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Vaccinated and Boosted
Join Date: Mar 2007
Location: Ohio
Posts: 1,595
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3.0 stumble
Hello All,
 Looking for advice from the brain trust! I recently converted my 1983 sc 3.0 over to PMO 40's. The motor has big port heads and 964 cams. It runs great...with an exception in the 2K to 3K range. There is a stumble or a stalling in this range. I can accelerate through this and it pulls great, but when driving civil it can be a PIA. How can I cure this stalling / hesitation??? Thanks for the help!!!! Mike
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2013 911 Turbo S 2025 Lexus TX350, 2024 Honda CRV Hybrid 1930 Cadillac V8, 1991 Ford F250, 2023 Chevy Colorado And some garage space.... |
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06-21-2012, 11:44 AM
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1968 2.0s
Join Date: Oct 2008
Location: UK
Posts: 116
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This stall, low down in the rev range, can be caused by a number of factors.
The valve clearance may be incorrect. The ignition timing may need adjusting. The carburetors may need adjusting. Between 2 to 3k it's the progression circuit that's at work. Beyond these initial tuning procedures, there are a number of variables but attend to these first and see how things are. Regards, Mike.
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1968 Super. 1959 Super. http://tinyurl.com/pbvfl5w Will this learning curve ever level out ? |
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06-21-2012, 12:30 PM
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Vaccinated and Boosted
Join Date: Mar 2007
Location: Ohio
Posts: 1,595
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I recently had the carbs adjusted and the timing checked. All looks good. One thing not checked is the valve clearance, thanks for this tip, I will look into this.
Thanks! Mike
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2013 911 Turbo S 2025 Lexus TX350, 2024 Honda CRV Hybrid 1930 Cadillac V8, 1991 Ford F250, 2023 Chevy Colorado And some garage space.... |
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06-21-2012, 12:42 PM
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Registered
Join Date: Oct 2008
Location: Nash County, NC.
Posts: 8,520
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The float bowls might be low because youre in transition at 3K to the mains and if they cant suck enough gas off the level because its too low, the venturi effect isnt kicking in enough.
Bruce |
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06-21-2012, 12:47 PM
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Registered
Join Date: Oct 2004
Location: Fayetteville, NC
Posts: 680
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I have the same problem but with EFI (megasquirt) and think also it is a lean condition like Bruce said.
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'77 911S w/'81 3.0/MegaSquirt/MSD '81 911SC stock Last edited by EPorsche; 06-21-2012 at 05:01 PM.. |
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06-21-2012, 01:08 PM
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Registered
Join Date: May 2003
Posts: 3,346
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Most likely a lean stumble. A wideband O2 sensor will make it much easier to diagnose. You could also use a dyno to check it. Carbs are hard to get smooth throughout the rev range without being excessively rich in some RPM regions. Might need a bigger set of idle jets, but if the stumble is not too bad, it might be better to just live with it. My car runs very rich at low throttle cruise, but with smaller jets it has a lean stumble.
-Andy
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72 Carrera RS replica, Spec 911 racer |
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06-21-2012, 03:29 PM
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Registered
Join Date: Feb 2008
Posts: 1,438
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Even though your carbs were recently adjusted, if the side-to-side balance wasn't performed correctly then one side may be leading the other and cause a stumble. When I say correctly I am thinking in HOW the throttle is applied during the 3000 RPM balance check; if the cross bar is grasped by the tuner then this is not how the throttle is applied when driving (throttle pedal activates linkage) then this could affect the adjustment. Another item is it could be a rich stumble. I think PMO is plugging the third progression hole on those carbs with four holes as the progression is too rich at transition.
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Paul Abbott Weber service specialist www.PerformanceOriented.com |
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06-21-2012, 04:43 PM
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