300 rough idle help please..

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300 rough idle help please..

Post #1 by 81F150 » Thu Jan 30, 2003 9:37 pm


I have a '81 F-150 Lariat with a 300. Im having some problems with a rough idle, so far ive replaced the fuel pump, fuel filter, plugs, distributor cap, rotor, put a new remanufactured carb on (stock 1bbl), and checked for vaccum leaks. The truck ran fine with no problems at all and just started this problem over the past week when stuck in stop and go traffic.

Edit: Also, the engine only has a year on it since the rebuild.

Any help would be greatly appreciated, Thanks...
Last edited by 81F150 on Thu Jan 30, 2003 9:56 pm, edited 1 time in total.


Post #2 by Guest » Thu Jan 30, 2003 9:54 pm

Might want to check your intake gasket..

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Post #3 by SuperMag » Thu Jan 30, 2003 10:39 pm

How long ago did you put the carb on, and where did you get it?
"...and he that hath no sword, let him sell his cloak and buy one." --Luke 22:36


Post #4 by 81F150 » Fri Jan 31, 2003 1:01 am

I just put the carb on last night thinking that would solve the problem since the old one was beat anyway, But it ended up not solving the problem. Checked the intake gasket and it was good.....so im still stumped :?:

Emerald 74 4X4

Post #5 by Emerald 74 4X4 » Fri Jan 31, 2003 6:33 am

When running...pull off one spark plug wire at a time off the spark plug...(be careful and hold only the rubber boot to prevent electricution :shockin: )..if a cylinder is not firing..the engine will not change in RPM..if it is..then itll run slightly rougher....you could have a bad plug wire. ..

i just did this on my friends chev 350, wwe traced it all the way back to a worn out distributor bushing. The bushing is worn out, and even with new points (which yours isnt im certainluy positive), The plastic follower on the points was geting worn out and onlythe #8 cyl. did not have spark..i adjusted the points dwell and got some spark out of it..but heck need a new dist.

anyways...good luck


Rough Idle

Post #6 by Guest » Fri Jan 31, 2003 7:20 am

Check for;

a) set/check timing; make certain that base timing is right (~6-10 degrees btdc)

b) vacuum leaks at gasketed joints & vacuum lines; use water mist/spray or equivalent to identify leaks (also, check w/ guage)

c) bad plugs can also cause, along with bad wires, cap, rotor, etc.

d) also, a bad batch of fuel can cause this; it did me once (try some fuel addative)

e) if engine will run well at high rpm but idles poorly, it is not the fuel pump! bad fuel pumps cause poor performance at high rpm no matter what the idle performance is!

f) a worn out carburetor or plugged fuel injectors can cause this; rebuild or replace


Post #7 by Unproffesional-Steve » Thu Feb 06, 2003 12:49 am

A buddy of mine had a rough idle (a loping idle actually) and he traced it back to a worn distributor. His vehicle was a 1978 E150 300-6.

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