High idle in 1980 L20

[Humboldt]

I am trying to correct high idle. I saw clogged Hitachi Idle something-or-another as a source of high idle and then this suggesting stretched timing chain
"Advancing the cam back into correct time will improve idle, off idle and low mid range power and torque."  Does "improve idle" mean tame it down? I am thinking of the troubleshooting advice to suspect carb last.  BTW I removed the Cat. (regretting, with backfires). Thanks for help!

 

[datsunaholic]

1) Make sure the throttle plates are actually closed.  Stuck linkage/cable or the idle speed screw set too high is the usual culprit.

 

2) Choke not opening (choke heater disconnected, not working, or misadjusted) also a common cause

 

3) If the above aren't the cause, and/or trying to turn down the idle speed screw makes it die at a too high level (such as idles at 1200 rpm but dies just below that) it's usually a vacuum leak.  Fixing that usually then means resetting everything because it ends up rich (due the the vacuum leak making it lean then correcting the wrong symptom)  Vacuum leaks are often caused by cracked vacuum hoses, carb base not tight, leaky intake manifold, bad brake booster, or owners removing emissions stuff without understanding what they were doing and causing a vacuum leak via unplugged/misrouted hoses.

[datzenmike]

Primary throttle shaft hole worn oval and throttle shaft or plate rubbing casing an sticking

Be sure to check the secondary plate as well, it can stick or have something jammed in it.

 

Choke on will engage the fast idle cam. Cam may also be stuck or the linkage.

 

Throttle cable too tight, holding it open.

 

BCDD set too sensitive and not shutting off after deceleration. Turn adjustment screw counter clockwise to lower trigger point vacuum.

[Humboldt]

I cleaned the carb plates with Q tips and gumout carb cleaner. They were filthy with black carbon. I tried to lube the throttle cable. I scraped the corrosion off the rotor and points in the distributor using a file. There was a little play in the dizzy shaft. Normal? I bought a new gas cap, because the old one had the gasket too worn to seal. I thought it might  be messing vacuum up if the fuel line/return was a closed loop. I bought a valve cover gasket because it appears to be leaking oil. Would that screw with the vacuum?

So far, no success. My headache began after I returned from Sturgis mini-tour drumming with a rock band and the truck had sat for 3 weeks. The truck ran like shit until I realized the distributer timing screw was loose and the thing was wobbling all around. I set the timing by what seemed by ear to be the best and tightened it back up. I will get a timing light and get back at it tomorrow.

What and where is a BCDD?

 

Thanks, again for the tips for a novice mechanic.

[Humboldt]

Here's the progress report: As well as the stuff mentioned above, I wd40'ed all cables and moving parts. I advanced the timing. I tightened the one 12mm nut I could get at that holds the carb down. I plugged two threaded holes that had nothing in them (10 mm.)  I don't think the 1980 720 Hitachi has a BCDD, which I did a search and found out what it was. It seems to be running better. Idle settles down and I didn't have run-on when I shut it down. (This time . . . keeping fingers crossed)

Thanks again for all suggestions and help!

[datzenmike]

Federal and California L20Bs did have the BCDD on the left (driver's) side of the carb. It's the thing sticking out at the bottom of the carb with all the Philips screws around it and the black rubber plug in the center.

 

[Humboldt]

It's there, phillips screws and all but there's a vacuum line coming out of it where the screw would be.

[datzenmike]

Yup some have that instead.

[Buzzbomb]

Be sure to check the secondary plate as well, it can stick or have something jammed in it.

 

I've had this happen more than a few times.. It's the first thing I check if I feel my idle is "sticking". You pull the throttle forward with the engine off and then push the linkage for the secondary down to make sure it's closed.