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Have 1972 454 recently acquired . Runs on for about 2-4 seconds after shut off. Had carb rebuilt and that helped a bit but still does it Ideas or suggestions?
Is the idle stop solenoid set properly? Frequently people will set the idle with the screw instead of using the plunger
(was the car working correctly and suddenly it's not or has it been this way for a long time?)
Very good question. Answer is yes BUT 1972 compression is 8.5. Was told by mechanic that with that low compression low octane was not needed. Is that wrong?
Check your timing... check your distributer for sticking vacuum advance weight and springs.
Does it idle faster when hot? If so lower your idle speed a hair
In the meantime yo avoid embarrassment in public.......... shut it off in drive then shift to park.
Centrifugal advance mechanism needs to move freely. Need to make sure of that first. Vacuum advance hooked up to manifold vacuum will help too. Once the distributor is working right, you can reset the timing and close down the idle screw setting.
The extra timing at idle will allow closing the idle screw setting while keeping the RPM at the same speed. less idle screw setting will help cut off the extra fuel that is allowing the engine to run on.
Have 1972 454 recently acquired . Runs on for about 2-4 seconds after shut off. Had carb rebuilt and that helped a bit but still does it Ideas or suggestions?
Very good question. Answer is yes BUT 1972 compression is 8.5. Was told by mechanic that with that low compression low octane was not needed. Is that wrong?
Not wrong. With 8.5/1 you should be fine on pump gas.
From: At my Bar drinking and wrenching in Lafayette Colorado
Run-on after ignition shut-off is caused by the throttle opening being excessive at idle/shutdown. You need to lower your idle speed and decrease the throttle opening. Excessive throttle opening is often caused by retarded timing, which requires the idle speed to be set higher in order to maintain engine idle rpm. Make sure your timing is far enough advanced, and run your vacuum advance off of a manifold vacuum port to allow you to back off your idle speed screw. If this does not fix the problem, you need to install an idle solenoid. This was the factory fix for the run-on issue. The solenoid establishes the idle speed you desire, and the carb's idle speed screw is set to a very low idle speed (about 600-650). Upon ignition shutoff, the solenoid collapses, and closes the throttle to prevent the run-on, as Mooser correctly stated in his first post to you above. Again: The problem is caused by excessive throttle opening at shutdown. Mark (stingr69) nailed it correctly in his post above - everything he stated is correct.
And low-octane gas is all you need in that engine - higher octane gains you nothing.
Lars
Since you're not sure, here's the idle stop solenoid, when the key is on, that brass colored hex plunger sticking out the right end would press against the plate in front of it that the throttle cable is hooked to and raise the idle higher than where it is on the screw stop down below it in the background there
(ignore the colors and everything else that's not "correct")
M
I can only relate what I found. My 350 developed a run on problem twice.
Holley 4150 w/vacuum secondaries.Mine's a 4-speed. Going through the distributor and timing didn't net me anything. Yes a high idle can cause it but I couldn't get it low enough to solve the problem and I was worried I might ignite my air cleaner some day.
Wondering if it always did this or did it suddenly start ? Mine had been fine and then wasn't - something changed. Then one day I noticed a very slight dribble of fuel at idle out of the secondaries. I eventually found the vacuum pod for the secondaries wasn't tight to the carb body. Tightened it up with a screw stick and it was fine. A more obvious example that did the same thing was when the input side of that 3 port vacuum stop near the master cylinder let go. Big open leak caused big fuel dribble for whatever reason. Last was the intake manifold fitting that appeared fine but would leak if you touched it a certain way.
You go tearing into things you could easily insert a problem without realizing it. My best troubleshooting tools are my pockets. I put my hands in them until I had a definite plan and just watch the thing go around and around for some time.
All I'm saying is that something changed and it's not all that unlikely something due to age, maybe from sitting. I'd just have a good long look around for something along those lines. I did have the hold down yoke for a distributor actually snap on me once and it took me a while to notice that but I'd bet something more common and perishable is the culprit.
