Stumble off the line...please help - Chevelle Tech
Troubleshooting Diagnosing problems done here.

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post #1 of 9 (permalink) Old Dec 15th, 04, 9:51 PM Thread Starter
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My 68 with a slightly built 350 has developed a stumbling problem when I jump on it off the line.

It does not do it if the RPM's are up.

It has MSD 6al and MSD distributor and coil. Holley 750 double pumper with Mech secondaries with no choke.

I also have begun to notice some black soot on the ground when warming her up in the garage (mostly from passenger side exhaust).

What do y'all think it is.

Thank you in advance for and advise.
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post #2 of 9 (permalink) Old Dec 15th, 04, 10:05 PM
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Its the Holley. I have the same problem.

Mike Smith
72 Chevelle, LS 6.0 LQ9 4l80e
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post #3 of 9 (permalink) Old Dec 16th, 04, 12:54 PM
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very common.too much fuel too soon at low rpm. thats why you don't experience this problem at high launch rpm.the black soot is from overly rich air/fuel mix at idle. mine does the same thing when cold.there are lots of carb tuning shops around that can customize your carb to your engines requirements and your driving. kind of like dyno work. i've been quoted 200-300 bucks to make it the best it can be. they change metering rods,jets,accelerator pumps, ect.
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post #4 of 9 (permalink) Old Dec 16th, 04, 11:54 PM
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Without you postting specs,it sounds like you need more initial timing at idle and a faster curve.

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post #5 of 9 (permalink) Old Dec 18th, 04, 12:22 AM
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I had the same problem and there can be alot of solutions to this problem. Like the other guy mention, check you inital timing. Also check this, with the car off take the air cleaner off and grab on to the throttle and lightly inch it back and pay attention to when the fuel squirts out of the nozzels. If it is coming out right as you barely move it that should be OK. Give the throttle a full pump and see if the gas squirting in comes in a steady stream. If it is not squirting until you have moved the throttle a good ways then try adjusting your accelarator pump to give a shot earlier. Another thing to check is how far are the plated on the primary side are open. There are little holes in the ventura and the plates shouldnt be too far open passed the holes. If they are too far passed the holes then you could be getting into the transitional fuel. If this is happening then you are using the fuel for the transition and then when you need the fuel its not there and that can make the car stumble. Good Luck! John
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post #6 of 9 (permalink) Old Dec 18th, 04, 2:18 PM Thread Starter
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excellent advice gentlemen...thank you very much.
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post #7 of 9 (permalink) Old Dec 23rd, 04, 8:46 AM
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Gorgeous cars,are they both the same color?got a bad monitor,but look perfectly straight.
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post #8 of 9 (permalink) Old Dec 23rd, 04, 6:06 PM
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Everytime I see this post, Stumble off the line.., I want to say don't drink so much! [img]graemlins/beers.gif[/img]


70 Chevelle SS396(454), M20, 3.73

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post #9 of 9 (permalink) Old Dec 26th, 04, 12:00 AM
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I don't think it's timing, I'd say it's too much carb. If you want a mech doublepumper, a 650 is enough. A 750 vac would be an even better choise on a street engine.

1969 Malibu in progress
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