Gentlemen: My 502 runs all right be deisels quite a bit when shutting off. In fact, it has been necessary sometimes to cover up the carb with rags to get it to stop. Can someone please explain what causes this, and how to adjust it out. Thanks in advance. R. B.

To much timing.........What is your 's set at......

My 350 does this also...its rather embarassing. Im running 18* initial timing and 36 total. What do you think?

Simple question, but difficult for me to give a straight answer, I was inside the car, my friends were trying to dial in the distributor and carb. It is a new crate engine first start up. We began with 10 initial. We have had it all the way between 30 and 40 total. ( book says 36 ) and it still displays the same annoying deiseling. Actually, when taking some timing out, it seems better. We are starting to wonder if we got he distributor off by about one tooth. Any further thoughts based on this additional info. All help is gratefully accepted. Thanks R. B.

the timing was way off from the factory. I got mines up to 42 total. Definently bump it up from 10 initial

My personal experience is that this is caused by having the throttle to far open at idle. The factory dealt with this by adding an anti-dieseling solenoid, which has a plunger that controls the idle speed. When the car is shut off the plunger retracts and the throttle closes completely. Advancing the initial timing actually benefits this condition. As you advance the timing the idle speeds up and you therefore reduce the throttle opening. My Chevelle will do this when I set the carb for an emission test. To keep it from running on you can either shut the car off in gear (AT) or turn the steering wheel as you shut the car off. Either will puts a little load on the engine and has always kept it from dieseling.

Bad or cheap gas will cause it too.

I have had the same dieseling problem with a 383 stroker that ended up with about a 10-1 compression ratio.I used 89 octane no lead pump gas, with vortec iron heads,and this was a regular problem from the start. As I am in the aviation business I have access to 100oct low lead aviation fuel, and this fixes the problem. I would have to think this is more of a fuel problem, rather than timing, because when you turn the ignition off you have no timing. On older engines that may have carbon build up, hot carbon deposits may cause this run on problem, mine has done this from brand knew.

I was helping with rb's engine today (I hope I am not the problem :D )....
It seemed that advancing the timing reduced the dieseling.

Initial 10° and as high as 20° (but that made it harder to start). Holley idle screws @ 1-1/2 turs, idle ~900RPM.

In what order would you adjust idle, idle screws, and timing? I am thinking first timing, then idle then idle screws for best vacuum, then reset idle and recheck idle screws....

I found advancing the timing and bring the idle back down with the idle speed screw works very well. However, my current bracket engine will diesel with 18 initial unless I mix a little race gas in (1 gal to 4 of 91). I tried more advance but it was too much total and the car slowed down. Guess I should shorten up the mechanical advance a bit. In my case, 20-21º initial would probably stop the dieseling.

As stated above ensure your timing is not too retarded,idle is not too high,engine is not running too hot. Also,i have seen a couple times where a motor with a lot of carbon in it will get that carbon build up lowing red hot to where it can ignite the incomming fuel
with the ignition off. So if you have an old motor and all the suggestions mentioned in these replies don't correct the problem then try some additive that you puor down the carb with the car running to clean your combustion chambers to see if that helps too.

I am assuming your running a reasonable comp ratio like 9:1-10:1 and not something wild like 11:1-12:1 with high cylender pressure doe to cam selection on the street with todays 91-93 octain fuel which in it's self could cause this to happen.

Scott

Originally posted by SWHEATON:
As stated above ensure your timing is not too retarded,idle is not too high,engine is not running too hot. Also,i have seen a couple times where a motor with a lot of carbon in it will get that carbon build up lowing red hot to where it can ignite the incomming fuel
with the ignition off. So if you have an old motor and all the suggestions mentioned in these replies don't correct the problem then try some additive that you puor down the carb with the car running to clean your combustion chambers to see if that helps too.

I am assuming your running a reasonable comp ratio like 9:1-10:1 and not something wild like 11:1-12:1 with high cylender pressure doe to cam selection on the street with todays 91-93 octain fuel which in it's self could cause this to happen.

Scott On behalf of rb, thanks for all the replies.

it is a b/n zz502. It had been stored for a bit with oil added to the combustion chamber. Perhaps the oil in there created some carbon deposits. We will re-adjust the idle speed etc and maybe try some carb cleaner.
Thanks,
Ron

Well i am running 11:1 compression. Is this dieseling a sort of detonation? My cam is enourmous by most peoples standards for street and i can run pump 90 octane without detonating, even with a load on it going up a steephill in 5th gear and putting her to the wood. I guess im saying should i be worried about this? I have CNC chambers in my aluminum heads and the motor probably has right at 1,000 miles {probably less} on it. Right now im shutting it off by putting the car in 5th and shutting it off with the key and slowly releasing the clutch at the same time.

Back off the carb butterflies, it will stop the dieseling. As mentioned, some cars used a solenoid, but this is not a solution for a performance car. One way I've done in the past, and it's worked great, is to drill a small (start with 1/16") hole in the front side of the front butterflies. What this does is allow you to close the throttle down and still get the air you need to idle. When the plates are open too far, you start to uncover the transition slots, which add fuel for small part throttle operation where you don't have the throttle open enough for the main boosters to start working. You'll find that doing this will let the car idle way down, and alot smoother too. This is basically an older trick, nowadays with four corner idle circuits, it's not needed, unless you're running a monster roller. If you pull the carb off as is, without touching anything, you should see two slots, one in either primary bore, in the baseplate. If they are uncovered more than .040" at idle, this is usually your dieseling problem. The whole procedure is covered in any good carb book, such as the HP Holley carb book by Mike Urich and Bill Fisher. I'm not just quoting this info from the book, I've done this quite a few times, both for my own cars, and for friend's cars. This trick is not for a carb you like to swap around to different cars/engines, unless you have spare butterflies to reverse the procedure for a smaller cammed engine. Hope this helps.

Thanks Darren and all. My pal Bow Tied and I will take a hard look at this. May need to make some modifications, eh? thanks again. R. B.

Well im going to be getting an 850 annular mighty demon for my 350 anyways. But on the topic, i currently have all 4 butterflies drilled at .095 inch. This helps it idle alot better and alot leaner but i still cant get it to consistantly idle. This may have to do with my new junk mallory stuff.

My 502 with EFI will diesel if I run 87 octane. If I run 93 it does not diesel anymore.

Andrew

Another method if your timing and carb is to your satisfaction... shut the engine off with the tranny in gear if an automatic, or if a stick, turn the key off and gently let the clutch out a bit to "drag" the motor to a stop.

These methods will work, but definately do some work and find/fix any other problems which may be causing it to do so.

Troy

Originally posted by andrewb70:
My 502 with EFI will diesel if I run 87 octane. If I run 93 it does not diesel anymore.

Andrew This is due to the lower octane gas being more volatile, which (not to change the subject :D )will also give your car better gas mileage, as long as it doesn't ping. I laugh at the people who use premium in a grocery getter 4cyl, not worth it. They just do it usually because someone told them "it's better" for the car. I also can't understand why people use full service around here when self service is $1.99/gal, full service is about $2.39/gal, and that's for 87 octane. graemlins/clonk.gif

I would actually prefer to run 87 octane. I have 8.75 static CR and about 7:1 DCR. The only reason I run premium is the dieseling.

Andrew

Thanks to Bow Tied and some good advice from this excellent club, we got most of the deiseling under control. Seems a lot of the problem was too much idle speed, and also to cover some other bases I added high test gas. We still have some dialing in to do. So far, I have been able to drive it up and down the driveway ( but fast! ) We are at 12 degrees initial and 36 total for advance. More tweaking?!

Thanks for advice and interest shown. I will drill the holes as suggested if it is absolutely necessary. At present, I have my hands full with the fundamentals.

Thanks again. R. B.