backfires only when shutoff

[chefplw]

I new and just learning, so please be gentle. about half of the time when the truck is good and warmed up, when i shut it down the last thing i hear is a gunshot loud backfire. I read some of the old posts on backfires and most seem to be while running. Is the exhaust system still the place to start.

[scott]

i could be wrong but i think backfires occur when a detonation happens with a valve openned. noise & flames shooting out of the carb an intake valve was openned, out the tail pipe an exhaust valve.

so i would check timing first.

[Jeff Aronson]

It's sometimes a sign of hot running, not at the radiator, but inside the engine itself.

The backfire is usually unburned fuel igniting in the cylinder or exhaust pipe because something is rather hot. So check your timing [too advanced?] and make certain that your choke is closing properly so you're not running too rich.

Which carb do you have on the car, a Zenith or a Weber? A Zenith can run rich [see the sticky on the Forum main page].

Also, if you pull the plugs, you can see what color the tips are. If they are whitish, you are running too hot, and maybe, too lean in the mixture. If they are wet or really black, then either the mixture is too rich or you should check the spark plug wire at that cylinder. The right mixture will also reduce any backfire.

Once in a while, the problem is in the PCV valve, although that ususally causes plumes of oil smoke from the exhaust as a symptom. Do you have a valve on your engine? It's next to the carb if you have one.

What I'm suggesting is that while the problem might be unburned fuel sitting in the exhaust system and igniting, you want to track down what's causing the unburned fuel at all.

Good luck with your diagnosis - let us know what you find.

Jeff

[greenmeanie]

Also check your exhaust and manifold flanges. A hot engine on shut down can eject a cylinder or so of charge that did not get ignited into the exhaust. On a well sealed unit this is not a problem as there is not enough oxygen to allow it to burn and it will have become cool enough or diluted enough by the time it exits the pipe. On an engine with a leaking exhaust you have a source of oxygen.

[I Leak Oil]

It could be many things. As others have suggested check the ignition timing, points gap, make sure the carb is adjusted and functioning properly, check your manifolds for leaks. Could also have the wrong plugs or a hot spot in the one or more cylinders. Make sure any vacuum ports are plugged tight and make sure the vac. advance on the distributor and power brake booser (if you have one) isn't leaking.
My neighbors lawn tractor does the same thing, scares the heck out of his 80 year old mother every time.
Jason T.

[CliftonRover]

first check the air mixture screw then the timing. the mixture can be set by sound. with the hood open all the way, you can stand back by the carb and play with the mixture screw. you can adjust the screw and then push the throttle linkage open all the way for half a second and then let go. the engine should be able to rev up quickly but also not backfire when coming back down to idle. while doing this intermittently check the timing. repeat the process until you get the best results. this should work for either carb, but with the weber I adjust the mixture screw in until the engine starts to bog down then turn it out until I have the desired tune.
this may not be exact but I have been tuning this way for years and watched my dad do it for years before that. it works. good luck
I drive my series truck 6 hours every weekend on all highway and it works well for me.

[chefplw]

got a chance to look at a couple of things as suggested. very thing seem tight in the exhust which I guess is why the backfire only seems to happen when shut off engine. last gas builds up hits air boom. pulled plugs all were white at tip. so runnig to lean and hot. next question is which screw to turn to adjust. I got weber 2bbl. not sure of model. thanks for the tips. hope that with time I'll be returning the favor.

[KevinNY]

It could also be as simple as too fast an idle.

[Jeff Aronson]

Wow, the 2 bbl, I have the regular Weber 1 bbl. With luck, Weber puts them in the same places on both carbs.

On mine, the mixture screw [lean and rich] is on the back side of the carb, with the slotted end actually facing the firewall. The idle adjustment screw is on the driver's side [LHD] facing the fender. It's right where the throttle linkage attatches to the carb.

The white tipped plugs are signs to running too lean or running too advanced in terms of timing. That's something to pay attention to because if you're running too hot for too long a time, you risk burning valves. It's worth gapping the points first and then timing the car, too. Then, if it still backfires, adjust the mixture. Play with the carb last - it's the least likely cause.

Jeff

[scott]

got a chance to look at a couple of things as suggested. very thing seem tight in the exhust which I guess is why the backfire only seems to happen when shut off engine. last gas builds up hits air boom. pulled plugs all were white at tip. so runnig to lean and hot. next question is which screw to turn to adjust. I got weber 2bbl. not sure of model. thanks for the tips. hope that with time I'll be returning the favor.

this is the web i have. i've copied the pdf at this site to my computer. it helps a lot

http://www.racetep.com/weber/32_36DGV.pdf