400 Dieseling problem
Thread Starter
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
400 Dieseling problem
Hi all,
My 73 400 will not stop dieseling. My carb uses an electric solenoid to hold the idle when the engine is running, then the solenoid retracts and the throttle plates close when the power is cut. I've got the carb body-mounted idle screw so that the plates can close all the way when the solenoid cuts out...I even took out the fast idle cam to give the plates complete room to shut. Still it diesels when I shut it off, like 1 out of 4 times.
Why?! Timing is at 12 BTDC as instructed by the engine decal. I think this all started when I had changed the timing a bit earlier this summer to see if I had any spark knock. Chilton's manual says 6 while the engine decal says 12, so I've tried the timing at several degrees in between the two and I can't get it to quit dieseling!
Any help would be GREATLY appreciated.
My 73 400 will not stop dieseling. My carb uses an electric solenoid to hold the idle when the engine is running, then the solenoid retracts and the throttle plates close when the power is cut. I've got the carb body-mounted idle screw so that the plates can close all the way when the solenoid cuts out...I even took out the fast idle cam to give the plates complete room to shut. Still it diesels when I shut it off, like 1 out of 4 times.
Why?! Timing is at 12 BTDC as instructed by the engine decal. I think this all started when I had changed the timing a bit earlier this summer to see if I had any spark knock. Chilton's manual says 6 while the engine decal says 12, so I've tried the timing at several degrees in between the two and I can't get it to quit dieseling!
Any help would be GREATLY appreciated.
Thread Starter
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
I checked for some vacuum leaks. I was looking around the carb, and noticed that the end of the throttle shaft (with the fast idle linkage) looked a bit wet. Not very much though...it looked like an accumulative thing. So I ran the motor and I sprayed carb cleaner on that area and the idle went up a bit.
Is this bad enough? I'd hate to junk this carb because I rebuilt it not two months ago and there didn't seem to be any play in the throttle shaft when I checked it over. I've got an old parts carb that I can check over in the basement but it'd need a complete rebuild.
Thanks
Is this bad enough? I'd hate to junk this carb because I rebuilt it not two months ago and there didn't seem to be any play in the throttle shaft when I checked it over. I've got an old parts carb that I can check over in the basement but it'd need a complete rebuild.
Thanks
Lead Driver
Joined: Nov 2000
Posts: 6,635
Likes: 349
From: Albuquerque
Not sure the shaft would cause enough leak. Are you sure the vacuum advance hose is on and connected to the right port? If it's auto check the hose from the tranny modulator.close it off and see if idle speed changes.
Elder User
Joined: Sep 2003
Posts: 908
Likes: 4
Wear around the throttle shaft is very common and the main reason that carbs are eventually junked. If the idle went up when you sprayed it there, it's leaking plenty enough to cause the dieseling.
You might try putting some thick grease around that shaft and see if the problem improves.
You might try putting some thick grease around that shaft and see if the problem improves.
Last edited by scroob; Jun 12, 2004 at 11:55 AM.
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