Exhaust Popping after Ignition turned off

Looking for advice on how to remedy the exhaust popping after ignition is turned off.
It happens a second or two after the engine dies, I am using a Pertronics ignition module and it seems like it only started after the outside temp has gotten cooler.
I only have around 400 miles on a new 350 crate motor and it wasn't doing it for the first 200 miles or so but it was alot warmer outside, not sure why that would make a difference anyway.
When it first did it I thought some raw fuel might have been lighting off inside the Lake Headers but it has been suggested that it is probably ignition related.
It doesn't sound like it is Dieseling at all it just does it once out each side.
I just want to make sure that it isn't going to damage anything.
Hope the video works, first time using youtube.




http://youtu.be/0rMavJ_dD8g

 

I have a lawn mower that does the same thing. When you shut the engine off, you are shutting the ignition off. It keeps pumping fuel/air mixture through the engine and into the exhaust. Eventually it explodes in the exhaust.

I cured the mower by shutting off the fuel to stop the engine. The mower was gravity feed. If you do it on a car, it will empty the carburetor and it will take a while to fill it to start.

You likely have a fairly straight through exhaust that allows air to come in via the tailpipe. Adding some bends in a vertical plane can help.

 

This MIGHT be an indication of the fuel/air mixture being a little lean (while running), causing a hot spot in/on an exhaust valve/manifold/header, when the above mentioned rich mixture of fuel settles in, the hot spot ignites the mixture.... BOOM...

 

Air is getting in the exhaust system somewhere, and near the engine. Then there is some unburned gasoline in there, too. A hot spot will touch it off. How fast is the idle? The engine windmilling to a stop from a too-fast idle will blow a lot of fuel out the exhaust.
Check for exhaust leaks, and an over-rich idle.
You might try letting it idle for half a minute to cool everything off and see if that stops the banging.

 

Thanks for the responses, how about the possibility of damaging something?
Heres a pic of my headers, would it make a difference if it was hot or cold outside? It did not seem to do it in the summer.
I don't think running the carbs dry is a solution.
I adjusted the idle adjustment screws with a vacuum gauge hooked up but temperature changes seem to make a big difference on the Holleys and how they idle !!

 

My mower will do it also unless I idle it down for awhile before shutting it off that is why I originally thought it might be fuel in the headers.
The car pretty much idled down my street and into the shop before I shut it off.
It was probably idling around 1000 rpms when I shut it off, I know that is a little high.
Still dialing in the carbs, this is my first time around so I am learning as I go.
First time it has been out of the shop with the outside temps just above 40.

 

Sounds like the throttle blades may be open a tad too far at idle. Idle a little too high. Maybe slow the idle a little? My mower does that too. LOL. Scares the shit out of ya sometimes.

 

I have just about pissed myself a couple of times when the mower did it, it was louder than the car.

 

.....as stated, idle your mower down THEN shut it down...if that isn't working replace the "fuel shut off solinoid," ay the bottom of the carburetor...(about $30 bucks)

 

djust, I just looked at your pics of your build, Very nice work.

 

If it's a proper transmission car, try this: When you stop, put the trans in high gear and stand on the brake pedal. Let the clutch out to stall the engine and see if it still pops. If not, it's run on letting fuel through. If it's an auto, obviously you can't try this.

 

Usually the exhaust system if full of burned gases that prevent ignition of the small amount of mixture that gets into the exhaust system. If the engine takes too long to shut down, it pushes the burned gases out so it is full of combustible mixture. Cooler weather might allow some carbon to build up in the exhaust that could stay hot enough to ignite the mixture. The large diameter pipe on your exhaust might allow fresh air to creep in after shutdown allowing the mixture to ignite.

 

It may also be the ignition fires one more spark as the voltage decays on shutdown and it jumps to a cylinder with an open valve firing the bit of fuel left there.

Or you have a bit of hot carbon buildup in the exhaust manifold / header which cooks the bit of unfired mixture pumped in after shutdown and it fires in the pipe after a few seconds.

 

Crap I never looked close at your pics. You are running straight out of the headers? Cold weather can mean denser air, hence a leaner mixture. It may be a lot touchier with the short pipes. Fatten it up just a tad on the idle screws and try it. May be a lean pop when you shut her down. Possibly. Maybe. Sort of. Lippy

 

Tuning issues are interesting (frustrating)...lol. It cracks me up that some of you guys are saying that your mowers do this...'cause mine does too...lol. It's a big gas rider. Two steps away, and BAM!...and I jump...lol. I acually opened this thread to see what I could learn about it.

About your car, djust...cold air carries more oxygen. I wonder if that's why it's happening now, and not when it was warmer. I also imagine that the extra cold air tends to exchange faster with the hot air in the exhaust. Although...my mower does it when it's nice and hot outside...so maybe not.

I agree that it seems like fuel-vapor migrating to the hot spots. A rich condition. I know that my mower has dirt in the fuel-system...and I'm compensating by running it with the choke partially on. Take that for what it's worth. Dirt in the fuel, carb inlet valve not shutting off, carbs too rich, etc.

I've had a little difficulty with the Pertronix ignition system. Wired correctly with their coil, etc...but it acted up sometimes. I think though, that regardless of any ignition issues, your back-fire wouldn't happen without a the rich condition. Good luck. Let us know when you figure the darned thing out.

Edit: Like Lippy and others mentioned...it could be a lean condition too...I might be full of hot air. Now I'm gonna have to see if my mower does it when I shut off the fuel first. I think I always shut off the fuel right before I turn off the ignition. That would be a lean condition. Get the carbs dialed in, and maybe she won't pop no mo'.

 

Sugestion for mowers.Go to your local parts store and buy a can of Seafoam engine and carb treatment .works wonders on small engines..
As for your exhaust pop ,,excess fuel in a hot spot.

 

Ok, I got maybe lean and maybe rich, I can play with that some on carb adjustment.
I appreciate all the replys.
How about the possibility of it hurting something in the motor at all when this is happening ?

And thanks for your kind words BISHOP, took me over 5 years to put it together and its my first, it has things that could have been done alot better but I am pretty happy about the way it came out.

 

Not going to happen. No way that is going to hurt your engine, just your pride.

 

Pull your plugs and check their condition. Chances are your are running rich, and with the idle that high you are not on the carb's idle circuit. So its probably the last rich charge getting enough air (passing by the throttle butterflies, then cooking of in a still hot cylinder. You need to get that idle speed down to 600 or so.

What is your engine's operating temperature? Are you choke's opening all the way when at temp?


I had a rotary engined Mazda that had a thermal reactor as well as a cat converter in the exhaust. It also had a manual choke. When it was cold I remember taking it one day to go to the grocery store. The choke was still out a bit when I pulled into the parking spot. I got out locked the car walked about 25 feet away and it let off an impossibly loud back fire. Other folks in the parking lot were ducking and running for cover, while I continued my walk to the store doors. The car did the backfire thing pretty regularly, but this was a mega ton rated deal.

 

that lawn mower shit is funny!! when we bought our rider i took it around the yard at full throttle and went to the shop and turned the key off.as i was gettin off this damn thing it backfired so loud my very first thought was someone was shooting at me so i went head first for the ground. my wife laughed her ass off and i felt like a dumbass.lol

 

How much timing is in this thing? The popping reminds me of Onslo on Keeping up appearances. You know, Mrs. Bucket? I know, not much help. Throw some cheap mufflers on it and see if it still does it. Lippy

 

The twin cylinder flathead Briggs&Stratton engines fire both cylinders at the same time,just like a Harley. A loud backfire is the nature of the beast,when shutting down.

 

I've got a snowmoble that does it, especially if I hit the kill switch before it idles down for a moment. Sometimes it takes a half minute before it explodes, so it catches me (and friends) by surprise. The snomo is a two-stroke though, so I know it's unburned fuel being ignited in the hot exhaust pipe.

As for your problem, I'd blame the carburetor before I'd blame the ignition. Turning the key off kills the ignition.

 

The idle is down around 7 or 8 hundred when it was warmer out it was idling a bit fast as I came into the shop even though my foot was off the gas pedal.

Initial timing is around 8 or 10 degrees and 34 or 36 total if I remember right vacuum advance is not hooked up.
It has 3 94's on it and only the center one has idle screws the others are blocked off, also it has a manual choke that is not hooked up to anything.

Might be something I have to live with in cold weather which is ok I guess as long as I am not doing any damage to anything.
thanks again for the advice.