Big Bang Theory help
 

I frequently had this same problem in the early sixties with my stock motored Model A. The car now has what is a pretty typical touring motor: high compression head, FSI distributor, Weber carb, performance cam, etc. The car runs fine while driving.

...and the problem I would like to solve is: what causes a loud BANG about a second or so after pulling into the garage after an hour or so drive and shutting the engine off? The strength of the bang will vary, and usually it does not seem to happen after short local drives, but yesterday it popped a bit after just a trip to the gas station for a fill up.

Thanks for your thoughts.

Re: Big Bang Theory help
 

Hey Russ, the big bang is caused by unburnt fuel in the exhaust system. The secret to finding the cause is tricky. Eg..exhaust leak, low carb float, too rich a mixture, timing off slightly.....ect. When the engine gets turned off, the manifold heat is enough to ignite the unburnt fuel. Jeff

Re: Big Bang Theory help
 

You turn the key off but the engine keeps spinning for a few revolutions until it stops turning. That pulls in air/fuel mixture from your carburetor but there is no spark in the cylinder to burn it. The air and unburned fuel go out the exhaust valves, into the exhaust manifold and into the muffler. In one of those places, probably manifold, a small speck of glowing carbon ignites the fuel and you get a backfire out the exhaust.

This is a running joke with one of the guys in our club because his engine pretty much always backfires when he shuts it off; it startles most the ladies :eek: (and some of the men :D )!

To address this:
- lower your idle RPM as much as you can, lean your mixture out as much as you can
- turn the fuel off before the ignition switch and run the fuel out of the carb to kill the engine, then turn key off
- put the transmission in high gear and (with your foot firmly on the brake) let the clutch pedal out to stall the engine, then turn the key off

Others may have creative ideas to address this, too.

Re: Big Bang Theory help
 

Sometimes this is caused by the mixture being a little on the lean side . Before switching off unscrew the GAV a couple of turns it could well do the trick . Of course return it to the original setting afterwards but I bet you knew that !!!

John in worst snow in 4 years BRRRR Suffolk County England.

Re: Big Bang Theory help
 

You might also try revving the engine up a bit and holding the throttle a full as you turn off the key. This will pump a good volume of cool air/gas through the header, possibly cooling it to the point it doesn't ignite. It also might make a bigger bang, so be prepared.

Re: Big Bang Theory help
 

Back in the eighties I had a Jeep with the AMC 6 cyl. I would get to work in the morning, hop out and get about 30 feet away when the BIG bang occurred. I finally put a wrench on the nuts holding the manifolds to the block. Got nearly a quarter turn on some of them. Never a bang again.

Re: Big Bang Theory help
 

Not to make fun of your problem , but this reminds me of the John Candy film " Uncle Buck " here .
https://www.youtube.com/watch?v=5ibO5kob3OQ

Re: Big Bang Theory help
 

Weber 32/36 has no GAV..though popular in the model a world that carb is notorious in other applications..for just that reason,over fuel,poor economy and bad power..

http://www.aircooled.net/making-webe...led-vw-engine/

Re: Big Bang Theory help
 

Seeing you have an FSI ignition module you need to check and reset your timing. You could be slightly too advanced. You might have a bad key switch that makes an intermitent contact when you turn it to the off position. As many of the threads are saying fuel gets pulled into the engine when you shut it down. If the key momentarily switches on it could cause a backfire. Case in point the FSI ignition puts out a hefty spark to the point where I have had my engine after it has been running a while actually start without the starter motor. Hopefully you allow your engine to come to a low idle speed before turning off the key. Cheers

Re: Big Bang Theory help
 

Is your tune spot on? Adjustments to mixture screws could correct it.

Re: Big Bang Theory help
 

Check the tappets.

A too close a tolerance on the exhaust valves can do this.

Re: Big Bang Theory help
 

Chief used to IRRITATE Momma, at about 30, he turned off the key, floored the gas for a distance & turned the key back on----"BANG" like a CANNON!---"Hey, Chief, how did you get that BRUISE on your Right Arm"???--LOL--
I'm sorry my posts are SO STUPID, I'll "TRY" to control myself???
Bill Inthebackseatlaffin'

Re: Big Bang Theory help
 

Slow the idle as much as possible That fixed my prob.