Dieseling issue - Team Camaro Tech
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post #1 of 34 (permalink) Old Aug 3rd, 06, 06:38 PM Thread Starter
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Dieseling issue

Ok so i have a new 383, 100 miles on it in a 68 bird. vortec heads, edlebrock 650 carb, msd ready to run distributer and blaster 2 coil. 9.5-1 compresion. 405 hp at the crank. tremec tko 600 5 speed. 1 wire 100amp alterator.

when warm and only when warm the car will diesel at shut off, Not run on but diesel. I have had the timing anywhere from 6-12 degress BTDC at idle, and adjusted the idle as well, I still have the prolem. at 12 degrees the car runs like ***. at 8 it runs great but none the less it still diesels.
The idle is set at 800 rpm, I have gone as low as 650 but still have the problem.

A friend stated that the wire to the positive side of the coil on the old cars was actually restricted down about 9 for the points style setups. and that with the new pointless systems a true 12 volts is needed and that with not enough voltage the car would run a little rich by not being able to have a hot enough spark to burn all the fuel, I do have a little smoke from the pipes that would make me believe it is rich and also it takes a bit to get it started when cold as if it were not getting enough juice to fire.

any thoughts
thanx
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post #2 of 34 (permalink) Old Aug 3rd, 06, 06:51 PM
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Cool Re: Dieseling issue

Dieseling isn't a function of timing. The plugs are not firing at all with the ignition off so it makes no difference if it is advanced or retarded. You are getting a combustible mixture at idle, which means your idle throttle is set to high. A simple fix (not the solution) is to bolt on an A/C solenoid found on a bunch of pre computerized cars with carbs that allows the throttle to fully close when the ignition is turned off.

The solution is to adjust your carb to idle with the throttle blades below the off idle transition slots which you have uncovered by setting your idle throttle speed to high. If car needs more air and you have adjusted every thing else then bore two small holes in the throttle plates to let air in and slowly enlarge them until you get a smooth idle were you are not running rich.

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post #3 of 34 (permalink) Old Aug 3rd, 06, 06:59 PM
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Re: Dieseling issue

First describe dieseling because to my knowledge it's the same as run on... Next you do want a full 12 v to the dist but it won't cause the engine to run rich or diesel. What is your dist curved like? How much timing is built into the dist? You need to know that so you can decide where to set your initial timing.

The dieseling (keeps firing after the key is shut off) is usually because the throttle blade is open too far at idle. If your idle mixture is off and your timing isn't in the ball park you could be idling at 600 with the throttle blade open too much. You need to look at the entire tune being close to clear this up.

More questions for you, what kind of carb are you using? How much messing with the carb have you done?

Pinging and pre-ignition and not good and happen when the engine is running. It can happen at part throttle, just off idle or wide open throttle. Timing plug heat range and fuel octane effect this. Run-on or dieseling happen when you shut the engine off and it tries to keep running. Throttle blade adj and crud build up in the cyl head usually cause this...

...Dennis

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post #4 of 34 (permalink) Old Aug 3rd, 06, 07:00 PM
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Re: Dieseling issue

Dave you type faster than I do...

...Dennis

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post #5 of 34 (permalink) Old Aug 3rd, 06, 07:18 PM Thread Starter
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Re: Dieseling issue

Ok so here is msd's definition of run on, the car idles at 800 when you turn the key off it continues to run at 800 as if the key had not been turned off and this is caused by voltage still being in the line and would necesitate a 0.8 ohm ballast resistor or resistor wiring MUST be installed. but this is only an issue on points style distributors.

The issue i am having it that when you turn off the car instead of the prrrrrrrrrrrrrrrrr of run on or the silence of it shutting off it goes bump pppppumppppp bump bubububumpffffffff. this is what i refer to as dieseling and is way different than run on, This is the kind that will destroy the lower end..

Now as stated above it is a edelbrock 650 carb, If it were the throttle blade i would expect the same issue at cold as i get when warm.

It is a blueprint motor that was dyno run before being shipped here. the carb should have been set well.
That being said i have adjusted the 2 idle screws and the throttle linkage stop screw to let the thottle close more, I am out of adjustment on the idle screws anymore and they will fall out. so i can not adjust the throttle linkage stop out any more.

so after checking all these i still have the issue in every configuguration possible and it still diesels. I took it back to how it came from blue print as i documented all this before i started messing with it.
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post #6 of 34 (permalink) Old Aug 3rd, 06, 07:26 PM
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Re: Dieseling issue

for an engine to run, you need air, fuel, and an ignition source.

Nothing you can do about fuel, if air is passing thru the carb, it will draw fuel, period.

The others have addressed the issue of too much air providing a combustable mix, and I agree with them completely. What dave and dennis described is the most common cause for dieseling, and the right way too fix it.

The last issue is ignition source. Something in the chamber is hot enough to light the fire even without electricity generating a spark. Logically, that something must be the spark plugs. Pull a couple, if they are way stark white or show lil dots (use a magnifier) on the center porcelain, try a colder plug.





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post #7 of 34 (permalink) Old Aug 3rd, 06, 07:29 PM
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Re: Dieseling issue

oh, and the "run-on" that msd describes on thier site has nothing to do with dieseling. Some cars, only with an MSD 6 or 7 ignition box, may continue to RUN when you turn off the key. The msd 6 recieves it's power source directly from the battery. The ignition lead to it is just a switch to turn it on and off.

If you have an MSD box (not an MSD ready to run distributor) it would bnot be a waste of time to disconnect the battery cable while it's deiseling and see what happens... doubt that's the problem from what you described, but ya never know.





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post #8 of 34 (permalink) Old Aug 3rd, 06, 07:30 PM
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Cool Re: Dieseling issue

You are correct in classifying your condition as dieseling; it is not the ignition run-on MSD is talking about with voltage bleed over. You have to much air getting in the engine. If the throttle blades are indeed closed then the air is getting in via a vacuum leak.

Once car is idling again. Get a propane torch and crak open the valve and after experimenting with flow rate at top of carb (don't want it so rich it kills engine), go on a vacuum leak hunt around manifold, base of carb and vacuum lines.
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post #9 of 34 (permalink) Old Aug 3rd, 06, 07:48 PM Thread Starter
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Re: Dieseling issue

Jim I was just explaining what my definition of run on and dieseling were as i was asked by DJD. It is a ready to run set up and does not have the box.

I will check for a vaccum leak but i have all new lines and have checked all the bolts on the carb.
As for the throttle blades being open is there another way of adjusting those besides the the linkage stop screw that the cable connects too,
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post #10 of 34 (permalink) Old Aug 3rd, 06, 07:53 PM
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Re: Dieseling issue

The msd defined run-on is something that only happens with an msd box and is fixed with a resistor. Your dieseling is the same as run-on in a car not equipped with a msd box... Check with msd and see if you need to replace the resistor wire like when installing an hei dist, I'm not sure if you need to.

Sorry I missed the carb info already given... Carbs and engines don't run the same when cold as they do warm so tune when completely warmed up. Close both idle mixture screws and open them up about 1 1/2 turns. Set your initial timing with the vacuum advance disconnected and plugged at the source if you have it to 8 degs btdc. Now using a vacuum gauge adjust the idle mixture screws for most vacuum. You can tune for highest idle if you don't have a gauge. You should end up leaning out the idle mixture to achieve this. My guess is 3/4 to 1 full turn out will do it. Your idle should have gone up so adjust the throttle to bring it back down to maybe 600rpm. Check your timing again and set it back to 8 degs and check the idle again. You should be able to rev it to about 900 rpm without the timing changing if you can't the dist needs curving. You should be able to start and shut the engine off at will now without issue...

...Dennis

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post #11 of 34 (permalink) Old Aug 3rd, 06, 08:06 PM Thread Starter
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Re: Dieseling issue

i will try that and let you know what happens
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post #12 of 34 (permalink) Old Aug 3rd, 06, 10:59 PM
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Re: Dieseling issue

A ittle futher to Jim s comments on hot spot causing ignition run on
Plugs in a temp too hot for the engine can cuase it
Check plugs as he descibes above...Also us a maginfing glass and chech the eletrode tips...a good heat will show the tip of the elecrode burnt a bluish colour, about 1.5 to 1mm of the tip...any more and it it shows the electode doesnt conduct enough heat from the tip, takes too long to cool..and remains red hot for the next cycle, igniting any mixture in the cylinder..run on.

My Spelling is not incorrect...it is creative

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post #13 of 34 (permalink) Old Aug 4th, 06, 12:55 AM
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Re: Dieseling issue

i agree with what is said above... but im gonna state the oblivous (forgive me) is the fuel octane too low for the amount of compression??
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post #14 of 34 (permalink) Old Aug 4th, 06, 03:16 AM
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Re: Dieseling issue

try a can of seafoam
i had same problem years ago and it was caused by carbon buildup.
1 can of seafoam and dieseling went away for good!


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post #15 of 34 (permalink) Old Aug 4th, 06, 05:03 AM
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Re: Dieseling issue

seafoam kicks a$$ i would highly suggest it... it will make old gas burn, i use it every time i store a vehicle.
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