Having some intermittent idle issues with my 1984 F150. It has the 300 Straight Six with a mostly stock ignition and fuel system (EEC IV/TFI, Carter YFA feedback carb).
Symptom: sometimes it idles very low to the point of stalling (especially if the A/C is on and in gear, no surprise). I misdiagnosed this as a throttle kicker issue (which uses four wires and is ECU controlled, not a single wire A/C only kicker), and have verified that the new kicker is indeed working.
I’ve rebuilt the carb, and adjusted the idle mix screw as Eric demonstrated in one of his videos (done during a warm idle that wasn’t low and messed up, adjusted for max vacuum). I also replaced the fuel line and filter for a bit of hose and a clear Cartridge-style filter since the old line went directly over the engine and was shortened with a bad flare job (trying to catch/counteract heat soak). The hose does get hot (carb and mechanical fuel pump are on opposite sides of the engine) and have seen fuel boil off; however, I’ve also had the idle issue with the filter full of liquid so I am lead to believe that such isn’t the issue.
The choke is also modified a bit. Originally, it used a dual-heat setup (like Eric’s original Fairmont engine). However, when I bought the truck last month I found that the heat tubes were rusted away so I Capped/plugged the lines to prevent a vaccum leak. If anything it seems like the electric half of the choke opens too quickly. I kept the original adjustment from before I rebuilt the carb (indicated with paint marks)
Also, it seems like the jump between the fast idle (only on a very cold start) and the next step (supposed to be four steps, but I only ever see two of them. The first is screaming fast (no Tachometer, but sounds like over 2000RPM), but as soon as I tap the gas it drops a lot lower. During a long (ish) trip today it went from bad idle, to working fine, to bad again all within a 1 hour drive.. so I think this “issue” isn’t related since all of that was at operating temp. Usually the issue is at operating temp, although it happened this evening with a cold carb and warm engine (had the hood up while swapping the throttle kicker and doing other unrelated work).
When throttling up it usually does fine… occasionally it lugs but letting off the gas and punching does the trick.
Ignition: replaced the cap/rotor, wires, and plugs (all needed replaced in a very bad way), used stock motorcraft copper plugs. Replaced a failed (and incorrect) TFI module and repaired some messed up control wiring. This corrected an intermittent no-start condition. Due to one of my timing lights being on “permanent” loan and the other MIA, I haven’t set base timing yet. As I type this I realize that this might be more of an issue than I thought???
Engine health: It has 102K on it, one cylinder might have a slight amount of blow by (based on the intermittent puffs of oil vapor from the vent tube when the hat is off). When I swapped the plugs all looked healthy (medium-dark brown). I haven’t done a compression test or leaked anything down (I have no tester or compressed air). It does sometimes try to diesel on power off (I assume this is somewhat normal for a carbed engine, right?).
Suggestions on where to look next?
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