390 miss need help
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From: tucson usa
390 miss need help
Hey all,
I've got a 71 f100 4x4 390 4bbl-edelbrock 600, 4spd and i've got a miss--not much of a miss at idle-a little rough. when accelerating it misses just enough to where you can hear it and feel it, not really bad though. I went through everything i could think of, new cap rotor, plugs, wires, air filter, fuel filters, installed pertronix w/coil, did compression check 135 all even. checked for vaccum leak w/carb cleaner everywhere, vaccum isn't good, fluctuates between 7-14 rapidly, and the plugs show signs of running a little rich- not fuel fouled or anything though. the truck runs great going down the road-no miss then. any suggestions?
thanks
david
I've got a 71 f100 4x4 390 4bbl-edelbrock 600, 4spd and i've got a miss--not much of a miss at idle-a little rough. when accelerating it misses just enough to where you can hear it and feel it, not really bad though. I went through everything i could think of, new cap rotor, plugs, wires, air filter, fuel filters, installed pertronix w/coil, did compression check 135 all even. checked for vaccum leak w/carb cleaner everywhere, vaccum isn't good, fluctuates between 7-14 rapidly, and the plugs show signs of running a little rich- not fuel fouled or anything though. the truck runs great going down the road-no miss then. any suggestions?
thanks
david
Elder User
Joined: Jan 2002
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390 miss need help
You've either got a semi bad plug wire or plug,
or a sticky valve/bad lifter, as it's just at idle
I'm betting on the weak lifter...
Steve & the Rockette
'63 F100
'68 F100
'72 Capri 2L
'73 Capri 2.6L V6
'73 MG B GT 2.6L V6(Ford)
'98 Contour SVT 2.5L V6 (Mods)
'01 ZX2 (No Mods yet)
or a sticky valve/bad lifter, as it's just at idle
I'm betting on the weak lifter...
Steve & the Rockette
'63 F100
'68 F100
'72 Capri 2L
'73 Capri 2.6L V6
'73 MG B GT 2.6L V6(Ford)
'98 Contour SVT 2.5L V6 (Mods)
'01 ZX2 (No Mods yet)
Posting Guru
Joined: Jul 2001
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390 miss need help
You have the indicator to look for with the bad vacuum. It can be several items from a cam going flat to a bad intake manifold. I would suggest pulling plug wires and narrowing it down to a specific cylinder. I had a 360 with a stock 4 bbl intake that had a crack between the heat for the carb and the intake runner, almost drove me nuts finding it. Also, just because you sprayed cleaner on top of the manifold doesn't mean there isn't a leak, it can leak on the inside.
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390 miss need help
One possiblity that I can think of is that the valves might be too tight. What type of rockers are you using? Are they adjustable? If you do not have adjustable rockers and if the heads have been decked it you might need shorter pushrods.
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From: buckley, wa
i agree with the intake... could be internally cracked... what do you have your timing set at? with the nylon-toothed cam timing gear, as old as the engine is, i would check to see if the teeth have worn off and see if the timing chain might have jumped a tooth or two... with timing retard/advance of a few degrees or so, you could get 7 or possibly all 8 cylinders to fire great at idle, but at part/mid throttle you could notice a misfire...
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Joined: Feb 2000
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From: Sun River St. George
Try re-torquing the intake bolts! Should be 35fp with cast iron intake. Fluttering vacuum is an air leak. Do you have an EGR on this truck? Loose carb? Could be a lot of things but do the easy stuff first. My problem turned out to be the intake gasket!
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