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Vaccinated and Boosted
 
mgatepi's Avatar
 
Join Date: Mar 2007
Location: Ohio
Posts: 1,588
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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1930 Cadillac V8, 1991 Ford F250,
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Old 06-21-2012, 11:44 AM
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1968 2.0s
 
Join Date: Oct 2008
Location: UK
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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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Old 06-21-2012, 12:30 PM
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Vaccinated and Boosted
 
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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....
Old 06-21-2012, 12:42 PM
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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
Old 06-21-2012, 12:47 PM
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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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Last edited by EPorsche; 06-21-2012 at 05:01 PM..
Old 06-21-2012, 01:08 PM
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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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Old 06-21-2012, 03:29 PM
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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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Old 06-21-2012, 04:43 PM
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