum at idle on my 140. Its somewhere around 10-11 inches.

James
65 Corsa
SF Bay Area

Re: Idle concerns with center mounted 2 bbl
Posted by: Colorsarge ()
Date: August 23, 2022 05:35PM

Just to update my original post. The car has now been put back to the stock four carb set up.

Frankford, De., eight miles north of Ocean City, Md.
1966 Corsa convt.

Re: Idle concerns with center mounted 2 bbl
Posted by: Phil Dally ()
Date: August 23, 2022 06:27PM

The aforementioned oil heated manifold plenum.

Attachments:


Re: Idle concerns with center mounted 2 bbl
Posted by: American Mel ()
Date: August 23, 2022 09:38PM

That is a LOT of added restriction, in an already poor flow design plenum!

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
WA. state, 1 mile south of the Canadian border, I am not at the end of the world, but you can see it from here.

Re: Idle concerns with center mounted 2 bbl
Posted by: jamesolefjensen ()
Date: August 24, 2022 08:12AM

Colorsarge Wrote:
-------------------------------------------------------
> Just to update my original post. The car has now been put back to the stock four carb set up.


I am curious to know if your idle troubles went away when you swapped out the centermount.

James
65 Corsa
SF Bay Area

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