[Mertz]

I was testing my 428 Catalina yesterday and it was running much faster than normal so much so that I had trouble stopping it. When I got home it was still running fast so I turned of the key. It continued to run fast with the key off. I opened the hood and it slowed down and finally died. It has always been a bit slow to die but not like it was dieseling just and smooth prolonged shut down. I notice some vapors at the base of the carb which might be a leak but it was fine days before with not vacuum leak. I will pull the carb and change the gasket.

Any ideas why it wouldn't die?

["70" Cougar]

i could be wrong but sounds like its getting spark to keep it running after you cut the ignition. Some residual power getting threw... . When you say running fast was do you mean it was idling high or just running amazing ( more so then usual). I don't have much jumping out at me but might as well get all the variables on the table to look at.

[JoshF]

Just a shot in the dark, do you have electric fans? Mine backfeed power when I shut it down and the car will run about 3 or 4 seconds after I turn the key.

[matt167]

ignition is probably back feeding off the alternator. probly a vac leak too.

[T-bucket23]

If it is idling to fast you could get run on after the key is shut off. You need to get the in park idle down to about 8-900 rpm or so. Once you take care of this your issues should go away. It doesn't sound like an electrical back feed as it does eventually die on its own.
Try adjusting the idle speed screw, make sure the choke is opening and the fast idle cam is out of the way. A vacuum leak could also cause high idle and run on. I would be willing to be the fast idle cam is hung up. It was a very common issue. The reason you see the fuel vapor is usually when it finally stops it turns half a turn backwards and blows the fuel back up through teh carburetor.

Chet

[Jmark]

Also, if you are running an old carb, the primary shaft may have worn the base plate severly (VERY common on Qjets) and it's hanging up and not allowing it to return to idle. Next time it does this, remove the air cleaner and try pushing the linkage shut. If it drops back to normal, you either have like Chet mentioned, a sticking choke, or a worn out base plate. They can be bored out and bushed to bring them back to good. The shaft rarely wears, just the aluminum bore in the base.

[j.d.brown.042964]

Does this setup use an idle speed solenoid (boosts idle when ac in use, or sometimes used to raise idlespeed when engine temp.gets too high)? If so it will be attached to bracket which holds it in position for an electrically activated plunger to act upon the throttleplate arm. If yours is so equipped, check to see that in the ignition off position the plunger is retracting. If it is sticking (staying extended) it will cause run-on (dieseling) from keeping throttleplates held open too far at shutoff.

[carsavvycook]

I agree with jmark and t-bucket, that the choke linkage is hanging up. Since it has always been "running on" after shut down, from the choke linkage hanging up. Another problem that comes to mind, while I am posting this, is the float, or it's level in the carb.

More thoughts;

Is it Points and Condenser, or HEI?

Does it use a ported vacuum switch for spark advance, or not?

Is your choke pull-off functional, or is the diaphragm blown?

[jimfulco]

Might also be a broken accelerator return spring. Many cars use two, and if one of them breaks, the throttle might not close completely.

[Mertz]

I have had 2 Qjets on the car. I took the first one off to put on my truck. It ran fine but still had a little run on. I do have an electric fan hooked up. I switched to another Qjet that has a temperature choke. The choke stays open all the time but it starts just fine. There are no other attachments to the carb. I have not checked it for throttle shaft movement but it did seem pretty tight when I checked it over when I got it. The vapor appeared at the base of the carb so I think there is a vacuum leak. It was at the right front corner of the carb. I have an adapter plate to go from spread bore to square bore. The first carb didn't have the fast idle problem but did run on just like the second one. Could that be the electric fan? Idle was set to 800 but doesn't want to stay there. I have a new base gasket I will install and see if that helps.

This car started with a 800 Holley which I changed out to an 600 AFB and it ran much better but still didn't seem to have the power that a 428 should have. I am trying the Qjets because I believe they can handle the bigger displacement. The first one didn't improve the power situation any. It is still slow. The car is a heavy car so I didn't expect to much but my 72 Chevy pickup with a 350 is much faster.