[DIYGuy]

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?

[MikeR64]

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.

[DIYGuy]

Originally Posted by MikeR64

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.

'Glad' to hear I'm not the only one experiencing such a strange symptom. Weird, huh?

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...

[DIYGuy]

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.)

[DIYGuy]

I should mention that this stumble has been with us since we bought the car in 1996...

[DIYGuy]

A quick G**gle search returns the following interesting bits:

Edelbrock carb, hesitation around corners?

Living With A Carburetor…Every Day

Know Your Enemy: The test drive
No amount of explanation will substitute for a test drive to diagnose driveability problems. Patterson took the 'Stang for a spin and reported off-idle stumbling, backfiring through the carb after making right-hand turns, and generally poor throttle response (that's probably more than Smith would have admitted). Now we had a baseline "feel" for the tune.

...snip...

Before the next test drive, we remembered the car's tendency to lean-backfire around right turns. The likely culprit was a low float level, starving the engine when the fuel sloshed around the bowl when turning hard. With the engine idling, Patterson unscrewed the sight plug, raised the float level slightly until fuel just touched the bottom of the sight plug threads, and called it good. The last test drive was the best, as we'd hoped. The 302 felt as strong as it did when it was new--well, as strong as a stock, low-compression small-block ever felt.
http://www.carcraft.com/howto/77958/index.html

Troubleshooting 101 - Stumble - My Carburetor Stumbles in the Turn

We are starting this out assuming your carburetor is in good shape; power valves are good; floats are set correctly and jetting is reasonably close.

First, ask your driver if it stumbles on a restart. If no, there is probably nothing wrong with your carburetor and the problem is fuel slosh or lack of fuel in the bowls.

If the answer is yes, your problem is the carburetor.
http://www.bakercarb.com/technicalinfo.html

EDIT: More...

9. Why does my engine stall/ hesitate when I come to a hard stop, hard acceleration, or when I turn a sharp corner?
Answer: Most commonly this is a float level issue. Confirm float level and retest the cause of the stall. In racing applications specialized floats are available for extreme versions of these situations. Also, the use of jet extensions may be required in the secondary side of a carburetor for extremely hard accelerations such as a drag car.
Source: 10 Common Carburetor Questions

and more...

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...

[DIYGuy]

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...

[MikeR64]

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....

[69Farburd]

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.