SBC Running Rough - Chevelle Tech
Troubleshooting Diagnosing problems done here.

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post #1 of 5 (permalink) Old Jul 14th, 19, 3:52 PM Thread Starter
 
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SBC Running Rough

Good Afternoon Everyone,

So I have a 1979 Chevy c20 with 350 sbc and Quadra jet. I just had the heads re done and put all new gaskets on because I had a burnt exhaust valve and engine was running really rough. I had the distributor pointing to #1, which was also at TDC. Fired right up but idles pretty rough, almost like a vacuum leak or timing is off. Played with the distributor cap a bit and that helped but still isn’t right. Revs fine also. No dripping from carb. Thoughts??
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post #2 of 5 (permalink) Old Jul 14th, 19, 4:02 PM Thread Starter
 
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Also note: I capped the EGR with an EGR delete plate and capped the line that used to be going to it? Did I miss something on this?

Gas is old but I don’t think it’s old enough to effect how’s its running. I had put a bottle of sea foam in it as well a little while back.
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post #3 of 5 (permalink) Old Jul 14th, 19, 4:23 PM
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Originally Posted by bdfb25 View Post
Good Afternoon Everyone,

So I have a 1979 Chevy c20 with 350 sbc and Quadra jet. I just had the heads re done and put all new gaskets on because I had a burnt exhaust valve and engine was running really rough. I had the distributor pointing to #1, which was also at TDC. Fired right up but idles pretty rough, almost like a vacuum leak or timing is off. Played with the distributor cap a bit and that helped but still isn’t right. Revs fine also. No dripping from carb. Thoughts??
Have you verified the base timing with a timing light?

72 Chevelle Malibu
Muncie M20
SBC 406
KB Pistons
PW Aluminum heads
RPM Air Gap Intake
MSD 6AL2
Holley Sniper EFI
Holley Dualsync
Stainless Long Tubes to 2.5 Exhaust
UMI Suspension Stage 2
CPP Steering setup
10 bolt rear - 3.55 with Eaton Posi
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post #4 of 5 (permalink) Old Jul 14th, 19, 4:38 PM
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Re: SBC Running Rough

Did you use the E-O, I-C method to set the valves, to 1/4 turn ech? If not, redo the valves using that E-O, I-C method, to 1/4 turn, and see if that helps.

Then, set the initial to 10 with a timing light with the DIAL BACK left OFF, OFF, OFF. P=lug the vacuum advance into a full manifold vacuum port on the base of the carb after setting initial, and see how many IDLE degrees of timing you have, good is between 22 and 24.
LeoP and DUTCH MAX HEADWORK like this.
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post #5 of 5 (permalink) Old Jul 30th, 19, 12:45 AM
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Frank
 
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Re: SBC Running Rough

As Dave said, initial ignition timing around 10 BTDC plus vacuum advance is important for idle quality.
Rough idling after replacing the heads could be due to setting valve lash too tight, which would keep valves from seating and mimic a vacuum leak. Hydraulic lifters work best with between 1/4 to 1 turn down from zero lash at the rocker arm.
I prefer to adjust the valves with the engine running. I use an old valve cover with the center cut out to temporarily replace the usual rocker arm cover. This give access to the valves while keeping most of the oil in the engine and off the exhaust. Back off one rocker arm nut until the rocker begins to make a clacking noise for at least five seconds to allow the hydraulic lifter to fully pump up. Then turn down the rocker arm nut until the clack noise stops, then another quarter-turn down. Adjust each rocker arm the same way.

-Frank
1970 Chevelle Malibu, one family owner
355 SBC, XE262H cam, Performer RPM heads, MSD HEI, TH200-4R trans, 3.55:1 Posi, digital gauges
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