dennisbernal91z

iTrader: (2)

Ok so I have a pro-billet dizzy, MSD Digital 6 Plus, and a blaster coil. All new wiring in the car that I did myself, so I guess there could be a short, but I have never had on of those and I literally did hundreds of solders, shrink wraps and so on.

The car drives great and makes great power, but 50% of the time when I try to shut it off, it sputters, revs a bit and may even make a pop (small backfire) through the exhaust. I hate this. The other 50% of the time it is totally fine and dies right away.

I am assuming the box is getting a little charge from somewhere and powering the coil still when it shouldn't.

I am tapping into the old 12V source for the old Malloy HEI I used to have in there.

Once the switched 12V source is killed, then the box is OFF. So how is this happening? Think I should wire the box directly to the keyed ign wire, right at the key, so I KNOW I am killing its power, maybe?

Any help would be great. Thanks.

-Dennis

GTA Jim

You don't say what intake you have. I'm assuming you're using a carburetor.

It's unlikely your ignition is causing it.. This is an old and common problem with carburated engines. It's called run-on or dieseling.

What happens is you get hot carbons spots on the cylinder heads and pistons. These hot spots ignite the fuel just like a spark plug would. The engine can run on for some time. It can even start turning backwards.

This can't happen in a fuel injected engine because the fuel stops squirting out the injectors when you turn off the key.

There's a good explanation of what is causing it and how to cure it at the link below..

http://mgaguru.com/mgtech/engine/ro101.htm

dennisbernal91z

iTrader: (2)

Thank you so much. I will read the link right away.

I am running a holly 750 DP, with an edel. vic. jr. intake and Aluminum trick flow heads. Thanks again.,

dennisbernal91z

iTrader: (2)

Well I read the link. Not bad. Only problem is that my car has less than 2000 miles on it, so the carbon buildup problem just isn't there yet. And the timing is right and the plugs are a little hot, but reading them and they look ideal. So really not sure what my deal is. I figure YES the carb is still dumping fuel in and there is air in there, maybe there are hot spots in the intake. think is the car never ran on with the HEI ignition. I Do have a return line from the from my reg, but not my carb. I run big lines though so I know there is good fuel circulation. The car doesn't over heat at all. If anything it is though to get it past 180. Is that a sign of a problem? Nothing is stock all aftermarket stuff. Except the cooling system is stock and doing more than enough. I guess I can do a compression test and see where I stand. and go from there....

strokermotor91

I've had the same problem with my MSD 6AL box. A "Diode" should have come with the msd box. This needs to be wired into the charging system. This should fix your run-on problem. Here is a link to help you out.
http://www.msdignition.com/1troub1.htm

Josh

dennisbernal91z

iTrader: (2)

Yeah, I have the diode. But my ignition system is far from stock and they say in the instructions that it needs to go in line with the voltage regulator and stuff like that. My car doesn't have that stuff. So I am super confused. Tell me where EXACTLY you installed your diode. Thanks.

Jza

Assuming an electric fan, does your car run as long as your fan is spinning? In other words you shut the car off, the fan will slowly spin down to a halt, and the car runs as long as it spins. If the ignition is wired into the same circuit for whatever reason, once the power is cut to the fan, the spinning fan effectively turns it into a generator putting power to the very circuit it was feeding from for as long as it has inertia left spinning. If you can recreate the run-on every time, try unplugging the fan and seeing if the run-on continues. If it does you have your culprit.

Mikz86TA

iTrader: (4)

How much initial timing are you running???

dennisbernal91z

iTrader: (2)

I will def check the fan idea. Thats just crazy enough to work.

Initial timing is at 17*. 35* total.

onebad82z

iTrader: (7)

Back the initial timing off 2 degrees or so for a few days and see how she acts. Yes it is not the optimal dyno tuned timing, but for testing purposes give it a try. If it gets better you have too much initial for the pump fuel you are running.

dennisbernal91z

iTrader: (2)

I run 93 all the time. I thought that would mean that I would be able to run the timing I wanted.... I dunno.

I guess I don't know much about the relation between octane and initial timing and all that. I will try 15 initial I guess.

onebad82z

iTrader: (7)

No unfortunately some 93 is crap.

dennisbernal91z

iTrader: (2)

Go figure.

I will play with timing this weekend. 2* at a time, drive around the block and check. Maybe it will fix it.

onebad82z

iTrader: (7)

Around the block may not reveal the problem as you know it. I am willing to bet it will only show up when the car is good and heat soaked, like after cruising around for awhile, romping on it here and there, etc. I would suggest to drop the timing 2 degrees and take it out for a nice cruise, get some heat in her. I would leave it that way for a few days of driving and see what transpires, if it (the problem) persists..drop another 2. No more than 6 total, anymore than that and something else is up. This is the easiest, so try this first.

JohnZ28

What RPM does the car idle at?

You might want to lower the RPM and also adjust the idle mixture screws to see if it makes a difference also.

Cflick

Is the ignition actually firing after key off ?
Put a timing light on it, and see if the thing is actually sparking.

If it is sparking, you can ignore fueling for now, and concentrate on making the ignition actually go off at key off.
If it's not, then you have dieseling, also called run-on, ( as mentioned by someone else above ) and you can ignore ignition for now.
This is what the dashpot on the old "electronic" carbs was for. To hold the throttle at idle when the key is on, and drop the plates totally closed at key off, hopefully cutting off fuel completely. ( it's not complete, but close )

Backing off the timing can help, maybe, as onebad82z suggests. That works by putting less combustion heat into the heads and cylinder walls, thus cooling the chamber to lessen the chance of detonating on low octane fuel later in the compression stroke after sparking stops. What also works in the same way, is punching the throttle at the same time you key off. This works by dousing the chambers with excess fuel, both cooling the chambers, and going a bit rich which is less prone to detonation. ( not *not* prone, just less )
Throwing a quick rev just before key off is the same objective, to flow cool fuel and air through the motor after sparking stops, to cool the chambers.
None of this has anything to do with water jacket temp. It's about combustion chamber temp. Head surface, piston top, valves.....

Hope this helps.....

dennisbernal91z

iTrader: (2)

Ok well I will do things in this order.

Today- I will check with a light gun if I am still firing. easy.
If I am, I am sure I can fix it somehow. If I am not. I will move on.

Then I will back the timing off and drive for 20 minutes. Hard. See how well she shuts off. I tried reving high once and cutting the key and a HUGE backfire shot thought the pipes like a gun shot. I had neighbors outside thinking a gun went off. So maybe the ign. is still going.... I run 93 and 3 fuel filters with almost 0 pressure drop at WOT, so I know I have a good supply of cool, clean fuel. I have a return line going to my reg, so that keeps that cold, but maybe my carb is getting hot. But I dont think that would be causing this, as I can pop the hood and easily touch it with no fear of burning.

I hope the timing thing fixes it. The car is going to get dyno tunned on the 10th so maybe they have some insight. And maybe after they re-tune the carb the problem will go away. I will keep this thread posted as I really wanna get this thing sorted out. Thanks for all the help so far.

-Dennis

Cflick

Don't recall if you mentioned a cat ?
A goose like that could dump unburned mix into the exhaust, which can get lit off by the cat.
In fact, I scattered ( literally ) a muffler that way just last week.

dennisbernal91z

iTrader: (2)

Nope, no cats of any kind. I reved up after a drive home from work, 20 min. and I reved it to 3 grand and keyed OFF. The motor came down and when it got to 1000 or so. BANG!!!!! So scared. My ears were ringing for 30 minutes after. (This happened in my one car garage). Now whenever I drive the car, I make sure I am at idle and sitting for about 5 seconds, then I turn her off.

It runs-on like 60% of the time. For example, getting to work today. No run-on, I will post if it does when I go out for lunch and get also when I get back to work.

Stupid problem. Ugh...