I've got a 1986 ford f 150 with the carbureted straight six 4 speed with the bull low. it idles fine but at low rpms when not giving it gas it stutters, it works itself out in the higher rpms and runs fine. so far I've put new distributor cap and rotor new wires new plugs. new fuel pump [mechanical], rebuilt the carb, fixed all vacuum leaks along with the vacuum advance on the distributor and got no where, any ideas?

 

A 1986 6-banger carb is computer-controlled, so pull the codes and see what the computer thinks is going on.

Here's how:

Fuel Injection Technical Library » How To Run a Self-Test


Note, there are three areas from which codes need to be pulled:

KOEO
CM
KOER

The order of codes appearing is important, as is from which category they appear. Post what you find....

 

Originally, on a 1986 4.9L, there would be no vacuum advance, since it was computer controlled as Chris stated.

Is this truck "new" to you? Has the "stuttering" just begun?

 

i bought the truck 1 year ago and have been driving it for 8 months or so it always had this stutter since i got it. there is no computer attached to the carb it is completely mechanical so i cant get any codes off it, it has to be somethin simple cause it is such a simple engine, unless something is wrong inside the carb like the pump [which i just changed] all im left with is timing or bad valves

 

I have a similar problem on my 1982 F100 4.9L. Less than 1500miles on rebuilt 4.9L, Carter YFA is rebuilt and totally mechanical/vacuum operated, new PCM on drivers' fender powering new Advance Auto Duraspark dizzy, MSD Blaster coil, 8mm wires, sparkplugs, etc...

At idle it'll "miss" a cylinder. I'm wondering if in the engine rebuild I missed a voltage regulator or something that's causing a momentary cut in spark. It's tough to start cold as well, even with the electric choke as well...

 

does yours run fine fine when accelerating, mine will only miss when accelerating when you first come on to it at 12 or 15 grand not sure dont have a tach but if i hold it at that specific rev then it ll keep on missing untill i either let off, as it runs at idle very well, or rev the engine more where it will also run better. its just such a pain in the *** because you cant just cruise along

 

Perhaps the idle air mixture screw is not adjusted correctly? If it is lean, you might have a low speed stumble. The idle circuit does not affect high speed operation.

 

i set the idle control srcew according to the truck at 1 and 1/2 turns out from completely closed off, there are only two screws to adjust on that carb that one and a idle speed screw

 

There should be three...

1: Idle Speed Screw
2: Fast Idle Speed Screw
3: Idle Fuel Mixture screw

1 and 2 are located on the left of the carb on the drivers side (USA), and the third is located on the right side in the throttle body.

On certain feedback carbs the Idle Fuel Mixture screw will set base fuel mixture and will affect all drive ranges.

 

this one only has the two screws one on the throttle to adjust idle speed and the other on the base of the carb

 

The 1 and 1/2 turns for the adjustment screw is a routinely recommended starting point. From there, each carburetor may require additional adjusting.

I normally use a vacuum gauge and adjust for the highest speed. Some then recommend turning the screw counter clockwise [lean] an additional 1/4 turn.

The carburetor on my truck [Carter YF from a 1970 F350] also has only two adjusting screws. The latter model carburetors do have the additional screw for fast idle adjustment.

 

Sounds like a plug issue to me. A miss could always be interpreted as a stutter and at low RPM's you will notice it quite abit this is why in high RPM's it seems to disapear. I would remove all of the spark plugs and see if the have a lot of carbon build up or even your coil may be bad and arching to the block somewhere causing timing issues.

 

i replaced the spark plugs in the truck with second hand ones out of the spare motor i got. but one of the old plugs the ceramic was extremely loose and a couple of electrodes were worn but replacing them did next to nothing so that led me away from thinkin it was plugs could be coil i suppose

 

Ok so here is how mine went...
I always would have to start it and almost have to warm it up before she would idle straight. So I tampered with the idle screw and got it where I thought it would be alright. then the next morning I would be going up this big hill about 15 minutes driving and the thing would bog down be irreversable and die and would take me forever to get her started again. To fix this I removed my choke plate now every morning I get to pump the thing oh well. So we were good for a little while, then I would get a little stutter and hesitation out of it so I did the same as you replaced cap and rotor thinking "Hey I whipped this thing!!!" Absolutely not I had the same problem opened my hood to hear a little arching tick replaced the wires and laid them so none were overlapping and well she runs great now.. But I have seen the coil arch to the block if the resistance in the wires is to great and if this happens once it can leave a carbon trail which allows it to arch with even new wires. I would just do plugs and if you buy a new coil an MSD is like 40 bux and you can gap your new plugs to a crazy amount and youll get a little better bang from your fuel. And my last thing timing I would recomend finding your timing mark and putting some silver finger nail polish or toolbox marker on it so you can see it well with the gun.

 

Fuel, air, compression, and spark are needed in the appropriate amounts at the appropriate times for a motor to run.

Start by checking for vacuum leaks. Make sure the carburetor is tightened down all the way. Check all of your vacuum hoses and lines to make sure none are cracked. If you hear whistling, hunt down the source of the noise. If your brake booster is leaking, replace it. Vacuum leaks can cause hesitation, stumbling, lean conditions which result in popping in the exhaust, and a lack of power. It can also cause your motor to run hot.

No vacuum leaks? Good. Check for spark. Are all your plugs in good shape? Wires? Cap and rotor? Good. Is the firing order right?

Now check for fuel. Look down the throat of the carb and work the throttle by hand to see if the accelerator pump is working. You should see a strong jet of gas every time the throttle is worked. How is the choke? Are all of the choke components there and functioning correctly? Is everything adjusted right? You'll want to use a vacuum gauge hooked to manifold vacuum to check your fuel mixture at idle with the motor fully warmed up. Adjust for highest vacuum.

Once you've got all these taken care of, your motor should run and run pretty decent. You'll want to get a timing light and check your timing. Or, learn how to time it by vacuum. Make sure your vacuum advance (if you've got one) is hooked to the right vacuum port (should be ported vacuum for this year of vehicle).

This is just a general guide to check all of the basic stuff. If, after checking all the basics, your motor still dies/doesn't run correctly/has a miss, etc. you'll want to inquire further and let us know what it is so we can diagnose it more appropriately.