Idle Issues
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Elder User
Joined: May 2017
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Idle Issues
I have 390 that has started to idle poorly. Did the brake cleaner test and did not find any vacume leaks around the intake and carb. It has an HEI dizzy, I have replaced the rotor and cap, all new plugs.
I tried isolating the cylinders by unplugging the wires and found on the right side #2, #3, and on the left #5 and #8 make no changes when unplugged. There is definitely plenty of spark at the wires, when I pulled them there was a big white spark.
Removed the valve covers and visually check for bent push rods, they all rotated smoothly and had just slight tension when slacked.
Any my tips will be greatly appreciated
I tried isolating the cylinders by unplugging the wires and found on the right side #2, #3, and on the left #5 and #8 make no changes when unplugged. There is definitely plenty of spark at the wires, when I pulled them there was a big white spark.
Removed the valve covers and visually check for bent push rods, they all rotated smoothly and had just slight tension when slacked.
Any my tips will be greatly appreciated
Tuned
Joined: Jun 2013
Posts: 441
Likes: 61
From: Craigmont, Id
With those 4 cylinders out of the circuit it would indicate you have a carb issue. If you notice those 4 are feed by one half of the intake manifold. The other 4 are feed by the other half. I would pull and clean the carb as it would appear that only half of it is feeding on the idle circuit. I remember when I was first starting in the trade years ago customer brought in a full size Ford with exactly the same issue. After checking it out I told him he needed a carb cleaning. He went to the lead mechanic in the shop who of course disagreed with what I had determined the problem was and told the customer he needed a valve job. Well after the valve job was done then the carb got cleaned as the problem was still there. But who would believe a 20 year old wet behind the ear kid when the old timer said other wise. Sad to say now I am the old timer how ever no longer working in that trade. My back long ago told me to quite bending over a fender all day long!!!
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Elder User
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With those 4 cylinders out of the circuit it would indicate you have a carb issue. If you notice those 4 are feed by one half of the intake manifold. The other 4 are feed by the other half. I would pull and clean the carb as it would appear that only half of it is feeding on the idle circuit. I remember when I was first starting in the trade years ago customer brought in a full size Ford with exactly the same issue. After checking it out I told him he needed a carb cleaning. He went to the lead mechanic in the shop who of course disagreed with what I had determined the problem was and told the customer he needed a valve job. Well after the valve job was done then the carb got cleaned as the problem was still there. But who would believe a 20 year old wet behind the ear kid when the old timer said other wise. Sad to say now I am the old timer how ever no longer working in that trade. My back long ago told me to quite bending over a fender all day long!!!
Fleet Mechanic
Joined: May 2001
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From: Cupertino
Try removing the idle mixture screws. Splash the holes with Gumout and the nozzle. Clean needles, put back in. Back out two turns after reseating and try again. You may have crud in one idle passage.
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Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Originally Posted by pchelp
Checking now
You don't need (or want) the key on or ignition energized during a compression test. By hooking up a remote starter you can crank over the engine from under the hood. Very handy tool that belongs in your toolbox. Connects to a small terminal and the hot side of the solenoid.
When performing a compression test first thing remove all of the spark plugs. The battery should be freshly charged and in good condition. I usually measure 4 cylinders and recharge, otherwise the 8th might read a little low. Block open throttle on carburetor all the way. Measure each cylinder reading with the same number of "puffs" say 5 or 6 - and note whether the cylinder comes up quickly.
It's kind of a pita to do this, but if you're going to do the compression test do it right don't half-*** it, you want good data. Write down your findings for each cylinder.
Post Fiend
Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
I lied.
They have one for $10.90, delivered.
Course, nobody yet has figured out how they can mail ANYTHING, even an empty box for $11 from the other side of the world to here, whereas it would cost YOU $25 to mail it just across town. What the hell. I'm sure the local businesses just love that. Hm.
They have one for $10.90, delivered.
Course, nobody yet has figured out how they can mail ANYTHING, even an empty box for $11 from the other side of the world to here, whereas it would cost YOU $25 to mail it just across town. What the hell. I'm sure the local businesses just love that. Hm.