What Makes An Engine Sputter Right At.....

horseballz · Sep 11, 2010

What makes an engine sputter right at the zero spot between accelerating and decelerating? Perfect, steady idle and smooth acceleration, but it sputters/stumbles at all RPM above 1100 or so. It does this with no load (car not moving/operating throttle by hand) and while driving at steady speed. While driving, the slightest bit of light acceleration makes it go away and aggressive acceleration is smooth. It does this in all 5 gears but is more noticeable in lower gears. I'm kind of stumped and it is frustrating to have my car run so well in all other ways, but sputter/surge at steady speeds! I suspect that it isn't helping fuel mileage either.
>Autolite 4100 carburetor
>Performer RPM intake
>Mallory Magnetic Breakerless distributor
>FRPP wires
>New spark plugs
>Good, steady vacuum and no vacuum leaks
All suggestions are appreciated.
Thanks In Advance,
Gene

jonfor · Sep 13, 2010

sounds to me like a gremlin in the ignition wiring or bad sparkplug gap or maybe accelerator pump.

MitchGT · Sep 13, 2010

Has the carb been rebuilt at all? If you have another carb handy, you might want to swap it and test to see if the problem is still present. I don't know much about your specific carb, but most have an idle circuit (which you car is fine at idle), an acceleration enrichment circuit ( also ok), and a main circuit. It might have some crud in that circuit, but gets enough fuel from the other circuits to mask the problem sometimes....

PM 65 · Sep 14, 2010

MitchGT said:
Has the carb been rebuilt at all? If you have another carb handy, you might want to swap it and test to see if the problem is still present. I don't know much about your specific carb, but most have an idle circuit (which you car is fine at idle), an acceleration enrichment circuit ( also ok), and a main circuit. It might have some crud in that circuit, but gets enough fuel from the other circuits to mask the problem sometimes....

Had the same problem on my car. Idle and acceleration fine. But when i was steady driving a light hesitation and mumbling was coming from the engine. Checked all things on the car i could think of and finally i cleaned the metering plate with air pressure and the main circuit was a little bit blocked with a dirt particel. Put it back together and problem was 100% solved.

Generally, did you do some work or changes on the engine or is the sputtering coming out of nowhere and the car was running fine some weeks ago?

PoppyMod · Sep 14, 2010

In addition to what others have stated, check your dizzy vac, both hose and the setting. If it has a hex snout, it's adjustable. There is about 10, or so, degrees that can be added or deleted that affects your mid- range. One full turn is equal to about 2 degs. one way or another. Start by turning the adjustment CW to close (0 affect on initial and mech. adv.). Then, CCW to add advance for fine tuning.
If when you mention sputtering as in "spark missing" then, check your wiring and cap, that it is in good condition and no sparking or tracking to ground. A good time to check is at night (wires).
Good Luck!

Hack · Sep 15, 2010

I suppose it's possible the problem is fuel - I'm not a big carb guy. However, every time I've had symptoms like you describe it's always been an ignition problem. Check for poor grounds or any other connection problems as well as looking at the actual ignition parts.

MitchGT · Sep 17, 2010

I would think that if it was a spark problem, it would show up on acceleration, when the most load is put on it. I guess at part throttle the ignition should be really advanced to burn the leaner mixture, and maybe a vacuum hose is blocked, so I guess it's possible.

What Makes An Engine Sputter Right At.....

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