Name that Idle!

Ok, it's stump the stars time, so you gearheads dive in. Here's the issue:

Started car today, ran fine as usual. Made a run accross town via freeways and car is up to temp on a 95' day. Not hot, in fact it runs about right temp wise all the time with the external cooler and thermostat.

Coming up an off ramp and stopping at a light the RPMs are whirring, I mean around 2500-3000. I'm instantly thinking stuck throttle cable and pull into the next open parking lot. Cut the ignition and check the cable.... no problem there, and it's still on my (painted) mark. But to make sure I loosened the cable retention screw on my linkage ar, (dual Weber 40s with Hex Bar). Started the car (the engine was still at temp) and the idle is still up there, but not quite so high.... 2000-2500. Again, there was no binding on the linkage and I hand throttled (revved)it back and forth, checking that it (linkage) was coming all the way back.

Now the car was running smooth all this time, no ignition or carb problem that I can tell. But I screwed in all the mixture screws, then came out to basic settings and it still idled at high RPMs, but seemed to be getting a lower RPM as the car cooled. I checked the idle stop screws, no problem there. Sat for a few minutes and talked to a buddy about it on the phone. While it cooled I checked for loose hoses, etc. (Distributor is 009 Compufire, no vacumn). Anyway the car cooled back to near OAT, and then started and idled at 900 RPM, where it normally does. Drove it back home and the idle now was at 400 rpm and stopped at the last two lights. Got home, added a quarter turn to mixture screws abnd got a nice 800-900 idle.

Again, the car runs smooth, no pops, backfires, misses. I am letting it (car and OAT) cool off before I mess with it. Any ideas. Vacume leak? Or maby the valves tightened up with the heat? I have ran about 3100 miles since my last oil and valve train adjustment, but never have had problems in that before. Ideas?

Jim Ward

Slipping fan belt?? Mine chucked a belt and the hotter it got the faster it idled. Just a WAG
Bill

Thanks, just checked that and belt is good. Besides the car never ran hot per se... it ran normal temps throughout. It's just the higher rpms occured after the engine was had ran up to op temps.

Jim

HMMMM - It sounds like a vacuum leak, but if the idle mix adjusters work while it has the attitude it sorta rules out vac leaks. That's why I thot maybe a slippy belt. Back to head scratching...if I have any revelations - will let you know.

I would also vote for a vacuum leak, probably between the manifold and head (highest temperature gasketed interface).

Anyone who read Flying Magazine in the 70s-80s recalls Duane Coles columns and the great "I Learned About Flying when..... variety of mistakes and situations)

Well I learned (again) about troubleshooting from: see the opening post. I too was locked into a vacumn leak... and proceeded to look hard into that. Retightened the mainifold and carb mount nuts on the 40IDFs set up. Replaced the 3/16" rubber vacumn caps on both carbs. And still after a warm up idle at the standard 8-900 RPM level, the engine would warm idle at 2500 RPMs!!!! I was perplexed and was about to resort to pulling the mainifolds and carbs and replacing all gaskets.... but sometimes I find stepping away from a problem helps. So, I put in new plugs, changed to some 8mm wires I had stashed, and changed oil * filters/screen. Opened a beer and was staring at the engine wondering.... THEN I SAW the obvious. I was staring at the idle screw setting on the right carb when I happened to see it RIGHT before my blind eyes. The right arm from the Hex bar to carb linkage was canted left (inboard... I walked up and felt it... loose! It was stuck on an angle and binding, also causing the whole linkage to engage the carbs early, even with the center cable arm in a full back position. NO friggin wonder I was idling at 2500 rpm! What a dummy thing to have missed. I had played with cables, mix and idle settings, timing, you name it.... and there was the problem plain in sight. Reset the arm, tightened it. Set the mixture screws for an initial 1.5 turns, got in and viola! Ran like a champ. Reset timing fine tuned mixture and she runs great again.

I learned (again) about trouble shooting this lesson. DO NOT assume a cause BEFORE you give the whole engine a visual. Stand back, look away, look again... sometimes the problem is so obvious that you look right past it. Of course I am admitting to a basic mistake... making myself a public dummy. But, maybe my lesson will help one of you when you start looking PAST the obvious because the symptoms seemed to point elsewhere.

Dummy from OKC
(Message Edited 7/31/2003 12:57:03 AM)

Jim,
Cut it out, bro, you're no dummy! Very good advice given, as always. BTW, I did used to read "I Learned About Flying From That" in the eighties. Good stuff.
John H.

Jim, thanks for the great advise. Too many times I have done the very same thing. Sometimes our brain works faster than our common sense,and the result is over looking the obvious. I appreciate you taking the time to offer this gentle reminder. Eddie

Jim cites a good exmple of "mental set", e.g., we assume one thing and so never see others that are right in front of us. It's a good idea to always look at everything related to the suspected problem source with "innocent eyes" before proceeding to any particular mode of action.