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Raize
Joined: 18 Sep 2013
Posts: 450
Location: Scotland
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 Posted: Sun Aug 10, 2014 5:37 am Post subject: Interesting effects from brake booster on idle speed |
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I recently discovered that if I repeatedly pump the brake pedal when the engine is warm, the revs will drop significantly from the standard 1000rpm idle until I stop pumping the brake pedal.
Strangely enough, if I do this when the engine is cold, the opposite occurs and I can make the revs increase.
I suppose that the additional air being allowed into the engine via the brake booster causes the mixture to be leaner than it normally would be.
Does this mean my engine is running too rich when cold (but that's normal of course) and too lean when hot? |
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morghen
Joined: 21 Jan 2005
Posts: 9102
Location: Romania
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| Posted: Sun Aug 10, 2014 6:13 pm Post subject: |
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Get a wideband and a gauge to check your AFR.
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Carrera RSR
Joined: 08 Jan 2010
Posts: 2312
Location: Somerset, UK
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| Posted: Sun Aug 10, 2014 9:44 pm Post subject: |
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My father would say this ....... 'stop pumping the brake pedal !!'
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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morghen
Joined: 21 Jan 2005
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Location: Romania
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| Posted: Sun Aug 10, 2014 11:45 pm Post subject: |
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| Carrera RSR wrote: | | My father would say this ....... 'stop pumping the brake pedal !!' |
 epic !!
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Raize
Joined: 18 Sep 2013
Posts: 450
Location: Scotland
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| Posted: Mon Aug 11, 2014 2:53 am Post subject: |
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There is that, of course.
Does the adjustment screw on the fuel distributor actually effect the mixture at any amount of revs other than idle? I was under the impression that it's just a stop for the air plate from looking at the k-jet diagrams. |
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Rasta Monsta
Joined: 12 Jul 2006
Posts: 11733
Location: PacNW
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| Posted: Mon Aug 11, 2014 3:25 am Post subject: |
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Screw alters mixture across the board.
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- WeiBe (1987 924S 2.5t) - 931 S3
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Ian89C4
Joined: 01 Apr 2011
Posts: 561
Location: North Carolina
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| Posted: Mon Aug 11, 2014 9:58 am Post subject: |
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I would check for vacuum leaks going into the brake booster. Or your may have a leak on the back end of it (inside the car attached to the brake pedal). I would leave the fuel and air alone until you can rule out a bad booster. If you play with the AF mixture without a wideband hooked up you will never be able to get it right.
Good luck,
Ian
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Ian Edgerly
North Carolina
Current:
1981 924 SCCA/ChampCar Weissach Race Car ("Serenity")
1987 924S ChampCar Endurance Car ("Der Weg")
1980 924 Euro ("Wahala") |
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Rasta Monsta
Joined: 12 Jul 2006
Posts: 11733
Location: PacNW
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| Posted: Mon Aug 11, 2014 11:37 am Post subject: |
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| Ian89C4 wrote: | | If you play with the AF mixture without a wideband hooked up you will never be able to get it right. |
And if you play with it without running baseline fuel pressure and delivery tests you will also end up frustrated.
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Toofah King Bad
- WeiBe (1987 924S 2.5t) - 931 S3
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Ian89C4
Joined: 01 Apr 2011
Posts: 561
Location: North Carolina
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| Posted: Tue Aug 12, 2014 7:10 am Post subject: |
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What he said 
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Ian Edgerly
North Carolina
Current:
1981 924 SCCA/ChampCar Weissach Race Car ("Serenity")
1987 924S ChampCar Endurance Car ("Der Weg")
1980 924 Euro ("Wahala") |
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