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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: fiddler on July 25, 2017, 05:10:32 pm
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My 1974 chevy C10 Stepside started missing at idle again today. I think it has developed a vacuum leak. I have capped the lines going to the brakes, fuel canister, pcv valve, vacuum advance, transmission, and environmental control canister. I think that's everything. I am reading 15 inches and fluctuating (I think the gauge is in inches), at idle. I think it should be about 20 and steady. Looking to verify the 20 number. Is that correct? I am going to pull all of the lines off and pull a vacuum on them one at a next. If all of this proves fruitless I am thinking maybe a manifold gasket or a carb mounting gasket. Any ideas??
thanks for your suggestions in advance.
ricky
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Your vacuum gauge reading is in in Hg (inches of Mercury)
If the engine isn't running correctly you won't get healthy vacuum readings. As far as your miss at idle, check your ignition system components and your ignition timing first.
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Where do you live?
Altitude affects vacuum readings.
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So I live on the California coast, 700 feet above sea level. (about 12 miles in from the coast as the crow flies). (And with the crow flies comment it becomes obvious that I am a Nebraska transplant...1979 time frame). Anyway, so I tried everything in the book to verify the vacuum leak. Couldn't find one. Then I removed a vacuum plug while the engine was running and the idle picked up....strange. So I reconnected all the vacuum lines and went back to the drawing board. Now the symptoms were identical to the last time the truck would not idle...same cylinder missing, compression good, sparks when I watch the plug outside of the cylinder....so now what? I was looking for carburetor torque specs when I came across an adjustment video from edelbrock for my carburetor. In it they had a common problem for a bad idle and how to fix it. Turns out the idle circuits in the carb get plugged with "dirt" often enough that they have a video and a how to to fix it. So the idle adjustment screw on one side was not responding to adjusting it no matter how far I turned it. The screw was on the same side as the miss. The other screw affected the idle. Classic symptoms for a plugged port according to Edelbrock video. So I removed the screw, cleaned the point and blew out the port with compressed air. Put the screw back in, adjusted all the way down by hand and then backed out 1.5 turns. Truck idles great now. Adjusted the screw down until idle started to go wonky again, and then backed it out a turn. Voila. Problem is I still didn't get to look at the vacuum reading when the truck was running normally. I am telling myself that I will test it here presently and discover the answer! Thanks guys!
ricky