runaway idle?

[rigel]

So the engine on my mostly-rebuilt auction car (87 carb'd 4at) starts at a really high idle (3-4000 rpm) and the rpm just keeps creeping up, even after the cooling fan comes on. I've connected all the vacuum lines correctly (I think) and replaced a couple that had obvious holes in them. The throttle stop screw doesn't appear to do anything regardless of which way I turn it. (Which way should I be turning it to lower the idle?)

At first I thought it was the pcv valve because it wasn't hooked up to the breather chamber, but it is now and it actually idles higher now.

Tapping the accelerator doesn't disengage the fast idle as it's supposed to.

The usdm manual isn't much help.

Short of a complete carb rebuild and testing/replacement of all the vacuum lines, where should I be looking?

[Dr_Snooz]

Check the adjustment on your throttle cable first. Also check its condition. If it is kinked or rusty, the cable will hang up and cause what you are experiencing.

[rigel]

throttle cable is slightly slack at idle. (eyeballed the 3mm but did that already)

[rigel]

Got an extra throttle cable, but I noticed at the jy that I was missing frequency solenoid valve a and air control valve a. (It did seem like I had a pretty remarkable amount of maneuvering room....)

Doh. reassembled. Testing shortly.

[rigel]

so, reassembled properly, and started digging into the USDM manual's carb/emissions diagnostics.

first thing i've found that is bad is the choke pull-off diaphragm. has a hole, wont hold a vacuum whatsoever.

went to the yard, pulled three of them. only one was obviously, definitely good.

installed. started engine, rpm's went up and it stalled. tried that a couple times and then realized i forgot to install the spring.

oy.

so this appears to me to be the most likely culprit, because at least it isnt starting with the idle rpms climbing.

i guess since the choke wasnt being released it just got stuck at high idle? not sure why it would climb but im still sorting out the vacuum line system.

if theres anything else wrong other than, say, adjustment screws after this i'll be surprised. i probably should rebuild the carb, but im not gonna until after it's on the road.

[Dr_Snooz]

A rebuild involves replacing the floats and needle valves. It's not a bad idea, but it's much more important to make sure that all the controllers bolted onto the carb work properly. Those seem to fail a lot more often than floats or needle valves.

Congrats on your good sleuthing, BTW.