I've got a 76 F-100 with a 390. I just recently finished putting a new clutch in it but during the process I broke off a bolt from the header going to the downpipe. I put on the exhaust with the one bolt I had and drove it with the leak for a total of about 10 miles.
The problem was every time I tried to slow down with the truck in gear it would start sputtering really bad. I finally drilled out the bolt and put a new one in so the exhaust leak is gone. But I can still hear it sputtering when I'm slowing down in gear!? It sounds muffled but it's still sputtering.
Any ideas as to whats causing this?
vaccume leak? Bad fuel mixture? I'm pretty sure I dont have a vaccume leak though.

 

Sounds as though the gasket blew apart when the bolt was missing. If the gasaket is gone, the leak will still exist. Might have to replace the exhaust manifold gsasket on that side.

 

You still have an exhaust leak. The name for that sound is actually backfiring.

 

Eric,

Is there any reason for the back firing after you let of the gas and let the engine slow itself down? I have always noticed that when hot rodders run glass packs

 

How is what Eric said off topic? The guy is asking what that sputtering sound is and Eric told him. Or are you saying your question is off topic?

In answer to your question its all about back pressure or lack there of...less back pressure = that wonderful burbble youre referring to...more = a quieter ride and more muffled. A regular muffler has baffels in it to bounce the exhaust and all the noise around before it exits...a glass pack is almost straight through...which means less pressure and more noise. Just for comparison...a blown out glass pack will end up sounding almost like you have no muffler even on there....a dead standard muffler will have a lot of resonance to it...kind like an echo inside a stadium...hope that explains it a bit...I tried to explain it as best I can w/o gettin into all the science stuff lol.

Anyway...back on topic...75F350 is probably correct sounds like you also took out the exhaust manifold gasket when you ran it with it partially connected. Just to be on the safe side I would replace the gasket on that side and maybe even add a little high temp RTV with the copper in it for some extra protection.

You mentioned headers...do yours have a little exhaust doughnut like the stock ones it do they just go on from the collector? If you have the little doughnut Id replace that as well just to make sure that it isnt warped at all from the uneven pressure from the single bolt.
-Chris

 

The additional air pulled in thru an exhaust leak by the flowing exhaust gas mixes with the unburned fuel in the exhaust and is ignited making loud explosions in the exhaust system. Many hot rodders with cobbled together exhaust systems have leaks.

 

I'd have to agree with the first guy... sounds like the gasket blew off... I just had the same problem when working on a buddy of mine's truck.

Christopher

 

Headers are notorious for leaks. Copper gaskets are a real improvement over the fiber types on headers. Locking fasteners will also help on header applications.

A broken bolt on a normal manifold will cause huge leaks and blown gaskets. The fiber gaskets work OK on normal cast iron manifolds. A broken bolt on the connection to the downpipe will also make for huge leaks and backfiring.

 

If there is an exhaust leak near the engine it can allow enough fresh air into the system to cause backfiring/sputtering sounds.

 

Thanks for the help guys. Yeah I'm pretty sure I dont have a gasket on it and I do know it's still leaking a little bit. It's alot more muffled now but this is beginning to suck! First the bolt breaks off, then I drill it out and tap it. Which was a huge pain and to top it all off I've still got a leak! I'll head out to the parts store and get some rtv and a gasket for it. Hopefully that does the trick.
When I had the downpipe halfway connected you'd be amazed at how many people told me "it sounds great!" I'm thinking are you kidding me? My exhaust is dangling by one bolt!?!

 

I don't think you'll want to use RTV on an exhaust gasket. It should go on bare.

 

I used the high temp RTV w/ copper...the kind for making gaskets like that on mine and it has done great...I uesd it primarially around the exhaust ports to insure a good seal...it also helped keep it in place while I was positioning the manifold back up. If you use it...make sure its the stuff like I said before.. that has the copper in it...anything else will probably melt and make a sticky nasty mess.
-Chris