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Dieseling- won't shut off

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Mudstud

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Got a 76 K5 with a 350 that doesn’t want to shut off. Factory HEI ignition. It runs like a beast but then sits there and diesels forever when I shut it off. I have adjusted timing several times/ different ways and confirmed it is holding the timing. I have an idea of what it might be but before I toss it out here I wanted some other opinions on what you guys think. Hate throwing money at a problem on little more than a hunch.

Thanks
 
Unless your dizzy is getting power while the key is off the only thing that can cause this is hot spots in your engine due to carbon buildup. Too high of an idle or a rich engine can cause this.
 
It's idling at about 600. I've recently overhauled the carb but this issue was happening before that. It's been adjusted by a guy I trust. The thing that I was told is that the springs under the cap may be getting weak and the cintrifigal force is keeping them stretched out allowing contact even after it's shut off. You thoughts on that?

How can I rule out the possibility the dizzy getting power after shut down?
 
This became an issue in the 70's after higher combustion temparatures were introduced to lower emissions..

GM used idle stop solenoids on carbs to set the idle speed,and allow the throttle butterflies to close fully when you turned off the ignition,and that would cut off any air supply,so hot spots in the combustion chamber were less likely to ignite any leftover fuel coming in the cylinders..

I doubt your distributor gets power after turning the key off,otherwise you'd eventually drain the battery down after sitting awhile,and it would not try to shut off at all most likely...

As for the advance weights,I think retarted timing increases the chances of dieseling more than advanced,I could be wrong,but retarted timing can cause things like exhaust valves,spark plug electrodes,and exhaust manifolds to get red hot..-also you'd notice hard cranking during hot starts,if the weights were stuck in the advanced position..it would probably ping under a load too..

The simplest "cure" is to leave the truck in drive when you shut it off,and decrease the idle speed as much as possible,without it stalling in drive helps a lot too...or if its a manual trans,let the clutch out in high gear after turning off the key..
 
diesel4me said:
This became an issue in the 70's after higher combustion temparatures were introduced to lower emissions..

GM used idle stop solenoids on carbs to set the idle speed,and allow the throttle butterflies to close fully when you turned off the ignition,and that would cut off any air supply,so hot spots in the combustion chamber were less likely to ignite any leftover fuel coming in the cylinders..

I doubt your distributor gets power after turning the key off,otherwise you'd eventually drain the battery down after sitting awhile,and it would not try to shut off at all most likely...

As for the advance weights,I think retarted timing increases the chances of dieseling more than advanced,I could be wrong,but retarted timing can cause things like exhaust valves,spark plug electrodes,and exhaust manifolds to get red hot..-also you'd notice hard cranking during hot starts,if the weights were stuck in the advanced position..it would probably ping under a load too..

The simplest "cure" is to leave the truck in drive when you shut it off,and decrease the idle speed as much as possible,without it stalling in drive helps a lot too...or if its a manual trans,let the clutch out in high gear after turning off the key..
Click to expand...


Good stuff!. The battery doesn't drain. It is an automatic. It sounds like you don't think its the timing weights then?
 
You can take a voltmeter to the dizzy power connector to verify it's not powered. For that matter, you can just unplug it and see if that is any different than turning the key off. You could try getting the idle lower and installing a throttle stop solenoid.

Then only time I had a SBC diesel was when it got really hot.
 
Mudstud said:
The thing that I was told is that the springs under the cap may be getting weak and the cintrifigal force is keeping them stretched out allowing contact even after it's shut off. You thoughts on that?
Click to expand...

I doubt it, but you can verify.

If the springs are that weak/worn, your *base* timing might be right but your *overall* timing should be whacked. Google "recurving distributor" and find the "Timing 101" PDF file, you'll learn more than you ever wanted to know.

Unless maybe whoever set the springs in there was getting clever with the springs and put in weak ones intentionally, but I've never heard of them being that weak from the get-go.

-- A
 
Thanks gents, I'll play with it some this weekend. Wouldn't my carb have a stop selinoid all ready? Its the original carb but came on a 400.
 
It might. I think some years they were only installed on A/C trucks.
 
I've seen some Q-jets with a idle stop solenoid,and a second solenoid that kicks up the idle speed when the A/C compressor comes on...

If you can even find an idle stop solenoid,you'll need the bracket for it also,which is a salvage yard only item I am pretty sure...the solenoids new can run over 100 bucks now,if one is available...
I'd try junkyards first,maybe you'll be lucky and fine one still intact on something--just about all the Q-jets in the 73-80 range should be the same or close enough to work..
 
Mine did come with factory AC but I don't have anything hanging off the front or side of the carb so don't think I have the idle switch.
 
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