Backfire on one side

[Steven]

On my 350 just put headers on around a month ago or so and ran them open when the weather was nice. Threw some exhaust on before the cold came through. Once the exhaust got on ran into a backfire only on the pass side through the exhaust. Figured it's a valve too loose or tight. Only happens in park or neutral at idle and doesn't happen every cycle. Pops a couple times stops and comes back after about 10 or more seconds. Rarely happens in gear at idle but with any throttle seems to disappear totally. No power loss in gear. Just an annoying backfire. Wanted to know if it sounded like a valve problem and how to go about seeing which would be the problem. I also believe timing is where it should be. Had a miss and adjusted the miss out

 

[87scotty]

Pull your plugs see if they tell ya anything!

 

[87scotty]

Hydralic lifters?

 

[rich weyand]

Sounds like a small air leak to me. When the throttle is open at all, you are running a lot of air, and the leak doesn't affect anything. When the throttle is closed, vacuum is high and air flow is down, and the air leak can cause a an intermittent lean miss followed by a backfire in the exhaust. Doesn't happen in the open headers case because the unfired lean mix doesn't hang around.

Check the carb-to-intake connection on that side. Spray some carb cleaner or WD-40 along the carb-manifold seam on that side and see if that temporarily clears it up.

If so, replace/add the gasket between the carb and the intake.

 

[rich weyand]

I talked to my bro, who has far more knowledge than I w.r.t. engines. He says you likely have an air leak into the exhaust system, probably where the header connects to the exhaust pipe. The exhaust gases at idle, flowing past the leak, suck air into the exhaust pipe, and create a combustible mixture with the unburned fuel in the exhaust stream, which occasionally ignites. When the exhaust flow speeds up and the pressure in the exhaust rises, the air leak stops. If you are running rich at idle, such as when the engine is cold and the choke is engaged, it will be worse.

The good news is that replacing the gasket between the header and the exhaust pipe is simple and cheap.

 

[Steven]

It shouldn't be an exhaust leak. Just put on brand new gaskets with the rest of the exhaust and made sure that the bolts were tight on. With the open headers i did have a backfire too, but my guess with that is the back pressure coming in.

 

[Rusty Nail]

That side is lean.

Or rich.

Edelbrock carburetor by chance?

 

[rich weyand]

It shouldn't be an exhaust leak. Just put on brand new gaskets with the rest of the exhaust and made sure that the bolts were tight on. With the open headers i did have a backfire too, but my guess with that is the back pressure coming in.

The time to suspect something is when that's the thing you worked on.

Pretty easy to check.