Dieseling on engine shut down

[ttraut@unlockit]

I have a rebuilt 440 with about 400 miles on it and every once in a while when I turn the engine off it spits and sputters and diesels for a second or two and exhales back up throught the carb. Why does it do this and how can I stop this?

Thanks! Terry

Here's my build up:

- Engine size: 440 bored over 0.030
- Intake Manifold type: Edlebrock Performer RPM
- Holley double pumper 850 cfm, mechanical choke
- Fuel Pump Size and type: Carter mechanical
- Fuel lines: upgraded to 3/8"
- Current distributor: Stock electronic unit + MSD 6A
- Head type and valve size: 906 with new valves and springs; ported slightly
- Cam duration at 50: Comp Cam's XE268H; Intake: 224 Exhaust: 230
- Lobe Centerline: 110 degrees
- Cam lift: Intake: .477 inches; Exhaust: .480
- Solid, hydraulic or roller: hydraulic
- Compression ratio: 10.3:1
- Headers: TTI ceramic coated; 1 7/8" primaries
- Pistons: TRW
- Rods: Stock

[Dr. Righteous]

Usually you running abit lean causing dieseling on shutdown.
FIRST, are you running a high enough octane fuel?? At 10:1 CR, that is 93 ONLY.
Pull your plugs and examine them for a lean condition.
If you have not already jetted your carb to match your engine try that first. The carb may preform well out of the box, but still need a little tweak.
Try a colder heat range in your plugs. (the plug is usually the "hot spot" in the combustion chamber)
Back off your total ignition timing a few degrees to see what effect it has on dieseling.
Of course double check all vacuum lines and gaskets for a vacuum leak.
If you are still running a vacuum advance, disconnect it and plug the line.
You should just use mechanical advance considering your engine build.
This is all pretty basic steps. But dieseling problems can be deep rooted, like poor combustion chamber quench, or carb problem that you cannot tune out.
Worse case can be massive combustion chamber deposits that have actually raised the CR and cause knocking and dieseling. BUT, these are usually on high mileage engines.

[jelsr]

Your idle speed is too high. Cut the speed a little at a time untill it will consistantly shut down w/o run on. As the Dr. mentioned you should have the idle mixture screws set toward the rich side. Timing is not an issue when the switch is in the off position. (no spark, period)

[ehostler]

Originally Posted by jelsr

Timing is not an issue when the switch is in the off position. (no spark, period)

That's what you would think. The fact is, to much timing WILL cause an engine to diesel.

[DanL]

Originally Posted by ehostler

That's what you would think. The fact is, to much timing WILL cause an engine to diesel.

Explaine, please?

[ehostler]

Too much intial timing causes the spark plug, chamber, and piston to get too hot in idle conditions. When you remove the spark, the engine still rotates for a few revolutions. During the compression stroke, and the excess heat will casue the compressed air fuel to detonate. This is dieseling.

An engine that is too lean at idle diesels much in the same manner. The over lean condition heats up the chamber and the plug. Remove spark, and this heat will casue detonation, when the compression hits a certain point.

[Dr. Righteous]

Originally Posted by jelsr

Your idle speed is too high. Cut the speed a little at a time untill it will consistantly shut down w/o run on. As the Dr. mentioned you should have the idle mixture screws set toward the rich side. Timing is not an issue when the switch is in the off position. (no spark, period)

I assumed the most basic carb setup was done. Mixture, idle speed ect.
But you are right, too much idle speed will cause some run-on.

[ttraut@unlockit]

Thanks!

Here are my replies:

I use only 93 octane gas.

I did turn up the idle to 900 rpm (warm) since I don't have an electric choke and do have a lumpy cam -- it won't idle cold.

And it DOES only diesel when hot (but not all the time).

Other than idle, I have not touched the carb since it was dyno'd. What exactly am I looking for in adjusting the idle mixture? Am I simply looking for the dieseling to stop? Is there something more scientific that I should be doing or looking for?

I'll check the spark plugs for a lean condition.

I'll look at the timing as well and give a report out.

Thanks for your tips on where to look.

[23T]

Originally Posted by ehostler

Too much intial timing causes the spark plug, chamber, and piston to get too hot in idle conditions. When you remove the spark, the engine still rotates for a few revolutions. During the compression stroke, and the excess heat will casue the compressed air fuel to detonate. This is dieseling.

An engine that is too lean at idle diesels much in the same manner. The over lean condition heats up the chamber and the plug. Remove spark, and this heat will casue detonation, when the compression hits a certain point.

Ed's right on that's why its called dieseling a diesel runs of heat of compression you are running on letf over heat.
Wonder who came up with run-on? Guess it just ran on with the key off!

[DanL]

I fully understand what dieseling is, and what "run-on" is.

What I don't buy is the timing bit. At idle, the timing should be as late as it will ever be; the mechanical advance shouldn't be coming in, and the vacuum advance, if run from ported vacuum, shouldn't be at work, either. Setting the base timing requires that the advance isn't active. Now, if somebody has his idle set too fast, things can change in the timing arena, but even if it's idling at 1000 RPM, the advance shouldn't be doing a whole lot yet, at least on most streetable engines.

I've been hearing for years that timing can cause this problem; I guess I'm saying that I just don't believe it. I don't believe that excessive base timing causes the mixture to get that hot. The cylinder pressures at idle are so (relatively) low, that I just can't see that much heat buildup due to an early spark. Anyhow, if early base timing can cause this diesling problem, why isn't there detonation at idle?

Carbon build-up in the chamber, valves or piston? Sure, because they cause hot spots. Plugs that are too hot? Yep, for the same reason. A throttle butterfly that isn't closing enough? Absolutely, but there has to be a source of ignition, even if the throttle doesn't fully close, and as I said, I don't think there is enough cylinder pressure for a (now absent) early spark to cause that much heat.

And, believe it or not, I'm not really trying to take a personal swipe at anybody here. I'm just trying to stimulate some thoughtful discussion.

[ehostler]

All that I can say to your disbelief is that I have experienced it and that I have duplicated it.

[dart joe]

I believe with your compression and cam your pushing it with 93, i personally wouldn't do it, check your cyl. pressure, not enough octane with cause this, and more likely when its warm, i myself will not run just 93 if my cyl. pressure is above 165-170 on a BB, but thats just me. and not idling cold tells me theres some tuning that needs to be done, your too lean, it should fire up and idle cold if everything is tuned properly, especially in the summer months.

Theres plenty of racers out there that have there timing locked out at 35-37* and have no dieseling problems, if your carb is tuned properly you shouldn't have deiseling, and yes, everything else needs to be right also.

Keep the cylinders HAPPY, thats what i say.LOL

[ttraut@unlockit]

I tried fattening up the mixture on the carb (Holley double pumper) by adjusting the mixture screws.

I first bottomed the screws and then opened them 1 1/2 turns as suggested by Holley. The engine smelled like it was dumping gas straight into the exhaust pipes! I checked the new plugs after about 3 miles and they're black and sooty.

I leaned the mixture a half a turn. New plugs still sooty.

Another half turn lean. Haven't checked the plugs but the engine is definitely running crisper and snappier. No gas smell. That means that I'm running the carbs with a half-turn of the mixture screw (Holley recommends 1 1/2 turns as base). THIS IS ABOUT 1/4 richer than it was set at by the dyno guy.

Timing is about 14 idle and 34 advanced.

I tried setting the idle at 800 and the car will not idle at that speed. 850 warm is low; 875-900 rpm is best. At 800, the car doesn't diesel (but often dies on its own anyway).

The dyno guy increased the jetting by two over stock. Would the jets be too big? (Forcing me to run leaner mixture.)

I was playing with the manual choke and the butterfly valve doesn't seem to want to move. I swear the choke valve moved freely before. Is this how I control idle when cold?

My old thermoquad had steps that would change the idle speed when warm versus when cold; is this the "electric choke"? If so, should I just go to an electric choke?

Thanks for answering my questions.

Terry

[dart joe]

If your plugs are black and sooty, you are way to rich, i'm surprised that you can't get a good idle with that small cam, i had 11 inches of vacuum with my XE284H and 10.25 comp. you also could have too much of the transfer slots exsposed and causing it to run rich, what is your manifold vacuum at in gear, if its lower then 7 then most likely the PV is opening, this will reek havic on the plugs, i have my timing set at 20*int and 35* total, i have never had deiseling problems, and i shut it down at 1100 rpm, but i run racing gas, also, are you using adj. rockers or stock, were the heads milled or the block decked, if so, how much, if you have too much preload on the lifters, you will have a hard time getting good manifold vacuum. IMHO, that is a baby cam, it should have good vacuum at idle, like 14in, and very crisp throttle responce, and it should still run great with 14* int. timing. I would check the preload on the lifters first to rule them out.

[dart joe]

A couple things, jet it back down, try 78/87 PV up front, you may have to put in a fresh set of plugs, check your cyl. pressure(cranking pressure) warm, all plugs out, throttle wide open, good strong battery, i'm curious what it is, is your ignition system in good shape, what kind is it. You really shouldn't need a choke, if it were mine, i would remove the choke flap.

[ttraut@unlockit]

Thanks Joe. Here are some answers:

I have an MSD6A with a MSD coil and stock electronic distributor. MSD 8mm wires.

I have never checked the vacuum and -- I feel like an idiot -- don't have a vacuum guage (got every other damn tool!!!) The one's I've seen seem like permanently-mounted devices; are there vacuum guages that are used strictly for diagnosis (like what I'm doing)? If so, please advise on what I should get.

Regarding idle - it idles fine at 900rpm when cold and okay at 850 when warm. The reason for trying to get the idle down is because someone thought that a faster idle would cause dieseling; made sense so I turned the idle down. However, when I have the idle set at 850rpm warm, the car will not idle at all cold -- I have to keep it revved to 1000rpm cold.

Throttle response with the mixture screws at a half-turn (or quarter turn) is awesome. No stumble, no bog, just go.

Regarding the rest of the questions, this is a street engine so:

- I have stock rockers (not adjustable)
- The heads were milled and block decked just to ensure true - negligible.

The more I think about this, the more I go back to my thermoquad with its temperature-controlled idle. THAT's what I need - a faster idle when cold and a slower idle when warm. Does the Holley come with such a mechanism?

I'll call them today as this sounds like a Holley question.

Thanks for your insights; it'll help when I call Holley.

Terry

[dart joe]

You can get a vacuum/fuel tester at any parts store like Auto Zone, O'Reillys, Advance Auto, let me know what Holley says, what jets are in the carb, check for any vacuum leaks, you may need to convert to 4 corner idle, i think that would help you alot. keep me posted.