350 chevy run-on (dieseling) problem...

[Black1]

I posted this on the Vette forums, but I know there are some "guru's" on here as well... so here goes.

_________________________________

Hello all,

I have a 1967 with a ZZ4 crate engine. I'm having a run-on (dieseling) problem. I was wondering if any of you had any ideas of how to stop it.

Here's what I have installed:

ZZ4 350ci GMPP crate engine
650cfm Demon carb (crate engine model)
MSD 6AL ignition box
MSD Pro Billet Distributor (tach-drive)
MSD Baster 2 coil
Stock GM (2 wire) distributor
Stock(ish) ignition relay box

I've installed the little capacitor supplied with the 6AL box. I went back and double checked that it was installed correctly (soldered in-line on the green wire (4th pin on the ignition box), and that the line on the cap was closer to the plug going to the alternator). ... This didn't solve the problem.

My timing is set to dead-nutz "0"... should I advance it and see if that helps?

The carb is running a little rich, but all the floats seem to be set properly and the car runs fine....

Any help or suggestions would be great.

 

[TNVIPER]

"dieseling"...hadnt heard that problem in a long time.....dont let it continue cause it is hard on the engine....I remember putting the car in gear to stop it.....is the engine high compression?......the problem may be the gas doesnt have enough octane rating....maybe a octane booster like Lucas will solve it......carbon buildup on the pistons will cause it too....

 

[scoobert]

check the carb? dieseling would suggest fuel was getting in after power off, and the heat was igniting it. is there a way to cut fuel? dont know much about carbs myself.

 

[Black1]

check the carb? dieseling would suggest fuel was getting in after power off, and the heat was igniting it. is there a way to cut fuel? dont know much about carbs myself.

You are partially correct... It has a mechanical fuel pump, which runs off the rolling assembly in the engine. So, when the ignition continues to "run-on" (MSD box continues to get a faint signal), it will keep current going to the coil, which in turn fires the spark plugs.... This keeps the pistons moving (due to combustion) and keeps the fuel pump going. This causes a small amount of fuel to be pushed into the cylinders via the carb's idle setting. .... The engine stops when the fuel in the system is not enough to run the process.

Follow me so far? :dontknow:

 

[JTS VENOM PERFORMANCE]

its too rich, too much fuel is still n the combustion chamber after shutoff, and with the msd, it will try to keep firing, either dissable the msd or lean out the carb or swap to vaccum secondaries and take off the double pump,
or lean out the carb,
also make double sure the blades are fully closing.
but since its a new engine, its most likely too much fuel left in the chamber, thus needing leaned out.

that is my take anyway, but fstjack is a super guru when it comes to carbs, probably one of the best in the country, I would check with him

 

[Black1]

....
that is my take anyway, but fstjack is a super guru when it comes to carbs, probably one of the best in the country, I would check with him

Thanks, Tony... I was hoping he would chime in... But, I will look at leaning out the mixture.

 

[JTS VENOM PERFORMANCE]

check the linkage on the secondaries, they should be adjustable, adjust them out a bit so they dont engage so quickly off idle, they could be at a touchy adjsutment and still dripping in a bit of fuel even at idle

 

[FATJACK]

Put some timing in that thing!!!

 

[scoobert]

You are partially correct... It has a mechanical fuel pump, which runs off the rolling assembly in the engine. So, when the ignition continues to "run-on" (MSD box continues to get a faint signal), it will keep current going to the coil, which in turn fires the spark plugs.... This keeps the pistons moving (due to combustion) and keeps the fuel pump going. This causes a small amount of fuel to be pushed into the cylinders via the carb's idle setting. .... The engine stops when the fuel in the system is not enough to run the process.

Follow me so far? :dontknow:

if that's the case can you put a remote ground on it?
when he shuts off the key, it activates.
no more spark means no dieseling?
i am really trying to learn carbs, seeing as how any good boat will have one.

 

[Black1]

if that's the case can you put a remote ground on it?
when he shuts off the key, it activates.
no more spark means no dieseling?
i am really trying to learn carbs, seeing as how any good boat will have one.

It's not really a ground thing... The signal through the wire is very very weak. But, the MSD box takes such a small amount of power to run, it's sensitive.

I don't think this is the issue, as I've already done the fix for it. That is, unless I just got a bad capacitor. :dontknow:

I might up the timing and see what happens.

 

[ntw0rk]

Lean it out...

And like Tony was saying double check all of the linkage and make sure it's all adjusted properly.

But, I know you have it running rich for the break-in too.... lean it out.

 

[FATJACK]

It's not really a ground thing... The signal through the wire is very very weak. But, the MSD box takes such a small amount of power to run, it's sensitive.

I don't think this is the issue, as I've already done the fix for it. That is, unless I just got a bad capacitor. :dontknow:

I might up the timing and see what happens.

Now yer talking..... I don't see how it runs at all at 0 degrees, advance that bitch.

 

[Begood]

Jake, is it a stick, or an automatic?

Bill.

 

[Black1]

Jake, is it a stick, or an automatic?

Bill.

Stick..

 

[FSTJACK]

If it is actually "dieseling", there are several things you can do.

Dieseling or run on is caused by hot spots in the piston crown and or combustion chamber. Di3eseling is not caused by a continuing spark but the mixture being ignited by hot spots in the combustion chamber area.

If you are running a projected tip spark plug you can change to a shorter tip plug and maybe a heat range colder.

If this does not do it you can install an idle speed adjustment solenoid. This is a electrically energized solenoid that is energized with the key in the run position.
You actually set the primary idle speed with the solenoid. You do not use the carburetor idle speed screw for idle speed and it is backed off and not touching the throttle lever. When you turn the key off the solenoid is denergized and this allows the throttle blades to close completly. This cuts off all the air flow through the butterflies and the motor just stops running. When the air stops the fuel also stops and it can't diesel.
When using this type of solenoid you need to depress the gas pedal slightly as the solenoid will not open the butterflies when closed. Once energized the solenoid will hold the the butterflies cracked and you adjust the idle speed with the solenoid.

 

[scoobert]

If it is actually "dieseling", there are several things you can do.

Dieseling or run on is caused by hot spots in the piston crown and or combustion chamber. Di3eseling is not caused by a continuing spark but the mixture being ignited by hot spots in the combustion chamber area.

check the carb? dieseling would suggest fuel was getting in after power off, and the heat was igniting it. is there a way to cut fuel? dont know much about carbs myself.

that's what i thought.

 

[Begood]

A quick easy cure is, while the engine is running, put the car in any gear, press on the brakes, turn the key off and let out the clutch.

Bill.

 

[WOT]

There are 2 types of dieseling that can occur. One is as FSTJACK says, where the motor runs on due to auto-ignition. The other is as I have experienced, and you would have to see it to believe it.

My motor is a '69 LT1. It would diesel when I shut it off. While trying to troubleshoot the problem I noticed that the when I would shut off the ignition and the motor would diesel (for up to 30 seconds or so), the motor was actually running backward!

The motor is 11:1, iron head, and has enough cam to to have a rough idle. What was happening was the partially unburned remnants getting deposited in the exhaust system (due to the cam and rich carb setting) get broken down enough to have easy auto-ignition properties. During the hottest summer months, the conditions were just perfect for the motor to diesel during shut off. Every single time, the motor would kick off and start running backward. If I turned the key on, it would fire over forward and idle as normal -- turn the motor off, and it would stumble and then immediately kick-off and run backward.

I first noticed it when I let the clutch out when in first gear trying to kill it when it was dieseling, and the car started backing up. It was a trip. After that, I had a Friend toggle the key on/off letting go from idling to dieseling and you could see the motor spin one way and then kick over the other way. It was bizarre.

I fixed the problem by leaning out the idle mixture and blowing all the carbon out (top speed run). It kept it from dieseling for a couple of months until the soot built up again. Then, I would just romp on it some more and it would fix the problem. Maybe this will help your problem?

 

[Black1]

Welp... I added about 12deg of timing... runs GREAT now. :dontknow: It was previously at -3deg for some reason.

Took it for it's first highway cruise today. It rocked. :rock:

 

[FATJACK]

Great!! Glad it worked...Used to run my old street/strip small blocks around 30 degrees advanced have run as much as 42, no shit.