Recurring cold idle issues in '83 after Duraspark conversion

[Fords4Us]

Hey everyone. I've got a recurring issue with my '83 manual shift Ranger's cold idle, and it has continued despite a lot of attempts to fix it. Basic behavior is this:
1. cold start any given day, let it warm up for 2-5 minutes (the colder it is outside, the longer I let it warm up), drive away.
2. first five minutes of driving is fine. Engine is on standard high idle when I put in the clutch, and idles normally. Engine temp gauge is working up through the "low normal" range.
3. The next five minutes' worth of driving, all bets are off. Sometimes the idle runs super high (higher even than normal driving RPMs), or it's so low it almost stalls out. When it's high, sometimes I can tap the gas pedal to kick it off the high idle setting on the carb. Other times that doesn't work, and it sticks at that really high RPM despite repeated taps on the gas pedal. This period of time corresponds to the engine temp gauge having been at normal for a few minutes.
4. That behavior continues for about 10 minutes at normal temps. Then the idle settles down again such that it runs at a nice low idle without any help from me, no tapping of the gas pedal required.

Here's what we've done so far to address the problem:
1. did the Duraspark conversion in 2015, except for the carb replacement. We kept the Motorcraft 2150.
2. Instead of merely disconnecting the vacuum lines and sensors, we physically removed all the vacuum lines and sensors, and removed the actual computer itself.
3. Looked high and low for vacuum leaks. Found several, blocked them all off.
4. Tested the engine compression (both in-cylinder compression and leak-down tests) and the engine scored surprisingly good for both. The mechanic said it was the best compression test he'd ever seen on an engine this old.
5. Finally replaced the Motorcraft 2150 with the Holley carb recommended in the Duraspark conversion guide, listed in the Tech section of this website. Got the Holley dialed in and adjusted so that it runs like a top once it's nice and warm. Also note that particular model of Holley has a manual choke. That choke is generally set to half-open for startup, then moved to the wide open position before I pull out of the driveway.

We're not sure what to check next. I'm thrilled with the power and fuel efficiency I get once it's warm, particularly with the Holley. I guess I could live with the cold idle issues, but there's gotta be a reason for it. Stop-n-go driving during that behavior gets really unnerving. Any ideas how to diagnose and fix this once and for all?

 

[85_Ranger4x4]

Have you tried different choke settings?

Is there any vacuum stuff for like the evap system yet?

 

[rusty ol ranger]

Long shot....but your throttle isnt sticking is it?

 

[kimcrwbr1]

Did you block off the egr with a metal plate? I used some high temp exhaust manifold gasket material the intake manifold gets real hot from the exhaust bypass crossover from the heads! Also felpro spacer #60529 with new gaskets on both sides cures all of the leaks at the base of the carb. I make the gaskets from high quality material better than th cheap prefab gaskets. Another common leak is on the back of the egr spacer point at the firewall under the brake booster hose is easy to miss. I just use a vacuume reducer and connect it to the vapor cannister and float bowl vent I hate the smell of fuel vapors under the hood.

 

[kimcrwbr1]

Just thinking about it I found mine runs best with rich slow needle adjustment. Preset the idle air needles out 3 1/2 turns from lightly seated let it fully warm up and disconnect thevac advance and plug the hose check the timing is at exactly 10dbtdc and set idle speed around 750rpm. There should be no vac going to the distributor. Once its running good check fuel mileage and black smoke pulling a hill. Mine is running great with #42 high speed jets. i get around 17mpg winter and closer to 19mpg in the summer.

 

[4x4prepper]

Have you checked to make sure your distributor is not sticking and is working properly on vacuum?

Replace any vacuum caps with a hose, bolt inside it, with a small clamp. The vacuum caps today are junk and will crack.

 

[Fords4Us]

And the award goes to............. sticky throttle cable. We finally punted and took it into the shop, and they found that the throttle cable wasn't hanging up per se, but it was sluggish when the foot came off the gas. The return spring wasn't strong enough to overcome that sluggishness, and the carb acted as if the throttle hasn't changed. The shop put on a stronger return spring, and voila! Problem solved. Thanks for all the ideas.........