Backfire after acceleration

[65Comet200]

Hi all,

I am having a little trouble figuring out a popping backfire I get after coming off the accelerator. The time is 10-12 deg. increase by 18-24 deg or so at 2000 RPM. The dwell on the LOM is 38, the idle screw is 1 1/2 off seated and the spark plugs are set right. I am running a Holley 1940 with SCV (it is the DOPF "Y") model off 63-64 223.

I do think that my exhaust system gets a little tight. I just noticed that it down sizes twice to what looks likes close to 1 in. dia. at one point. So maybe back pressure???

Not sure if that is enough information but any help would be great.

Thanks and have a good one.

 

[FalconSedanDelivery]

Deceleration Backfires are almost always caused by an exhaust leak that is before the muffler, if you have trouble hearing where , temporarily stuff a wet rag in the tailpipe and listen

 

[rbohm]

8) is the backfire coming out of the exhaust or the carb? it does make a difference. if it is going out the exhaust, then likely you either have a rich mixture and it is still burning after it leaves the exhaust valve, or you have an leaky exhaust valve doing the same thing. if it is coming out of the carb, then you either have a valve timing issue, or a leaky intake valve.

 

[65Comet200]

Thanks for the replies.
I am almost certain that my backfire is at the exhaust and not the carb. I should be putting the transmission back in tomorrow and will try the wet rag idea. There was a hot air choke port drilled into the exhaust and maybe that is the source of the leak. The rest of the exhaust is very rusty and fragmented. There are a lot of welds there so I will check out the whole system.

I will report back with any findings. Thanks again.

 

[ludwig]

If you are not replacing the hot air tube from the manifold, you need to plug the hole. I tapped it from above and below and screwed two screws in about 1/4".

 

[65Comet200]

Good news is I have the transmission back in and it is working great. Bad news is I am pretty sure the plug in my exhaust manifold is leaking. I wasn't able to get to the wet rag test to see what else might be happening but I will get to that tomorrow....I hope.

Thanks again and I will get that plug taken care of.