Exhaust Soot
Thread Starter
Instructor
Joined: Sep 2013
Posts: 118
Likes: 1
Exhaust Soot
My 1971 Stingray often participates in local parades where you putt down the road for a couple miles or I go to car shows. Not a daily driver. (350 w/mild cam/4-barrel Holly carb w/Edlebrock intake. MSD ignition/header side-pipes) Veteran's Day parade I noticed I was blowing black smoke. Not super heavy, but noticeable. Also could taste it. (I have side pipes) Got her home and pulled the plugs. All were covered with black soot. No mis-firing & performance is the same as always. Could it be caused by the hi-test non-ethanol gas I have put in the tank the last (3) times? I usually run the mid-grade. Just trying to figure what happened all of a sudden. Have another Christmas Parade this weekend & am trying to participate in that as well. Thanks in advance for replies.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
It is not the gas.
Check the idle screws and set with a vacuum gauge.
What heat range of plug are you currently running?
Look down the carb throat when idling.....is there gas trickling out of the boosters? If so.....you need to change out the metering plate/block gasket on that side of the carb......this is VERY common for engines that sit a lot.
Jebby
Check the idle screws and set with a vacuum gauge.
What heat range of plug are you currently running?
Look down the carb throat when idling.....is there gas trickling out of the boosters? If so.....you need to change out the metering plate/block gasket on that side of the carb......this is VERY common for engines that sit a lot.
Jebby
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Your engine is running ver "rich" (excess fuel being burned) when at low speeds. This is usually a carb adjustment or setup issue. Your engine should be set to idle at about 700 rpm. If yours is set at a higher rpm, you may have been feathering the clutch to keep from running too fast in the parade line. You probably need to drop the curb idle speed and adjust your idle needles to allow more air into the fuel mixture at idle. This is an iterative process, as adding air to a rich engine will also cause the idle speed to increase. Continue those adjustments until you get it to idle well at 700 (or so) rpm and so that any adjustment either way on the idle needles will cause a slight drop in rpm.
Race Director
Joined: May 2006
Posts: 18,736
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How did you end up with NGK UR5 plugs? They cross reference to AC plugs in the 42 and 43 heat range, way too cold. You need to install 44 or even 45 range plugs if you intend to do lots of low speed driving.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,212
Likes: 9,352
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Take it out and drive it like you stole it. Just blast all the carbon out of there that idling and putting around builds up. I bet it'll clear up.
