Weber Backfire on Shutoff

[Brett!]

Hey y’all.    

I have a Weber in my Brat and despite best efforts to tune the carb and set the timing, I always get a cannon blast out of the exhaust when I shut off the car.    

The timing is currently set at exactly 8* and  the carb feels well dialed in when driving under all conditions.    However, when the engine is warm and you shut it off.....BAM! Always a single gunshot that terrifies anyone in ear shot.   

 

So, any ideas of things to check / change / tweet.    Do Weber’s have an anti back fire valve?  

[moosens]

Timing

[Brett!]

I get it, 4 sentences are hard to read.  Did you miss that part where the timing is dialed in, checked and rechecked?   

Its ok if you did buckaroo, what else do you think it could be?

[moosens]

Yup read it all.

Had too many of these cars. Is this your first ? 

[moosens]

Not insulting you or anyone else so why make that comment ? Relax. 

 

It’s the timing and that’s that. Best of luck with it. Check it all again. 

[Subasaurus]

sometimes timing isn't where it's suppose to be, the flywheel isn't perfectly accurate, my 79 Wagon liked being at 9 1/2 degrees while my 80 Brat likes being at 7 degrees, just go by feel and sound, and keep an eye on that temperature gauge!

your jets may be too large or you need to close off that fuel mixture more, check with a vacuum gauge as you're tuning it down some, you want stable 17-18 inches of Mercury (or more! if possible). you're dumping an excessive amount of fuel so it flash ignites in the exhaust and BAM! 12 gauge shotgun.

check your wires that wire up to the ignition coil, you may have a wire on the wrong polarity on your ignition coil, this happened to me once and caused this shot gun like sound when shutting the car off, there's multiple ways to make the vehicle pop like this.

i had so much fun doing this on purpose in my 79 GL Wagon, i would floor it as the vehicle shut off to dump more fuel to make the pop even louder when i would arrive at work back when i worked at a shop, and once in a while police would arrive due to the report of gunshots fired, which weren't. i miss that wagon.

i know this because i would do it to my friends when they were following behind me at night, big flame blast comes out the rear and ofcourse a gun shot like sound, IT IS LOUD.

Edited September 16, 2019 by Subasaurus

[montermahan]

A thought.  My tractor does that.  I just let it idle for a few seconds befor turning off ignition.  The carb will keep putting gas in the exhaust system as motor spools down after you shut off ignition, them something hot will set it  off.   

[Brett!]

3 hours ago, Subasaurus said:

sometimes timing isn't where it's suppose to be, the flywheel isn't perfectly accurate, my 79 Wagon liked being at 9 1/2 degrees while my 80 Brat likes being at 7 degrees, just go by feel and sound, and keep an eye on that temperature gauge!

your jets may be too large or you need to close off that fuel mixture more, check with a vacuum gauge as you're tuning it down some, you want stable 17-18 inches of Mercury (or more! if possible). you're dumping an excessive amount of fuel so it flash ignites in the exhaust and BAM! 12 gauge shotgun.

check your wires that wire up to the ignition coil, you may have a wire on the wrong polarity on your ignition coil, this happened to me once and caused this shot gun like sound when shutting the car off, there's multiple ways to make the vehicle pop like this.

i had so much fun doing this on purpose in my 79 GL Wagon, i would floor it as the vehicle shut off to dump more fuel to make the pop even louder when i would arrive at work back when i worked at a shop, and once in a while police would arrive due to the report of gunshots fired, which weren't. i miss that wagon.

i know this because i would do it to my friends when they were following behind me at night, big flame blast comes out the rear and ofcourse a gun shot like sound, IT IS LOUD.

I had taken it into a shop to try to get assistance with the backfiring issue because despite my best efforts I couldnt' get it to stop.   They were not old school subaru folks, but they do restore MG's, Midgets, and Triumphs that commonly use the Weber we upgrade to.   They rebuilt the entire carb and it ran better, but they were not able to eliminate the backfiring.     Over the weekend, I decided to try again and fix the issue.   Turns out they had super advanced the timing and it was set well into the negative degrees, so much so that with a timing light, you couldn't see TDC.   I adjusted it back to approximately 8* which made a major difference in how smooth the engine would run at idol.      I thought this might fix the issue, but once the engine was good and hot and got switched off.. BAM!!!!!!  louder then ever.   

This is why I dont believe its the timing, although I havent tried setting the timing even further retarded past 8*.        

Excessively large jets and a build up of unburnt fuel is where I was leaning.   I am unfamiliar with the vacuum gauge test you are describing.   Could you elaborate a bit more, or give me a link to do some reading.  

[Brett!]

Oh, I understand what you were trying to get me to check with the vacuum gauge.     Ill check all of that this evening and report back. 

[Otterby]

Is it a backfire coming through the carb or an afterfire coming from the exhaust?

On warm days with the engine running at temp for a while I will get a good afterfire coming from the exhaust when I shut it down. My engine has some pretty good wear and the blowby is fed into the air cleaner causing a rich condition when the engine is hot.  My theory is that these gasses collect in the exhaust system enough that they detonate when the engine is shut down. I don’t want to vent my blowby outside of the engine so I installed a oil catch can and that helped slightly but my only real solution is to rebuild the motor.