Engine Deiseling Problems

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Moparman67

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I just completed a total rebuild on my '71 340 block that I installed in my 67 Dodge Dart. It was a mild build, .030 over, balanced crank, edlebrock aluminum heads and roller rocker assembly, edelbrock aluminum intake with a edelbrock 600 cfm carb, tti headers. Here are the specifications for the installed Comp cam at the link below. I have the intial timing set at 16 degrees advanced which unfortunately fives me a 45 degrees advanced a WOT. I have tried jetting the carb, adjustung the timing however once the car is warmed up it experiences dieseling(run-on) when I shut it down. Luckily it is 4-speed set-up so I just pop the clutch to stop the run-on. On the highway the engine runs about 200 degrees but can easily climb to 220 when at a stop light.
Anybody have any suggestions how I can solve this deiseling issue. I am in the process of finding a fan shroud because I dont have one installed in hopes to bring the temps down thinking that maybe the extreme heat is igniting the residual fuel. I also only run high-test 91 octane fuel.

http://www.compcams.com/Company/CC/cam-specs/print-specs.aspx?csid=627&sb=0
 
I'd say cooling is the issue. Might be a combo of shroud and other cooling problems, or a "new engine" that is a bit tight. Even so, that is borderline.

YOU MUST get that distributor recurved or replaced OR YOU WILL be replacing pistons or worse.
 
Vacuum leak is my guess.

16 Initial is good for that cam in most circumstances.
 
It sounds like your timing is too advanced, try retarding it a little.

also what is your compression? that could play a factor also.


Have you tried running racing gas in it to see how it acts?
 
The engine is definately tight, the machine shop reported 220 psi on their test stand when I picked it up. I am not exactly sure of the compression ratio.
Somebody did mention about having the distributor curved but i am not sure what that involves?
Advancing to 16 degrees at idle is the only way the engine would idle smoothly, somebody else also told me that was good for that cam.
Performance is great and have no other issues other than that deiseling?
 
You've got to get your mechanical advance down about 7 or so degrees. 16+29=45. Too much total timing. 16+22=38 is better. What dizzy are you using?
 
The mechanical advance should not be involved in the dieseling issue unless there is something else going on with the distributor. (Though the excess advance needs to be fixed.) How are you measuring total WOT advance? You need to do that with the any vacuum advance hose disconnected, just to be sure that is not effecting the measurements, especially if you are just 'blipping' the throttle open, setting still with no load on the engine.

I would first suspect that the carb throttle plates are not set up properly, and are set or hanging a bit open, allowing the dieseling. This would explain why the idle will not work with less advance. Check the secondary throttle stops to see if they are set too far open.

Also, see if the vacuum advance hose is connected to ported vacuum, not manifold vacuum.

Do you know what is the cam advance? Which Edelbrock heads, 63 or 65 cc? And what pistons? Just curious about the static compression ratio.

Decreasing the heat may help some. But the tendency to diesel points to something else needing to be changed. Is 91 octane the highest you can get? Tried any octane boost?
 
Moparman67 said:
Somebody did mention about having the distributor curved but i am not sure what that involves?
Advancing to 16 degrees at idle is the only way the engine would idle smoothly, somebody else also told me that was good for that cam.
Performance is great and have no other issues other than that deiseling?
Click to expand...

scooby67 said:
You've got to get your mechanical advance down about 7 or so degrees. 16+29=45. Too much total timing. 16+22=38 is better. What dizzy are you using?
Click to expand...

Do NOT take this lightly. You MUST get that distributor reworked or replaced or you WILL be buying engine parts, especially as hot as it is running Too much advance UNDER POWER is what kills engines FAST
 
Do you have positive control of idle fuel mixture? In other words are the throttle plates so far open to make it idle the transfer slots are being exposed? You may need to drill bypass holes in the primary throttle plates. With your compression an over rich idle could easily be the culprit. If you have a secondary stop screw you can use it to get more idle air bypass the adjust the primary stop back to where your idle circuit is effective.
 
Thanks for all the advice. The heads are 63cc. and vacuum advance is connected to carburator. I did remove vaccum advance when testing the WOT and ran it yup to 2500 on the tach to get a timing reading.

I will look at the carburator closer and make sure the carb throttle plates are functioning properly and adjustment on the secondary throttle stops.
 
One local mechanic told me to throw my Edlebrock carburator away and I should invest in a Holley Avenger carb. This carb is only 4 years old with very few miles on it.
 
Moparman67 said:
One local mechanic told me to throw my Edlebrock carburator away and I should invest in a Holley Avenger carb. This carb is only 4 years old with very few miles on it.
Click to expand...


My son's car had the same problems with the engine trying to continue running after he shuts it off. After trying every trick in the book I finally went to O'Rielly's and bought an "old school" anti deiseling solenoid. Don't get me wrong...make sure everything else is correct and if those things still don't fix the problem then you might consider the solenoid fix. It worked out great and the car has been running well and shutting off like it's supposed to.
The solenoid keeps the idle up at 800 RPM at idle when the car is running, as soon as you shut off the ignition the solenoid lets the carb lever completely close shutting off any/all fuel.

My son used to keep the car in gear (automatic) to make the engine stop but now he can put it in park to shut it off.

GOOD LUCK!!!

Treblig
 
Moparman67 said:
One local mechanic told me to throw my Edlebrock carburator away and I should invest in a Holley Avenger carb. This carb is only 4 years old with very few miles on it.
Click to expand...


You need to fix your timing before dialing in the carb...
 
curb idle rpm too high causes dieseling most cases.
 
What is the lowest idle speed you can get it to idle at, at 16 initial. Are you on ported (nipple on pass side). I'd say the high idle speed is making it diesel. What cam? You may need more than 16. In order I would set: (1) initial with vac gauge method (2) total (initial+slots with can capped) to 35 (3) mix/match springs/any combo but toss the heavy OE spring with the elongated loop on one end so that you are just under the pinging point at WOT up thru the gears on your hottest/driest day no matter what RPM the slots max out at but with your light/tack make a note of where you are at there then (4) plug in/adj the can & want to stay just under the pinging point in everyday driving under varying RPM/load (in hg) conditions & (CCW slows the rate). #1 get the idle speed down & there might be a vac leak. What "67dart273" said, get some changes done on this right away before you run it again. with over 200 cranking psi you need some good octane, If you build a monster you gotta feed it what it wants (needs). #1 enough octane (2) correct (lower) idle speed (3) fix vac leak if there is one (4) dial in dist subsystems systems IN ORDER EDIT if that cam is wild enough that it requires a high idle you may have to tune on manifold (nipple on Dr side), more on that later if needed, one thing at a time
 
I just gotta ask; Whats up with that 220psi? If thats accurate, you could be in for a tough tune. I think you're gonna need a tuner friend. Or an aftermarket, topside-adjustable, dizzy.
Make sure your metering rods are staying down at idle(shouldnt be a problem with that cam).
 
Sounds like a stuck choke I had that same problem with my first car and 88 accord carb car the choke got stuck one day and couldn't make the idle go down and when I shut the engine off it would deisel till I pop the clutch my dad found the choke stuck close causing it to draw loads of fuel and the problem. He bent it to be stuck open all the time, no more problem. He also makes about same on compression test with his 340 and runs a holly with no choke just two small taps of gas peddle and she fires right up every time even when she cold.

I'm with the guy get rid of that edlebrock carb see nothing but issues with those learn to tune a Holly much simpler there a reason all the racers use them
 
OP: Do you know what pistons are in the engine? Anything on deck height?

What plugs are you running? An excessively hot plug tip could contribute to the tendency to diesel.

Do you have a vacuum reading at idle?

Dieseling takes a combination of 3 things:
- Throttle a bit open (for whatever reason)
- Hot heads or hot spots in the heads or pistons
- Marginal fuel octane for the compression

(Dieseling was a common problem in the early smog era due to timing being retarded for emissions, resulting in a more open throttle at idle to keep idle speed up despite the retarded timing, and hotter engines with the 195 thermostats and retarded timing. The idle stop solenoid mentioned above was a cure.)

I have to wonder on the compression number being accurate. With the 63 cc heads and a typical 340 deck height (an assumption here), the SCR will be around 10:1. But the cam is a 268 degree intake duration so a 220 psi cranking compression seems a bit on the high side. Perhaps a lot of cam advance....the cam has 4 degrees of advance ground in, and if it was advanced some more, that could be excessive.

Just keep in mind that the ignition timing has nothing directly to do with the dieseling, since it is off, but may be a good indicator of something else wrong. The possibility mentioned by some of a vacuum leak makes sense.
 
Well I had a buddy drop by and give me a hand with diagnosing the problem. We placed a vacuum gauge on it and there is definately no vaccum leak. I then had him cover up the carburator with a rag as soon as I shut it off, with several different attempts and experiments this definately stopped the dieseling. We then backed the idle off enough so I had to keep feathering the throttle to keep it idling and when I turned it off there was no dieseling either. This must mean that the throttle plates are staying open enough to suck air and fuel in at shut-down. Other than installing an idle stop selenoid does anybody have some advice on adjusting the throttle plates on an edlebrock carb?
 
A picture from the bottom of the throttle plates in their current closed position against the stop would be helpful.
 
I just went through this on my 460. What everyone says is true. I had a horrible run off issue that even pooping the clutch would not stop. Once I got the correct mechanical advance in the distributor, set my initial, and lowered my idle to 700 rpms, it stopped. I have a very similar cam spec wise to yours, too.

Did you adjust you idle fuel after adjusting the idle? When the motor was assembled, was the intake installed per Edelbrock's instructions? I've seen intakes leak from the thermal expansion of the aluminum head. What was the rpm you got it too to stop the run off but struggled to idle? Is the choke hooked up and operating properly. If electric it must be powered to work. If manual it must be wire controlled, even if you want it open all the time.
 
I normally had the idle set for 850,hard to tell where when the die selling stopped because I had to keep feathering the throttle to keep it running but it was two complete turns out on the idle screw. I am confident there are no leaks on the intake and I have no choke connected.
 
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