My el camino has to run premium, if I try to run regular I get the afterrun. Easy way to get rid of it is to turn down your idle just a tad. Usually on a car in tune and running proper octane, if your throttle is open just a bit too far it can cause it to diesel. Remember when you turn the key off the spark plugs arent firing, its the hotspots getting fed extra fuel and air and self ingniting. jim

 

high idle speed is popular. retarded timing will cause hot chambers, maybe enough to help with the self-igniting.

 

You do not need spark to light off the mixture in the chamber as we know.
You can have preignition or ignition from heat ..High cylinder pressures put the squeeze on the mix..when you compress or squeeze air it gets hot..Hot enough to self ignite.

Now you know it is not too much timing..because you shot off the spark..took the timing out of the equation..

Now to keep the engine from running on you need to kill the air supply.

This is usually done by reducing idle speed by closing the throttle blades more.

You can use a solenoid to set the throttle against..when you shut off the ignition the throttle blades will shut completly and the engine will die..
Higher octane will burn slower and will also keep the diesel from happening as easy.

 

If it is being caused by running lean or hot engine temps, a couple other things can cause it. Worn out parts in the dist causing the timing to fluctuate improperly. Also too hot of a spark plug. Idle mixture screws misadjusted. Cruise a/f ratio off for the rpm that you run. Cooling system not up to snuff.

I get it with 87 octane. It likes 89 and doesn't have an issue. Same with 91. I run my setup rather lean with hotter plugs for the throttle response.

 

Also have noticed it with undersized stock style mufflers. That one is easy to test. Just disconnect the exhaust at the end of the headers. Put in a couple washers at each bolt to create a big exhaust leak. Will be noisy, but if back pressure was causing an issue this will get rid of it for testing.

 

I used to have this issue a while back before I went through the engine again but I found that if I kept the car in Drive and turned off the engine without putting it in park (that way the idle RPM would be lower) it solved that issue. I also went with a colder spark plug but still did it. I do believe it was my high idle RPM and the carb.

 

You may want to look at this, and any other problem by wondering about how things work, and what can make things happen.

Ignition is a product of a source of fire, air, compression (well, more for efficiency), and fuel.

Which is the variable you think is the problem?

 

whenever i have this problem, it's because the throttle blades are open too far.. some playing and tuning to get it to idle with the throttle closed farther has always stopped it for me.. this is done with more ignition timing and getting the idle mixture screws set properly.

 

Never had much of an issue with my Chevelle (stick car), but the Elco (auto) has had it's fare share of run on issues in the past. The one that helped tune my carb the most was drilling an 1/8" hole in the primary throttle blades of the Holley. This helps in getting the throttle blades closed more to reach the .040 transfer slot that Holley states you should have. I do the shutting it off in gear method also, or throw my ignition switch off.

 

I ran my 327 on premium fuel all the time until gas prices got crazy a few years back; I had the same problem running 87. One turn of the screw lowering the idle and I was good to go on the lower octane... That might be the simplest first step for you to try.

 

Thanks guys, I didn't expect this many responses!

I run 91 Octane. I was running 94 until one of the fellow Canadian members here advised against it due to ethanol issues with Sunoco 94, which is the only 94 octane available.

I'll look at the idle issue. It idles at 750rpm in gear, but yesterday it seemed to be running around 1200 in park, when it usually runs around 1000.

 

When you reset your idle speed, make sure you do it when the car is warmed up to operating temperature. Otherwise, you aren't setting it to run at X idle at X RPM. You're getting X idle and Y RPM.

 

GM put an anti-dieseling solenoid on some vehicles.
Lets the throttle blades stay shut with no power to it but during normal running ,it holds the throttle blades in the idle position.

 

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.

 

Holley 770 Street Avenger

 

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

 

what is your initial timing set at? can you adjust your initial timing so that you can use less idle screw on the carb?

 

Are you running vacuum advance at idle (manifold vacuum)? Lack of vacuum advance kills idle combustion efficiency, requiring more throttle opening, which then encourages run-on dieseling.

 

If you say this just started and are having a high idle.Maybe check for a vaccum leak.

 

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

 

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

 

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.

 

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.

 

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.

 

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

 

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

 

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.

 

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

 

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.