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saintjoelarry · 10-24-2018, 11:24 AM

I'm not sure I saw a response to JR1967's comment about a plugged exhaust

I had a very similar condition that you have - and I did the exact same things you did to find the problem. In the end, it was baffles in the later style muffler had collapsed and blocked the exhaust. At idle the pressure is low enough that it could get through the small gaps around the blockage, but at higher RPM, the pressure was too great and caused backfiring.

Here's a link to my discussion thread:

https://www.fordbarn.com/forum/showt...ferrerid=17981

Assuming you exhaust system is welded one piece from manifold to tailpipe, the only way to test it is to disconnect the exhaust pipe from the manifold. That of course has some risks, as any backfire is occurring right in front of the carburetor. And honestly, with NO exhaust back pressure, you may well get some backfiring when you let off the gas. But, when I tested it, I increased idle up as high as I felt comfortable, and had no backfire (that is, until I released the throttle and it felt back to idle - but that was minor).

Good luck! And keep a fire extinguisher handy!

10-24-2018, 11:24 AM	#56
saintjoelarry Junior Member Join Date: Nov 2011 Location: Grand Rapids, MI Posts: 25	Re: Terriable backfiring I'm not sure I saw a response to JR1967's comment about a plugged exhaust I had a very similar condition that you have - and I did the exact same things you did to find the problem. In the end, it was baffles in the later style muffler had collapsed and blocked the exhaust. At idle the pressure is low enough that it could get through the small gaps around the blockage, but at higher RPM, the pressure was too great and caused backfiring. Here's a link to my discussion thread: https://www.fordbarn.com/forum/showt...ferrerid=17981 Assuming you exhaust system is welded one piece from manifold to tailpipe, the only way to test it is to disconnect the exhaust pipe from the manifold. That of course has some risks, as any backfire is occurring right in front of the carburetor. And honestly, with NO exhaust back pressure, you may well get some backfiring when you let off the gas. But, when I tested it, I increased idle up as high as I felt comfortable, and had no backfire (that is, until I released the throttle and it felt back to idle - but that was minor). Good luck! And keep a fire extinguisher handy!