Technical 8BA Runs On "Diesels" After Engine Shut-off- Normal?

Hey Kats and Kittens!
It's your favorite Uncle ED. here hoping this finds you all doing well.

My 8BA seems to just not want to stop. I turn the ignition key off and she runs on until the fuel dries up. This is my first Flattie build and I'm not sure if dieseling is normal.
So, I'm putting it to the experts; is it normal?
She's a '49 Merc. 8BA with Edelbrock Intake and Heads and (3) Original 97s on top.

If it's not a normal condition, here's what I have...

• Plugs look good.
• Doesn't appear to be running rich.
• It did it with both points and Pertronix Electronic ignition
• Doesn't seem to be running hot.
• Timing appears to be close if not correct.
• Chamber Pressure and Bleed Down are all good
• No excessive smoke out of the Lakes Pipes.

Questions...

1. Am I running too hot a plug?
2. Living in Houston, what heat range and what plugs do you recommend?

Having spent most of my spare change on old Hemis and Nailheads, I know dieseling is not a good thing. It typically means your combustion chambers are too hot and you're pre-igniting your mixture amongst possible other things; but essentially, it's not good.
"mmm drugs and pre-ignition are bad, mm'kay"

So, I'm putting it out there for your knowledge and my "ED"ification!

Thanks, and Be Well.
Brother "Big" ED.

 

Do you have the alternator wired in with the ignition?

 

Doubtful. He said "I turn the ignition key off and she runs on until the fuel dries up."

 

Octane of fuel ??
6sally6

 

I seem to remember that if you use a "one-wire" alternator and connect it without a diode, it will "feed back" to the ignition, causing a "run-on" condition. When I built my old '36 3-window 30 years ago, there was a special plug available that made connecting the alternator straightforward and also incorporated that diode.

I think that's what @rusty valley was referring to.

Look at this : https://www.amazon.com/alternator-charge-diode-keeps-charging/dp/B084ZWTWZ8

 

alternator back feeding the system.

you need one of these
https://www.speedwaymotors.com/Spee...DVfwCV3tDUnGgsBa20mXw03n8joYxo9MaAltTEALw_wcB

 

I run both 6 and 12 volt one wire alternators on several of my cars. Never had this problem. They run from 8BA to 8CM to Model A and other flathead engines. Do you know the compression ratio of the heads? Are they new or used and possibly been resurfaced? Timing set too fast will usually be indicated by the starter hesitating and hard start. Stock timing on the 8BA should be set at around 4 degrees before top dead center. 8CM is 4 degrees, so the 8BA should be pretty close, also. Plugs were H-10 originally and were fine for your altitude and temperature, but hard to find. MotorCraft has plugs the mimic the same characteristics. Not very expensive and readily available from early Ford parts vendors. The only other thing I can think of is a sharp edge in a combustion chamber(s) that gets red hot. Being near sea level with high humidity has an effect on the combustion chamber pressure, so bear that in mind. You may need to tweek the timing a couple of degrees.
Edit: Your engine, if Mercury, is not an 8BA but is an 8CM. Longer stroke, a bit higher compression and a little more HP. Hope this helps you.

 

If you wire on the ignition circut, and you have the idle set to low rpms...it might shut off. Then again, if idle is high enough to keep a charge going, it will not shut off. If the idle speed is just perfect, mama bear says it will run on abit before loosing charge and die.

 

A one wire alternator doesn't use that plug nor is it wired to the igniton in any way, shape or form. The "ONE" wire is the Bat wire that goes to a hot all the time connection. Usually the junction block to the battery.

 

The one wire alternator suggestions could be true, but it all depends on how the vehicle is wired.

@"Big" ED when you turn off the ignition, does the idle change at all? If it is truly "dieseling" it will slow down a bit from an idle, and any that I've seen usually won't diesel on all cylinders, and there will be a distinct knocking sound.
You say it continues until the fuel is gone. I assume this means you have an electric fuel pump that shuts off the fuel supply. Is it connected to it's own switch, or does the fuel pump shut off when you turn off the ignition key?

 

Dang that engine is pretty.
Alternator feedback shouldn't cause dieseling, it should just not shut off because the alternator is still supplying power to the ignition through the exciter wire.

Normally dieseling is caused by:
Too high of an idle speed,
Carbon buildup in the combustion chamber, Doubtful in this case unless you are prone to let it idle for long periods of time or have been cruising at low speeds a ton.
Too low of an octaine level with the gas. If you aren't getting detonation on hard acceleartion I don't think that is the issue though.

 

Take the belt off the alternator and see if the problem goes away.

Terry

 

Don't forget that an idle rmps to high can do this also.

 

If that was the case it would run until the tank was dry .
Hard dieseling , pinging and that is very distinctive.

To rule out ignition have a buddy yank the coil wire off the coil or cap wire or pull the wire feeding voltage to the coil. If it keeps dieseling as you describe its not from spark firing it or some alternator issue. hook a volt meter to the coil wire and make sure it's not getting some voltage after you shut the key off . Possably a low voltage barely enough to keep it running

if its not spark caused Id try a cooler plug ( is it the correct type plug ) and or better gas first, richen it up if its lean . throw a temp gun on the plugs and see if there super hot or something.

Beautiful engine BTW

 

It is hard to see in your picture but do you run an electric fan? They can feed back into the ignition system and keep the car running. A diode can fix it.

 

Dieseling or "run-on" is usually the result of high idle speed and high compression chamber temperatures. But these two conditions alone will not cause an engine to diesel. The engine also needs at least some air and some fuel to continue running, even if poorly. And as already noted, removing fuel from the equation will stop the dieseling. Combustion can not occur with just excessive heat and air.

For a carbureted engine the easiest way to cut off the fuel supply, assuming the carb(s) are working properly, is to reduce or shut off air flow through the carburetor(s). Are your three carbs set up to work progressively or with direct linkage? If progressive, make sure that the throttle plates on the outboard carbs are fully closed at idle.

If direct linkage, disconnect the outboard carbs and make sure that their throttle blades are fully closed. For testing purposes leave the outbboard carbs disconneted. Either way make a long enough test drive to get the engine up to full temperature and see if it still diesels when you kill the ignition. If it does, turn the ignition back on and let it run at idle with no load on the engine for a minute. Sometimes this will allow the combustion chamber temperatures to drop enough so that it will shut off with no run-on. Avoid letting it diesel for any length of time.

Do whatever you can to keep the idle speed down and make sure it's not pulling fuel from the main nozzles at idle. Keeping coolant temps under control will help as well. If the plugs are sooted up badly the combustion chambers may be carboned up as well. As noted above you may want to run some top engine cleaner through it. Even some extended highway speed driving may clear out the carbon, especially if the car sees mostly in-town, stop and go driving.

 

My electronic ignition keeps my coil hot for five seconds after shutoff. I like it that way. It relieves fuel pressure and the carbs don’t leak afterwards. Dieseling is another story.

 

"Dieseling is a common problem with high compression / high performance engines, especially in warm weather or when the engine is hot. In this case, it's usually caused by running "cheap" gas with inadequate octane. In this case it can usually be remedied by switching to a higher octane fuel."

Hello,

Most hot rodders that build their own motors have the skills, follow the correct procedures and get everything to fire up with new plugs, timing and other things. But, back when the octane level changed, then the "dieseling" started for most of us.

At one time, Chevron/Standard Oil gas stations were the top of the line with three choices of gas. The top level was easily enough octane to keep our hot rods running well. We did not make mistakes in our builds or we just left the tuning to get us where we wanted to go. Whether it was the end of the dragstrip or down 40 miles to a Laguna Beach cove for some great times.

There was no reason to put in the cheapest gas or lowest octane because of the problems they cause our high performance motors. Some teens had to use the lowest octane levels due to the fact that it costs much less than the 3rd level or highest octane gas from the other pump. That was their choice.

We never saw dieseling at the drags because most used high octane rated gas. So, what is good for the powerful motors or any motors running in tense situations needed all the help they can get. Performance was not that far off, But when drag racing is measured in inches and bumpers, then everything is on the line... pun intended.

Jnaki

There was a time in our auto history when the federal government changed the gas quality levels, smog restrictions came on to the scene and then most cars had the problem. It was ok for all cars to use the lowest level of octane as the motors started and ran. But, the dieseling came with the low cost, low octane gas from the pumps. That caused many teen hot rodders and others to use those octane boosters with every fill up at any gas station. With the octane boosters, there was little to complain about, but costly. YRMV

It is/was a simple thing. The lower the octane level, the worse the high performance was hindered. It is not rocket science, but simple economics. With the cost of gas being so high, we also turned to mixing half and half to meet the high octane levels and still spend less money. Some magazine articles in conjunction with scientists studying the mixture's effects on high performance motors were amazed. Full power and no dieseling.

So, for a while, we were all running to two pumps to fill a quarter tank with the cheapo gas, and it was cheap. Then the rest with the highest level of octane available. For some reason, the mixture resulted in the same performance as using the high octane only gas. An empty tank gas fill up run was usually longer when we got gas at the local Chevron gas stations. Forget the middle level, it was low octane mixed with high octane that did the job. And, we still had plenty of tuned performance, with some savings on gas expenditures for a weeks worth of high school driving, cruising and racing.

 

has the one wire from the alternator gotten hot and fused itself into the coil feed wire

 

My old man had a Chevy 6 cyl pickup that would diesel when you cut it off. He wouldn’t run anything but regular gas, was still leaded back then. We’d just have it in gear when we stopped, cut off the key , hold the brake and slowly release the clutch. It’d jump and groan but it would go dead as soon as the clutch load hit it. After a few times, it just became natural to cut it off that way….

 

I've never heard of this feature before. What brand of ignition system are you using? Does the manufacturer verify and state that this feature exisits on this system? And maybe more importantly do they explain what its function is? How is the ignition wired up and where does it pick up its power?

 

I don’t believe it’s part of the distributor function. ????
It’s an MSD.
It’s wired directly to the ignition switch. I believe it’s more associated with the fact I have an electric pump wired directly to a power junction bar.
It seems like the switch cuts power to the MSD immediately but the engine runs for five more seconds before completely shutting down.
I don’t have a real answer, I’m just happy it does it.

 

Well it's been a week since "big" Ed checked in here. After re-reading his original post a couple times I'm not completely sure if what he's describing is dieseling or some kind of an electrical feedback problem supplying current to the ignition after turning off the ignition switch.

I keep coming back to his statement that "she runs on until the fuel dries up". Does that mean he's turning off an electric fuel pump before or after he cuts the ignition? Does he mean that the engine continues to run normally until shutting off the fuel pump? Or is it in fact chugging and clattering and trying to hurt itself, as in actually "dieseling"?

The more times I read his initial post, the more questions keep popping up. Like is the fuel pump wired up so that it only sees power when the ignition is switched on?

 

@"Big" ED How about an update? You've been given some suggestions, and also been asked for some clarification. These dead end posts drive some of us crazy.

 

He maybe waiting on that last tank of fuel to run dry.

 

Thanks Glenn. Like I said, this is my first Flathead as I’ve always been an early Hemi and Nailhead guy.
You’re right, I’ve got an 8CM. The ignition switch I used is a cool piece of a 40s Chris Craft boat.
Thank you for the help.
Brother ED.

 

Thanks for the advice everyone.
Like I was telling Glenn, I’m an old 50s Hemi and Nailhead guy and this is my first Flattie.
It’s an 8CM with 3/4 cam and higher compression Edelbrock heads. I had to deck the block so the compression is higher; once broken in I’ll get a final compression reading on all cylinders.
The keyed starter switch is an old piece of a 50s Chris Craft boat I found at a swap meet years ago which I rebuilt with some parts from a VW of all things. So there is a good possibility something in there is crossed where it shouldn’t be.
I can’t thank you all enough for the advice and once I get this gremlin solved, I’ll post my findings.
Be Well and Be Safe!
Brother ED.

 

Thanks Clay.
When I turn the key off, the engine continues to run like normal and slows as if it runs out of gas.
Thanks.
Brother ED.