Rough running 390 @ idle
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Freshman User
Joined: Jun 2004
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From: Washington
Rough running 390 @ idle
I have a 71' 390 which has 50k on a rebuild (I have no info on the rebuild) and it runs extremely rough at idle. It has a recently rebuilt autolite 2bl and pertronix ignition. To even get this thing to run I have to put it 30btdc and rich it wayyy out and run it at 900rpm idle. I had the mixture screws in all the way and then backed out 2.5 turns, the timing set to 12btdc and It went "Whump,bump,bump,bump.....Whump,bump,bump,bump " very painfull and slow rough idling. At higher rpms I don't notice any pinging (with it advanced to 30). Could it have been rebuilt with a different cam and the timing marks are off?? Vacuum leaks? I'm lost here, have searched the forums and am tired of messin' with this truck. If I didn't love it so much would sell it and buy a Honda.
Postmaster
Joined: Jul 2003
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From: iowa
It could be a serious vaccum leak or a chain that jumped. I think its a timing thing. Even if you had a leak you wouldn't need all that much timing. I would pull the front cover off and inspect and/or replace the chain. The gasket set is only $15 for the job. And by the way turns the screws back to 1.5 to 2 turns. Get a vaccum gauge to set it but that could be a pain in the but with an ill running truck.
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Freshman User
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From: Washington
Ok, so I took the DW-40 and sprayed it around the carb base and intake manifold and there was no noted bogging. There was only one Cap and it looked good too. As far as vacuum lines it looks like this: 1 from dizzy to carb, 1 from carb riser plate (I think this is what its called) to the left side valve cover. The intake manifold has one going to my tranny, one to the brake sys and one that is capped on the manifold. As soon as I removed the air cleaner to check things out, with the engine running, the RPM's dropped way down and it wouldn't idle w/o the choke on. Anyhow, sounds like vacuum leak is not an issue. I'm going to check the timing chain tomorrow. What should I look for? General slopiness? Also how do you reset the timing marks. Better yet is there a link out there that outlines this procedure? I have some digital pics of the motor, they are in my gallery if it helps to see what I'm looking at. And, to add I have no HV oil pump, just replaced the fuel pump, filter and intake manifold gasket 3 months ago.
Last edited by 71Goose; Jun 4, 2004 at 12:23 AM.
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Postmaster
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From: iowa
The front and back of the intake could be leaking. Not sure what gaskets you replaced. As for the chain you should check the slack and general codition. The marks on the cam gear and crank gear should be lined up bottom of cam and top of crank when the first piston is on tdc on compression stroke. that being said I think it would be best to have the First piston on tdc of comp stroke before you take it apart. That way you don't have to turn it later without the pulley and bolt. I just think it would be easier to do that first. And remember there are like 4 bolts on the bottom of the front cover that hold the oil pan to it. Because you go prying on it and you will have more problems. And you can check the dizzy when the piston is on tdc. Take the cap off and the rotor should be pointing to #1 on the cap. My guess with 30 degrees needed it might behind #1. And keep in mind the order is counter clockwise. And if thats good continue on to do the timing chain after that.
Last edited by fordeverpower; Jun 4, 2004 at 12:28 AM.
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Thread Starter
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Freshman User
Joined: Jun 2004
Posts: 29
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From: Washington
Ok, here's where I stand now:
Carb set to 2 screws out, 650 idle in drive, timing 30 btdc= very rough idle, loping, irregular.
I put the #1 @ TDC, pulled the dizzy cap and my rotor is pointing between 1 and 5, leaning ever so slightly towards 5. My next move is to re-position the distributer and see where that gets me, but I want some confirmation from you guys before I rip it out. Here's the other freak thing: when I step into it, the engine smooths out and sounds great. No pinging, good responsive power, but then at idle -ugggh.
Whadda Ya think?
Carb set to 2 screws out, 650 idle in drive, timing 30 btdc= very rough idle, loping, irregular.
I put the #1 @ TDC, pulled the dizzy cap and my rotor is pointing between 1 and 5, leaning ever so slightly towards 5. My next move is to re-position the distributer and see where that gets me, but I want some confirmation from you guys before I rip it out. Here's the other freak thing: when I step into it, the engine smooths out and sounds great. No pinging, good responsive power, but then at idle -ugggh.
Whadda Ya think?
Last edited by 71Goose; Jun 4, 2004 at 12:49 PM.
Thread Starter
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Freshman User
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From: Washington
I went ahead and reset my dizzy, got the rotor dead on #1, re-gapped my plugs to .40, (pertronix said to gap them .05 over stock), disconnected vacuum advance, set my curb idle and then timed it. Good news is that its at 12btdc, bad news is that it STILL won't idle smoothly at 650. It sounds great at 1000, but then my poor tranny is clunkin' when you go from park to drive. What else could I be missing??
Thread Starter
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Freshman User
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From: Washington
Now I really did it. This truck is going to drive me to drinking, actually it allready has...So this afternoon I retimed it and set my curb idle to around 1000rpm to get a smooth idle, drive around for about an hour, get gas, it diesels for about 30secs then shuts off and then the darn thing wont start. I go to open the hood and it is HOT HOT HOT. no juice, bypass the soleniod-nothing. I've got plenty o' fluid in the radiator, I know the timing isn't advanced too much. I had to leave and walk home. Visions of me throwing molatave cocktails throught the open passenger windows and watching the truck boil into an inferno race through my head. I've had it. Beer time. Maybe by then it will cool down, everything was too hot to touch. Maybe I just don't have what it takes to be an FE owner....
Junior User
Joined: May 2004
Posts: 70
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From: American Falls, Idaho
I found this problem on my truck just last weekend - pull the vacuum hose from the power brake booster and plug it with something as it could be a failed diaphram inside the booster.
I replaced my booster and it went from running like yours is to so smooth I can't hardly tell it's running!
I replaced my booster and it went from running like yours is to so smooth I can't hardly tell it's running!