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'New' Carb. Soot on driveway whilst on choke?

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Old 06-22-2013, 03:45 PM   #1
CF Senior Member
Member Since: Jun 2005
Location: Bovingdon Hertfordshire
Default 'New' Carb. Soot on driveway whilst on choke?

I've just replaced the QJet on my 1974 350 Auto. - It wouldn't idle when hot - turns out the air horn was warped.

So I've got a replacement from a 1975 Auto. This carb. is a big improvement, starts from cold first time, idles smoothly when hot (or cold) and the idle mixtures screws are responsive (which the old one were not).

The ONLY problem with it is that is leaves soot markson the drive from the exhaust tips when I back it out of the garage from cold. I guess it is too rich when on choke.

I have an electric choke fitted (which works fine in terms of getting the car running when cold) - I've tried truning it clockwise to 'lean' out the choke, but that doesn't seem to stop the black marks on the drive (that I'm sure the wife will not be impressed with when she works out what is doing it ).


Constructive comments please - (change the carb. is only partially useful!).

Oops - posted this in general - can I delete this ?

Last edited by langlj00; 06-22-2013 at 03:53 PM.
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Old 06-22-2013, 11:56 PM   #2
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Member Since: Jul 2011
Location: Tyler TX

Hmmm... On my Edelbrock electric choke clockwise richens the choke (leaves it on longer). I've found that to cure my Edelbrock off-idle flat spot I have to run the idle circuit a bit rich, meaning I need to lean the choke, especially in the warmer months (almost all year here in East Texas). Otherwise I get a bit of soot upon start-up.
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Old 06-23-2013, 10:09 AM   #3
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Member Since: Jan 2006
Location: Crossville TN

When the choke is set, it probably closes the upper choke flap too much. That can be adjusted by bending the link between the choke mechanism and the upper flap. Also, the "high idle" adjustment may have the throttle open too far.

Opening up the choke flap and backing off the high-idle adjustment should solve your problem. If you want to know how to make all the adjustments on your Q-Jet, you need the Chevrolet Chassis Service Manual for the year that carb was produced or for what year car that replacement was meant to be used.
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Old 06-23-2013, 10:09 AM
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