[SWHEATON]

this has happened twice now with my 69ss, both times after an extended drive ( 30+ minutes ). I've pulled it into the garage and it has run on for 3-5 seconds after shutting down.

Is this a timing issue or fuel related? Maybe this a dumb question, but i just don't know what causes it or how to attck the problem. The 2 incidents have happened about 6 weeks apart, so there has been trouble free driving in between.

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Its likely caused by 1 if not 2 or more of items listed below collectively together causing the dieseling.

* overly retarded base timing.(Often need approx 16-18 deg base timing with even mild aftermarket perf cams)

* no vac adv or running or running it on ported not full int vacuum so no additional timing at idle also retarding timing by a min of 10-12 deg if not more.

* too low of octane fuel for compression/cam combination causing too high of cyl pressure.

For example,GM stated even middle farly mild pre 71 motors like for ex 396-402/325hp-330hp-350hp bbc with 10.25.1 comp had to run on a min of premium 98 octane or higher leaded fuel and not todays non leaded lower octane pump fuel with 10% ethenol added that thse 40+ yr old gen muscle car sbc/bbc motors were never designed to run on in the 1st place!

* motor running a bit hot.

* spark plug heat range too hot.

* excessive lean fuel calibration setup or vacuum leak esp at idle and lower engine speeds just above idle causing higher cyl/engine temps.

* too high of idle speed.

* excssive carbon buildup in combustion chambers increasing compression & combustion chamber temps while also making the excessive carbon buildup in comb chambers turn cherry red creating hot spots that can ignite incomming fuel from carb only setups (not efi) even when power to ign is turned off for no spark at plugs.

Etc!.

Scott

 

[SWHEATON]

if your not getting detonation and its an automatic car just shut it off while in gear and it wont stumble it will shut off fast. i had a 72 duster with a 440 that did that.

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All thats doing is bandaiding the root cause which is generally issues with tuning/timing/fuel calibration/etc.

And when motors diesel when trying to shut down they are often more prone to detonation too due to out of wack/not optimal tuning issues which can destroy a motor in a heartbeat when under max/hard load @ WOT!

So going the shut down in gear route isn't the right way to go here IMHO,he needs to find root cause & fix it right which shouldn't be that hard to do .

Scott

 

[SWHEATON]

Are you running a Quadrajet? Some had a solenoid that closed the butterflies completely shut because of problems GM had with dieseling. Had a 71 Impala that did that, fixed the wire to the solenoid which is right on the carb near the choke and no more dieseling.

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GM often ran the solenoids back then (1971 & up) to stop diesleing because they were running hot 192-195 deg stats along with retarding base timing (some vac adv to ported retarding timing even more at idle too) and leaning out fuel/carb calibration too .

Why?

It was all done in an effort to collectively increase exhuast/motor temps enough to reduce certain exhaust emmissions due to EPA/Gov regs ,none of it was done to increase or help engine perf because doing all that decreased engine perf.

That was just another example of a bandaid to cover up the diesleing issues the detuning caused which GM along with other car mfg's did back then to reduce certain ex emmissions that significantly reduced idle/low speed perf & driveabilty.

Scott

 

[66 chevelle 496BBC]

As said before, I bet you have a vacum leak. idles don't just change by themselves. a high idle will cause your situation. hit the intake area with some starting fluid to find it.

jim

 

[Dan Orgill]

Thanks again for all the responses. As soon as the weather clears I'll take the car out of the garage and see what's what.

As far as vacuum goes, I'm running vacuum advance hooked up to the carb, like the factory did. The timing was originally set at 18 degrees initial, 36 total, seemed to like that best. I'll have to recheck that, also check the nut on the dist. hold down if the timing is off. I'll yank a spark plug as well. If I'm not sure what I'm looking at, I'll post a pic.

Anything else I can hit the intake with besides starting fluid? It's a detailing thing, I don't want to mess up under the hood ( see pic in my signature ).

Scott, the motor runs a steady 185-190 all the time, even in the summer heat we've had. As far as the carb goes, it's right out of the box except for a change to the secondary spring to the lightest one. No changes to jetting or anything.

 

[VORTECPRO]

This has happened twice now with my 69SS, both times after an extended drive ( 30+ minutes ). I've pulled it into the garage and it has run on for 3-5 seconds after shutting down.

Is this a timing issue or fuel related? Maybe this a dumb question, but I just don't know what causes it or how to attck the problem. The 2 incidents have happened about 6 weeks apart, so there has been trouble free driving in between.

You would either need a idle soleniod, or what I do, simply shut the engine off in gear.

 

[Gary56]

As said before, I bet you have a vacum leak. idles don't just change by themselves. a high idle will cause your situation. hit the intake area with some starting fluid to find it.

jim

Do not use starting fluid (ether) for this. Use carb cleaner. Ether is to volatile.

 

[1967 chevelle]

Same thing happens in my camaro, if i put 94 Octane she wont run on, but if i put 91 Octane she will run on for a good 4-5 seconds, when i run 91 Octane under load there is no detonation, so when i'm ready to shut the car off i have it in Drive and she wont run on

 

[Splitter]

Dad had a couple of 366 powered grain trucks that would diesel when shut off. The engine would spin backwards. Only way this is possible is if there is fuel in the exhaust. High idle speed puts more air and fuel into the exhaust when the engine is shut off.

 

[70-SS454]

Generally Timing, Carbon Deposits, & Low Fuel Octane...or a combination of all 3...

 

[HerrKooled]

SWHEATON, my chevelle would diesel a lot as well, and I ran cool plugs, lower base timing and 34 degrees of total timing. I had a 180 degree stat I switched from a 195, but it still ran around 190 which confused me. My NGK ur6 plugs had carbon deposits everywhere when I checked them, just black and rich. I imagine inside the cylinder heads is just nasty. Motor is a full roller and I always run premium 91, but it would not surprise me if it's watered down. Any recommendations? Also, whats the best way to clean the motor out of the carbon without taking it apart? Thanks.

 

[Dan Orgill]

I started simple first with this. I checked to make sure the vacuum connection on the carb and the distributor were properly attached. I took it out for a drive to get it well warmed up and adjusted the idle when I got home, so it's now exactly 1000 in gear and 700 in idle. Took it out the next night on another long drive and brought it home, no run on. I didn't mess with the A/F mixture, just the idle screw.

I will have a look at the timing next time I've got it out. I'm just wanting to try one thing at a time to see if it works, rather than do 3-4 things and wonder which one worked.

 

[richietables]

I think that 1000/700 rpm at idle sounds about right. If you're lucky the problem is solved...:thumbsup:

 

[nashville beth]

As far as vacuum goes, I'm running vacuum advance hooked up to the carb, like the factory did.

What the guys are asking is : Is the vacuum advance hooked to manifold or ported vacuum ?

You can tell by unplugging it at the carb. If it's manifold vacuum, the engine speed will change; the port on the carb will have a strong suction. If it's ported vacuum, there will be almost no suction at the port on the carb.

You need to have it hooked to manifold vacuum so the timing is advanced at idle. That will allow you to close the idle screw down and it will idle with less throttle opening, and hopefully that will stop the dieseling.

 

[Brian Mc]

Also make sure that the fuel tank has an open vapor vent. Tank pressure building up could cause it too. Sometimes the vapor line gets insect nests in it, or capped off at the tank port.