Small block diesels when I turn the key off

Posted By: VITC_GTX

Small block diesels when I turn the key off - 09/07/12 05:46 AM

5.9 Magnum that I built to copy the old 380 horse crate motor. It has an Edelbrock 650 AVS and runs great (12.76/104.5 in the quarter). When the car gets to 180-190 degrees it diesels when I turn the key off. Current idle is ~750 rpms. What's causing this?

Posted By: 70Cuda383

Re: Small block diesels when I turn the key off - 09/07/12 05:52 AM

Usually too much idle rpm or initial advance. What octane are you using?

Did you have to open the throttle too far to get it to idle?

Posted By: VITC_GTX

Re: Small block diesels when I turn the key off - 09/07/12 06:02 AM

Running 87 octane with no know pre-detonation. I think the initial timing is ~15 degrees.

I don't know about the idle adjustment. I took the carb out of the box and the only adjustment I made was the idle screw. Don't know how far I needed to adjust it (few years ago).

Posted By: 70Cuda383

Re: Small block diesels when I turn the key off - 09/07/12 06:38 AM

You say there's no detonation but that it's dieseling? I bet it is detonating but you can't hear it. I run my 5.9 with 93 octane. My cam is a little smaller, at 222/228, on a 107lsa, but I'm also running thinner than stock head gaskets and have heads shaved .030 to bump compression to 9.5.

I'd consider at a minimum to try 89 octane and see if it makes a change.

Posted By: 340SHORTY

Re: Small block diesels when I turn the key off - 09/07/12 06:41 AM

back the timing up a few degrees.

Posted By: Mebsuta

Re: Small block diesels when I turn the key off - 09/07/12 12:09 PM

My RR does that with 89 octane. Have to use 91 or 93.

Posted By: YO7_A66

Re: Small block diesels when I turn the key off - 09/07/12 12:18 PM

I would suggest the higher octane or a colder spark plug.

Posted By: MoparforLife

Re: Small block diesels when I turn the key off - 09/07/12 12:54 PM

The most common thing that causes engine run on is idel speed too fast. Timing retarding the timing slightly will tend to slow the idle speed down and is the reason that setting teh timing back at times works to control run on. As for timing itself causing it it can't. Went the key is turned off the spark to the ignition is gone so timing from that point is a none function.
Detonation or hot spots is another cause to look into.

Posted By: Osyman

Re: Small block diesels when I turn the key off - 09/07/12 01:13 PM

Is the throttle return spring tight enough to bring it all the way back? I think the heat from the engine can heat the spring up enough at op temp that it can loosen up a little.

Posted By: bigsbigelow

Re: Small block diesels when I turn the key off - 09/07/12 01:17 PM

My grandfather's MP Magnum 406 crate motor dieseled, also. I swapped what ever plugs it had in it for a quality brand stock 360 Magnum plug for a Durango and it stopped. We always run pump premium 93 fuel.

Posted By: radar

Re: Small block diesels when I turn the key off - 09/07/12 01:57 PM

If you shut off in Drive it will stop spinning immediately.

Posted By: 70Cuda383

Re: Small block diesels when I turn the key off - 09/07/12 05:23 PM

Quote:

As for timing itself causing it it can't. Went the key is turned off the spark to the ignition is gone so timing from that point is a none function.
Detonation or hot spots is another cause to look into.

Unless the timing is too far advanced that it's making the engine run hot, and creating hot spots in the chambers, which is causing pre-ignition/detonation at all throttle and load conditions, but he just doesn't hear it or feel it until after he turns the key off and it keeps running for a few seconds.

but, since he said he's on 87 octane with a "performance built" 360, I say first thing to do is jump up to mid-grade or premium to see if it cures it.

Posted By: kilroy

Re: Small block diesels when I turn the key off - 09/08/12 03:36 AM

Timing is not an issue when the key is off. There is no timing involved. THERE IS NO SPARK when the KEY IS OFF!!! REPEAT THE IGNITION SYSTEM IS DEAD WHEN THE KEY IS OFF. Its caused by to much air/fuel hitting hotspots in the chambers and combusting. The air/fuel excess is caused usually by the throttle blades being opened to far in an attempt to bring the idle speed up so the engine wont stall. This usually needs to be counteracted by (contrary to what most everyone here is saying) increasing, YES, INCREASING initial timing so that idle speed is brought up and throttle blades in the carb CLOSED down to stop a surge of fuel being pulled in at idle. Or by using an idle solenoid which releases/retracts when the power is off, there by allowing the throttle blades to close cutting off the air flow, there-by stoping any vacuum signal to the carb and stoping the fuel flow.

Posted By: kilroy

Re: Small block diesels when I turn the key off - 09/08/12 03:41 AM

Of course increasing intial timing will then lead to the distbutor needing to have total timing limited to stop the pinging/detonation....

or the use of the afore mentioned idle solenoid.

Posted By: RobX4406

Re: Small block diesels when I turn the key off - 09/08/12 03:50 AM

Quote:

Timing is not an issue when the key is off. There is no timing involved. THERE IS NO SPARK when the KEY IS OFF!!! REPEAT THE IGNITION SYSTEM IS DEAD WHEN THE KEY IS OFF. Its caused by to much air/fuel hitting hotspots in the chambers and combusting. The air/fuel excess is caused usually by the throttle blades being opened to far in an attempt to bring the idle speed up so the engine wont stall. This usually needs to be counteracted by (contrary to what most everyone here is saying) increasing, YES, INCREASING initial timing so that idle speed is brought up and throttle blades in the carb CLOSED down to stop a surge of fuel being pulled in at idle. Or by using an idle solenoid which releases/retracts when the power is off, there by allowing the throttle blades to close cutting off the air flow, there-by stoping any vacuum signal to the carb and stoping the fuel flow.

In my best sarcastic voice...

So what you're saying is it's the not timing, but, the timing!!!

I vaguely remembering writing the same type of suggestion recently. The timing settings have a lot to do with this.

Posted By: roe

Re: Small block diesels when I turn the key off - 09/08/12 03:51 AM

My 360 did that on 87 octane. Stepped up to 93 with no other changes and it stopped

Posted By: 70Cuda383

Re: Small block diesels when I turn the key off - 09/08/12 03:59 AM

Quote:

Timing is not an issue when the key is off. There is no timing involved. THERE IS NO SPARK when the KEY IS OFF!!! REPEAT THE IGNITION SYSTEM IS DEAD WHEN THE KEY IS OFF. Its caused by to much air/fuel hitting hotspots in the chambers and combusting. The air/fuel excess is caused usually by the throttle blades being opened to far in an attempt to bring the idle speed up so the engine wont stall. This usually needs to be counteracted by (contrary to what most everyone here is saying) increasing, YES, INCREASING initial timing so that idle speed is brought up and throttle blades in the carb CLOSED down to stop a surge of fuel being pulled in at idle. Or by using an idle solenoid which releases/retracts when the power is off, there by allowing the throttle blades to close cutting off the air flow, there-by stoping any vacuum signal to the carb and stoping the fuel flow.

Do me a favor, re-read what I said.

You are absolutely correct, when you turn the key off, the SPARK PLUGS STOP FIRING. so once the key is off, spark advance from the timing has NOTHING to do with run-on.

but, if the engine is running too hot and has hot spots in the chamber because the timing is TOO FAR ADVANCED, (admit it, if the timing is too far advanced, you're creating more heat than you need to be, and the engine will run hotter) then even if the throttle plates are completely closed, the engine can still diesel.

does the idle circuit not pull in fuel and air? I think the answer is yes, it does....otherwise, the engine would never idle even with the key on.

retarding the timing a few degrees can make the engine run cooler and stop the dieseling. --it will also hurt performance though, so another solution is to run a higher octane fuel, with more resistance to detonation, which can stop the dieseling.

again, go back and read my post that got you all fired up.

Posted By: Cab_Burge

Re: Small block diesels when I turn the key off - 09/08/12 04:08 AM

Quote:

If you shut off in Drive it will stop spinning immediately.

I have to do that with my pump gas street motor that has 10.78 to 1 comp. artio at the track after a run, leave it in gear and shut it off

Posted By: kilroy

Re: Small block diesels when I turn the key off - 09/08/12 06:30 PM

No I agree with the statement of hot spots, BUT, TO MUCH FUEL is the problem. Yes you pull fuel through the idle circuits BUT, again most of this is caused by the throttle blades being open to far up the transition slots in the throttle plate or even so far that the carb is even running on the main circuits which will pull, again, TO MUCH FUEL at idle causing said dieseling. The proper idle adjustment would be to take carb off and set front throttle blades so that transition slots are square or just over, than start INCREASING base timing somewhere in the neighbor hood of 13*-22* till the thing idles well. This will also clean up the idle emissions and help off idle hesitations. ITS ALL RELATIVE TO THE THROTTLE PLATE OPENINGS. This can also be caused if the rear butterflies are open to much. Just trying to help people tune their cars properly.

IT almost always because we all like our cams to rumble which causes us to open up the idle screw to make it idle which exposes these much misunderstood transition slot and then the motor just pulls way to much fuel all the time.

Posted By: kilroy

Re: Small block diesels when I turn the key off - 09/08/12 06:30 PM

Quote:

In my best sarcastic voice...

So what you're saying is it's the not timing, but, the timing!!!

I vaguely remembering writing the same type of suggestion recently. The timing settings have a lot to do with this.

Posted By: Kern Dog

Re: Small block diesels when I turn the key off - 09/09/12 04:02 AM

The 493 in the Charger had a dieseling problem for years. 10.8 compression and aluminum heads. I always run the highest pump octane which in CA is 91. I switched from RC12YC plugs to 9YC and the problem instantly stopped. Shutting off a car in gear feels like a cop out to me. an engine shouldn't sputter and run on after turning it off.