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#1
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Puzzler: hesitation/stumble at low speed on left turns only!
70 LeMans 350/2bbl, TH350, standard differential (not posi), Comp Cam 268H grind cam (previous owner installed.) Everything else is stock, including distributor mechanical and vacuum advance.
When off the gas heading into a 10 to 25mph left turn or curve (as from a left turn lane or two-lane country roads), if I hit the gas (part throttle, not mashing it) there's often a slight but noticeable stumble/hesitation from the engine as it comes off idle. The odd thing is: it never happens when making right turns at similar speeds. Note that the vacuum advance is connected to manifold vacuum, since the Rochester 2bbl has no ported vacuum source (this is the factory set-up as far as I know.) Also, I have bypassed the TCS vacuum switch. I've set the initial idle timing using 'best manifold vacuum' method, as the stock 9*BTDC timing seems to make the car more prone to off-idle hesitation even on straight-on take-off from stopped (I suppose that the cam requires a bit more static advance.) I set it to best vacuum, then backed off (retarded) the timing a few degrees. (The car has much better off-idle throttle response since doing this.) The timing mark seems to be at about 20*BTDC or thereabouts (wild guess, it's off the timing cover scale.) The 'left turn only stumble' problem also happened when I used the stock timing, so I don't think the current timing is the problem, but hey, what do I know? Any suggestions?
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Bought: 1970 Le Mans Sport Convertible (second owner,) Palladium silver, 350/2bbl (sigh) with Comp Cams 268H grind (installed by previous owner), TH350 Trans. Mods: Front suspension: Global West negative roll tubular upper arms, Del-a-lum bushings, Koni shocks, 12" disc brake conversion, 1.1" front sway bar, Rear suspension: Global West tubular control arms, springs, sway bar. Exhaust: Flowmaster 2.5" American Thunder system. Mostly complete: Firewall-forward engine compartment restoration Last edited by DIYGuy; 07-12-2012 at 10:13 PM. |
#2
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I'm really interested as to what the guys come up with for this one because I had the opposite issue with a stock 64 4bbl GTO; it would stumble ONLY when turning right. I know it was something in the carb because I recently changed to a tri-power setup and it doesn't hesitate any more. But I am like you - what the he!! causes a directional, low-speed stumble in the fuel system?? Two carburetor rebuilds over the years did nothing to fix it.
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Just another "member". |
#3
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At first I thought it might be due to fuel sloshing in the float chamber or floats adjusted too low. But if it was that, I'd expect other symptoms as well. Same goes for fuel pressure problems or ignition. I'd rebuilt the carb and checked float levels once, a long time ago, no change. What the heck is going on that only happens at certain engine RPMs, certain throttle openings, and only when turning a certain direction? That seems like it has to be something about the carburetor (since fuel moves around the float bowl in a turn due to lateral G forces...) But then, you've had several carb rebuilds and no effect. Curiouser and curiouser. Surely someone's identified and solved this problem before...
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Bought: 1970 Le Mans Sport Convertible (second owner,) Palladium silver, 350/2bbl (sigh) with Comp Cams 268H grind (installed by previous owner), TH350 Trans. Mods: Front suspension: Global West negative roll tubular upper arms, Del-a-lum bushings, Koni shocks, 12" disc brake conversion, 1.1" front sway bar, Rear suspension: Global West tubular control arms, springs, sway bar. Exhaust: Flowmaster 2.5" American Thunder system. Mostly complete: Firewall-forward engine compartment restoration Last edited by DIYGuy; 07-13-2012 at 10:17 AM. Reason: Correction: I did do a carb rebuild long ago... |
#4
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I suppose I need to tear down the carb and see what's inside - perhaps that'll give me a clue (look for something about the float chamber innards that could contribute to such a symptom.)
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Bought: 1970 Le Mans Sport Convertible (second owner,) Palladium silver, 350/2bbl (sigh) with Comp Cams 268H grind (installed by previous owner), TH350 Trans. Mods: Front suspension: Global West negative roll tubular upper arms, Del-a-lum bushings, Koni shocks, 12" disc brake conversion, 1.1" front sway bar, Rear suspension: Global West tubular control arms, springs, sway bar. Exhaust: Flowmaster 2.5" American Thunder system. Mostly complete: Firewall-forward engine compartment restoration |
#5
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I should mention that this stumble has been with us since we bought the car in 1996...
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Bought: 1970 Le Mans Sport Convertible (second owner,) Palladium silver, 350/2bbl (sigh) with Comp Cams 268H grind (installed by previous owner), TH350 Trans. Mods: Front suspension: Global West negative roll tubular upper arms, Del-a-lum bushings, Koni shocks, 12" disc brake conversion, 1.1" front sway bar, Rear suspension: Global West tubular control arms, springs, sway bar. Exhaust: Flowmaster 2.5" American Thunder system. Mostly complete: Firewall-forward engine compartment restoration |
#6
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A quick G**gle search returns the following interesting bits:
Edelbrock carb, hesitation around corners? Living With A Carburetor…Every Day Quote:
Quote:
Quote:
http://forum.ih8mud.com/40-55-series...eft-turns.html http://www.forwardlook.net/forums/fo....asp?tid=30128 http://forums.chevyhiperformance.com...urn/index.html --- So far, float level and/or fuel pressure seem to be the most popular suggested causes...
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Bought: 1970 Le Mans Sport Convertible (second owner,) Palladium silver, 350/2bbl (sigh) with Comp Cams 268H grind (installed by previous owner), TH350 Trans. Mods: Front suspension: Global West negative roll tubular upper arms, Del-a-lum bushings, Koni shocks, 12" disc brake conversion, 1.1" front sway bar, Rear suspension: Global West tubular control arms, springs, sway bar. Exhaust: Flowmaster 2.5" American Thunder system. Mostly complete: Firewall-forward engine compartment restoration Last edited by DIYGuy; 07-13-2012 at 10:13 AM. |
#7
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Ah, let's not forget accelerator pump AND float levels.
Suppose the accelerator pump is marginal or otherwise behaving differently around a certain turn direction...
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Bought: 1970 Le Mans Sport Convertible (second owner,) Palladium silver, 350/2bbl (sigh) with Comp Cams 268H grind (installed by previous owner), TH350 Trans. Mods: Front suspension: Global West negative roll tubular upper arms, Del-a-lum bushings, Koni shocks, 12" disc brake conversion, 1.1" front sway bar, Rear suspension: Global West tubular control arms, springs, sway bar. Exhaust: Flowmaster 2.5" American Thunder system. Mostly complete: Firewall-forward engine compartment restoration |
#8
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Well, with mine, the float levels were where they were supposed to be. I LIGHTLY ran a large flat file across the top of the fuel bowl to ehcck for straightness and did the same with the top, thinking possible sloshing was taking place. (This is an AFB, original equipment stuff.) all those fixes above sound like something for a Holley. The only thing I could think of that may be causing it was that the ball bearing in the cage at the inlet to the accelerator pump well may not be sealing as it should. But why would that only affect things when turning right? I even tried hard to duplicate the issue on left turns to no avail. Dunno....
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Just another "member". |
#9
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Mine used to die only when turning right. It turned out to be the insulation worn off of part of the positive battery cable, and it would short out when turning right.
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