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equality72521's Avatar
 
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Throttle bypass screw question

Does the throttle bypass screw (for adjusting idle) have any relation to AFR? At the correct AFR my bypass screw needs to be all the way in to maintain proper idle, I can't go any further. Is there a relationship between AFR and idle screw such that if I back out the screw and adjust the mixture screw at the same ratio will I get the same mixture?


BTW, if you ever have your AFR's swinging all over the dial along with super mega hella rich at the onset of boost causing the car to bog check power to the WUR. Ask me how I learned that one!
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

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Old 03-06-2010, 12:31 PM
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I don't think it does beyond once the AF is dialed in, you set the rpms.

I could be wrong though...
Old 03-06-2010, 02:40 PM
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Quote:
Originally Posted by equality72521 View Post
BTW, if you ever have your AFR's swinging all over the dial along with super mega hella rich at the onset of boost causing the car to bog check power to the WUR. Ask me how I learned that one!
Ok here I go. "How did you learn that one" ? :-)
Old 03-06-2010, 03:39 PM
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It is idle speed adjustment only.

Some multi throat Weber and Dellorto carbs also have individual throttle butterfly air bypass screws for synchronising the individual carberator throats only, and some of them whistle a little with each intake stroke at idle.

Those carbs also have air correction adjustment screws for idle mixture adjustment of each carb throat, but there is no such thing as an air "correction" CO adjustment on CIS.
Old 03-06-2010, 03:40 PM
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I wonder why I need to have that screw dialed all the way in to set the idle properly???? I've checked several times for vacuum leaks and can't find anything. I replaced all of the gaskets and seals recently.
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--clutch-monkey
Old 03-06-2010, 03:51 PM
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It's pretty difficult to tell if any injector blocks are cracked or one of the 12 gaskets surrounding them are leaking without removing them.
There are alot of places that could be a vacuum leak.
Old 03-06-2010, 04:00 PM
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I'm with Jim. Sounds like a vacuum leak. Try spraying flamable brake/carb cleaner at all joints and listen for an rpm increase.
Old 03-06-2010, 05:44 PM
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The idle adjustment scew is no more than a controlled vacuum leak (around the trottle), if it is all the way shut and still idles too high then there is a leak somewhere else. Do you have the stock BOV? A 930's best place to hide a vacuum leak.
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Old 03-07-2010, 10:39 AM
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Found it. The AAR was not operating and was staying open. I blocked it off and had to readjust.
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 03-07-2010, 11:56 AM
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Quote:
Originally Posted by RarlyL8 View Post
Do you have the stock BOV? A 930's best place to hide a vacuum leak.
How so?
Old 03-07-2010, 02:36 PM
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Quote:
Originally Posted by equality72521 View Post
Found it. The AAR was not operating and was staying open. I blocked it off and had to readjust.
Exactly the same issue I had, The AAR was not closing all the way and in order to get a 950 rpm idle the air adjustment screw was all the way in. I removed the AAR valve body and plugged the hoses that went to it and now run without the damn thing. It only requires a little effort withe the gas pedal upon starting the car in colder weather, but thats fine.
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Old 03-07-2010, 03:23 PM
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I thought about installing a 12vdc 30psi NC solenoid valve wired to a switch up front for cold starting. That way I could have the bypass work when I wanted it to without any issues. Sort of a manual electric bypass valve.
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"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 03-07-2010, 03:31 PM
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Quote:
How so?
The gaskets disentigrate in the stock BOV which allows air around the large piston.

I eleminated my AAR and AAV when I moved to AL. The idle is low (700rpm) for a couple minutes but no big deal. I have a manual throttle assembly from an earlier 911 that I wanted to install for this function but like everything else my car comes last so it hasn't been installed.
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Old 03-07-2010, 06:51 PM
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I've also replaced the BOV gaskets.
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 03-08-2010, 03:19 AM
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Quote:
Originally Posted by equality72521 View Post
I thought about installing a 12vdc 30psi NC solenoid valve wired to a switch up front for cold starting. That way I could have the bypass work when I wanted it to without any issues. Sort of a manual electric bypass valve.
I have started mine successfully in Michigan weather as low as in the 30's-40's without much trouble with the AAR removed. It just takes a little throttle action for 30 seconds or so to get a stable idle. Not a big deal compared to buying a new AAR if you can even find one.
Try it for awhile without it and see what happens.
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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
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Old 03-08-2010, 05:11 AM
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You could also install a BMW 320i heater valve in place of the cold start AAR, the 2 hose connections are the same size, and run a generic pull/push carburator choke cable to somewhere around the auto heat box between the seats and use it as a nice mechanical/adjustable cold start high idle device that wouldn't interfere with the throttle linkage.

They're kind of expensive new now.. part #64-11-1-366-669-BOE
Pelican Parts - Product Information: 64-11-1-366-669-BOE

...but there's lots of them in salvage yards for real cheap.

I think the 944 heater valve is the same and would also work and cost way less...
944-572-085-02-M53
The body is plastic and it has a connection and clamp for a cable, and 2 hose connections that are the same size as the CIS AAR valve.
Old 03-08-2010, 09:28 AM
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Quote:
Originally Posted by A930Rocket View Post
I'm with Jim. Sounds like a vacuum leak. Try spraying flamable brake/carb cleaner at all joints and listen for an rpm increase.
is this the best way to look for a vacuum leak? I must have one, too...

I've removed all of the AAR's and AAV's a long time ago, etc... All hoses are new along with intake blocks. The car starts and idles on it's own when cold. The idle starts out at about 500 RPM when cold [maybe this is normal with all accessories removed], but slowly goes to 1,200 over the next 10-minutes of warming up.

my idle screw is all the way in... i'd really like to have the car idle at 900 when warm, so I need to find the leak!!
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Last edited by mooney265; 03-08-2010 at 03:57 PM..
Old 03-08-2010, 03:55 PM
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Another place that can be a vacuum leak is the brake booster fitting and hose that is rubber hose with hoseclamp on to a metal tube through the center tunnel and then another rubber hose and clamp down in between the transmission mount and floor pan.
Then the hose eventually attatches to the intake manifold on the left side.
Old 03-08-2010, 04:01 PM
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Originally Posted by JFairman View Post
Another place that can be a vacuum leak is the brake booster fitting and hose that is rubber hose with hoseclamp on to a metal tube through the center tunnel and then another rubber hose and clamp down in between the transmission mount and floor pan.
Then the hose eventually attatches to the intake manifold on the left side.
that is something i definitely have not checked out... will do so this weekend!

*** could I just disconnect and plug at the intake manifold while idling and see if idle drops??
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Old 03-08-2010, 04:42 PM
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Shannon, here are a few of my favorite hidden sources of vac leaks:
> AAR that does not close all the way
> Throttle Bypass valve with a blown diaphagm
> AAV stuck in the open possition
> Injector blocks with spiderweb cracks
> Loose injector blocks
> Any vac hose that leaves the engine compartment (brakes, boost gage, etc)

Loose injector blocks are commonly overlooked.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 03-08-2010, 05:40 PM
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